British aerospace company claims biggest engine advance since the jet(uk.reuters.com)
uk.reuters.com
British aerospace company claims biggest engine advance since the jet
http://uk.reuters.com/article/2012/11/28/uk-science-spaceplane-idUKBRE8AR0R520121128
108 comments
The liquid nitrogen is just used for the ground tests. On the Skylon itself, this cooling capacity will come from boiling the liquid hydrogen fuel.
Skylon would basically use the cryogenic fuel as a giant internal heat sink.
A more fanciful application of the concept: http://tvtropes.org/pmwiki/pmwiki.php/Main/StealthInSpace
A more fanciful application of the concept: http://tvtropes.org/pmwiki/pmwiki.php/Main/StealthInSpace
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Elon Musk was specifically asked about this technology when he was at Martin School last week. He didn't hear about it, but he seemed skeptical that it would work better than rockets. His response here:
http://youtu.be/c1HZIQliuoA?t=48m55s
SpaceX might still be the first one with a re-usable rocket if they finish the Raptor engine in 3 years. Plus, his could actually be used to land on Mars, while this can only be useful where you still have atmosphere and oxygen - so only for launches to orbit.
But it's great to see more private companies competing in this area. And even if nothing comes out of it for space travel, it might still turn out be a useful technology for airplanes.
http://youtu.be/c1HZIQliuoA?t=48m55s
SpaceX might still be the first one with a re-usable rocket if they finish the Raptor engine in 3 years. Plus, his could actually be used to land on Mars, while this can only be useful where you still have atmosphere and oxygen - so only for launches to orbit.
But it's great to see more private companies competing in this area. And even if nothing comes out of it for space travel, it might still turn out be a useful technology for airplanes.
> so only for launches to orbit
Isn't that our biggest challenge right now? I like how dismissive you are of it, like we have somehow overcome that and are now on our way to Mars any day now.
If someone was able to reduce the cost of going into orbit by as little as 10% that would be a MASSIVE achievement. Getting out of the atmosphere is still a massively costly, logistical, and dangerous challenge.
Isn't that our biggest challenge right now? I like how dismissive you are of it, like we have somehow overcome that and are now on our way to Mars any day now.
If someone was able to reduce the cost of going into orbit by as little as 10% that would be a MASSIVE achievement. Getting out of the atmosphere is still a massively costly, logistical, and dangerous challenge.
I didn't mean to be dismissive about it, just trying to point out the major differences in how SpaceX's design is supposed to work (if it does work) and how this is supposed to work. And I agree, just being able to send them to orbit in a much cheaper way, would already achieve 90% of our current space goals.
But you don't reduce the cost of going into orbit by 10%. Not even in the absurdly best case.
Let us take Elon at his word that increasing the size of the booster stage by 5-10% can replace these engines entirely, and assume that it actually is 10%. The booster stage is mostly fuel, which varies as volume, but the costs are all in the metal part, which scales as area. So this size increase costs you at most 6.6%. It actually costs less because the complicated rocket thrusters are left unchanged. So let's say 5%.
But we're not reducing the cost of the whole rocket by 5%, just the booster stage. If we assume that the booster stage is, say, 70% of the rocket we're only saving a maximum of 3.5% in reductions on the existing rocket components.
But we haven't yet factored in the cost of the new component. Which is, after all, very much like the existing engines except more complicated. In a Falcon 9 we have 9 rockets, so each one is about 11% of the cost of the total. If we strap on 3 of these new things, which are much like rockets except more complicated because of the whole hybrid thing, they each can cost a maximum of about 1.2% of the total cost to stay within our budget.
So unless these things, which are more complicated than rockets, cost you 1/10th as much, you don't get any cost saving at all. It is difficult to see how they can be this cheap.
Meanwhile, back in reality, Elon has plans to reduce the cost to orbit by another factor of 10 in a similar time frame to when these might actually become commercially available. (He's already reduced launch costs by a factor of 10.)
A large part of how he's managed to reduce costs so much is that he does this kind of common sense analysis in his sleep, and uses it to ignore everything he has to ignore, and zero in on what he needs to pay attention to. The numbers on this technology simply don't add up for space.
(But they don't have to. A practical technology to double the speed of jets by a factor of 2 will be extremely interesting to various militaries. Plus the potential savings for the existing airline industry - which is a much, much bigger market than space right now - means that they are going to have no shortage of potential customers. Elon is simply not one of them.)
Let us take Elon at his word that increasing the size of the booster stage by 5-10% can replace these engines entirely, and assume that it actually is 10%. The booster stage is mostly fuel, which varies as volume, but the costs are all in the metal part, which scales as area. So this size increase costs you at most 6.6%. It actually costs less because the complicated rocket thrusters are left unchanged. So let's say 5%.
But we're not reducing the cost of the whole rocket by 5%, just the booster stage. If we assume that the booster stage is, say, 70% of the rocket we're only saving a maximum of 3.5% in reductions on the existing rocket components.
But we haven't yet factored in the cost of the new component. Which is, after all, very much like the existing engines except more complicated. In a Falcon 9 we have 9 rockets, so each one is about 11% of the cost of the total. If we strap on 3 of these new things, which are much like rockets except more complicated because of the whole hybrid thing, they each can cost a maximum of about 1.2% of the total cost to stay within our budget.
So unless these things, which are more complicated than rockets, cost you 1/10th as much, you don't get any cost saving at all. It is difficult to see how they can be this cheap.
Meanwhile, back in reality, Elon has plans to reduce the cost to orbit by another factor of 10 in a similar time frame to when these might actually become commercially available. (He's already reduced launch costs by a factor of 10.)
A large part of how he's managed to reduce costs so much is that he does this kind of common sense analysis in his sleep, and uses it to ignore everything he has to ignore, and zero in on what he needs to pay attention to. The numbers on this technology simply don't add up for space.
(But they don't have to. A practical technology to double the speed of jets by a factor of 2 will be extremely interesting to various militaries. Plus the potential savings for the existing airline industry - which is a much, much bigger market than space right now - means that they are going to have no shortage of potential customers. Elon is simply not one of them.)
A tank's weight is proportional to volume, not surface area, assuming equal pressure and material strength.
Also, you often can't just simply stretch tanks - you need to increase thrust. Otherwise your payload drops because of lower T/W and more gravity losses in early flight.
Also, you often can't just simply stretch tanks - you need to increase thrust. Otherwise your payload drops because of lower T/W and more gravity losses in early flight.
A tank's weight is proportional to volume, not surface area, assuming equal pressure and material strength.
The weight of a full tank, yes.
The weight of an empty tank, no. An empty tank is mostly a shell, and the size of that shell corresponds to area.
Also, you often can't just simply stretch tanks - you need to increase thrust. Otherwise your payload drops because of lower T/W and more gravity losses in early flight.
I am assuming that Elon Musk's 5-10% estimate takes things like this into account.
Incidentally "early flight" in this case is very early. At the ground, oxygen levels are a bit over 20%. But as you go up, oxygen drops off faster than nitrogen, so oxygen intake falls off slower than drag. At some point you'll gain nothing. I do not know what that point is, but the oxygen/nitrogen level is part of why it is most efficient for commercial airlines to fly at around 9 km high. So it is really just a few km that you get a potential benefit. But your top speed at that moment is a pretty small fraction of what you need to get to orbit.
The weight of a full tank, yes.
The weight of an empty tank, no. An empty tank is mostly a shell, and the size of that shell corresponds to area.
Also, you often can't just simply stretch tanks - you need to increase thrust. Otherwise your payload drops because of lower T/W and more gravity losses in early flight.
I am assuming that Elon Musk's 5-10% estimate takes things like this into account.
Incidentally "early flight" in this case is very early. At the ground, oxygen levels are a bit over 20%. But as you go up, oxygen drops off faster than nitrogen, so oxygen intake falls off slower than drag. At some point you'll gain nothing. I do not know what that point is, but the oxygen/nitrogen level is part of why it is most efficient for commercial airlines to fly at around 9 km high. So it is really just a few km that you get a potential benefit. But your top speed at that moment is a pretty small fraction of what you need to get to orbit.
Nope, the shell. In a bigger tank (similar material and pressure) the shell has to be thicker. It's mathematically trivial.
Here's why Elon Musk is more likely to be right: because fuel is cheap.
The cost of fuel and the cost of fuel tanks is an insignificant part of the cost of an orbital launch, around the 1% level. The major drivers of cost are overall system complexity and manufacturing cost of the engines. And here's the big problem for a Skylon spaceplane, rockets are fairly simple systems whereas hypersonic airbreathing engines are extraordinarily complex and difficult. And if you can manage reusability on your launcher then the ordinary rocket engine wins hands down.
The reason why the jet engine won out over the propeller in civil aviation is not because of the higher thrust or better performance of the jet, it's because of lower operational costs. A jet powered aircraft requires less maintenance per passenger-mile than a propeller driven aircraft does. Partly this is because, despite the design complexities involved, a jet engine is actually a much simpler system.
The idea of not having to haul up a full load of oxidizer on an orbital launcher is a tempting one, but it doesn't come easy. One of the big advantages of a rocket is that it can push up above the bulk of the atmosphere when it's still traveling fairly slowly and do most of its accelerating in a near-vacuum. This reduces aerodynamic drag, aerodynamic heating, and dynamic pressure forces. All of which are some of the most pernicious problems to deal with in a launch vehicle. No few launch vehicles have been lost just as they reach "max-Q" (the moment of maximum dynamic pressure), and for an air breathing launcher it would likely be forced to fly through even more severe aerodynamic regimes than most rockets for significantly longer periods of time. This is hard on the vehicle design, hard on airframe longevity, hard on the thermal protection systems, and hard on the whole vehicle in general.
So on the one hand you have a vehicle which requires significantly more robust engineering and significantly more complex engines and overall design while probably having a shorter total service life. And is perhaps some significant factor riskier to fly in general. And on the other hand you have dead simple basically 60 year old engineering that is just put together sensibly, flown within a familiar flight envelope in a way that minimizes risk and iteratively improved to continuously shave off operating costs. It's a pretty safe bet which one is more likely to actually lead to lower launch costs.
The cost of fuel and the cost of fuel tanks is an insignificant part of the cost of an orbital launch, around the 1% level. The major drivers of cost are overall system complexity and manufacturing cost of the engines. And here's the big problem for a Skylon spaceplane, rockets are fairly simple systems whereas hypersonic airbreathing engines are extraordinarily complex and difficult. And if you can manage reusability on your launcher then the ordinary rocket engine wins hands down.
The reason why the jet engine won out over the propeller in civil aviation is not because of the higher thrust or better performance of the jet, it's because of lower operational costs. A jet powered aircraft requires less maintenance per passenger-mile than a propeller driven aircraft does. Partly this is because, despite the design complexities involved, a jet engine is actually a much simpler system.
The idea of not having to haul up a full load of oxidizer on an orbital launcher is a tempting one, but it doesn't come easy. One of the big advantages of a rocket is that it can push up above the bulk of the atmosphere when it's still traveling fairly slowly and do most of its accelerating in a near-vacuum. This reduces aerodynamic drag, aerodynamic heating, and dynamic pressure forces. All of which are some of the most pernicious problems to deal with in a launch vehicle. No few launch vehicles have been lost just as they reach "max-Q" (the moment of maximum dynamic pressure), and for an air breathing launcher it would likely be forced to fly through even more severe aerodynamic regimes than most rockets for significantly longer periods of time. This is hard on the vehicle design, hard on airframe longevity, hard on the thermal protection systems, and hard on the whole vehicle in general.
So on the one hand you have a vehicle which requires significantly more robust engineering and significantly more complex engines and overall design while probably having a shorter total service life. And is perhaps some significant factor riskier to fly in general. And on the other hand you have dead simple basically 60 year old engineering that is just put together sensibly, flown within a familiar flight envelope in a way that minimizes risk and iteratively improved to continuously shave off operating costs. It's a pretty safe bet which one is more likely to actually lead to lower launch costs.
In the SABRE design you can pick whatever transition point you like to move from air breathing to pure rocket operation.
Certainly, but if they pick a flight profile similar to modern rocket powered launch vehicles the advantages of the engine almost entirely evaporate. In order for the engine to be worthwhile the vehicle needs to spend a lot more time in the troposphere and lower stratosphere than any other launch vehicle, and that gives rise to all the problems I described.
Your requirement that they follow a rocket flight path is arbitrary. They'll use the best profile for the technology.
You're also underestimating how hard rockets work while still in the atmosphere. For example, the shuttles SRB work entirely within the troposphere and stratosphere. They're about a million pounds of propellant each, and together they make up 70% of the shuttles lift off weight. If you eliminated the need for the oxidizer in the SRBs, you'd save nearly half the entire weight of the shuttle. Because of the non-linearity of the rocket equation, saving weight produces compounding advantage, so this would be huge.
You're also underestimating how hard rockets work while still in the atmosphere. For example, the shuttles SRB work entirely within the troposphere and stratosphere. They're about a million pounds of propellant each, and together they make up 70% of the shuttles lift off weight. If you eliminated the need for the oxidizer in the SRBs, you'd save nearly half the entire weight of the shuttle. Because of the non-linearity of the rocket equation, saving weight produces compounding advantage, so this would be huge.
I think you're misreading what I'm saying, let me see if I can be more clear.
The key goal of an orbital launch vehicle is generating the necessary speed for orbit (over 8,000 m/s, around mach 25). The difficulty of reaching the altitude of low Earth orbit is inconsequential in comparison. A rocket has the advantage that it can do its accelerating wherever it's more convenient, so the typical flight profile is first up and then over, because it's a hell of a lot easier to accelerate and travel at high speeds above most of the atmosphere. For example, the Falcon 9 reaches an altitude of 5km before it even goes supersonic, and will reach an altitude of 30km within the first 2 minutes of launch.
An airbreathing engine however needs to stick around in dense enough atmosphere for its engines to work. And if a vehicle relies on a significant amount of airbreathing then it needs to spend a significant amount of time in that denser atmosphere. And that means that it needs to do more of its accelerating in denser air, which means that it will encounter higher aerodynamic forces, higher drag, more heat issues, a higher max-Q, etc. Those sorts of forces tend to be the "long poles" that aerospace vehicles are designed around, it dictates everything from the materials used to the type of construction to the service life of the vehicle's frame, etc. This is something that positively cannot be avoided for an airbreathing vehicle.
Sure, the SRBs generate a ton of thrust on the Shuttle, but they also help push the Shuttle quickly to higher altitudes and lower air pressure. Before the Shuttle hits mach 2.5 (of 25) it is already at an altitude where atmospheric pressure is 1% of sea level.
As I said before, mass isn't the big driver of cost in orbital launch vehicles, cost comes from complexity which comes from operational complexity (flight profile, staging, etc.) and design complexity (engines, control systems, handling, etc.) A vehicle which saves fuel but increases operational complexity is not a cheap vehicle. Fuel costs around $1,000 a tonne, whereas an engine can easily cost $10,000 / kg.
The biggest win that a vehicle like Skylon would have initially is that it might make it easier to make reusable launchers. If that's the case then even an expensive launcher which can be reused only a handful of times might still be useful in reducing overall launch costs. But if an entirely rocket based vehicle can be made to be reusable then it's very unlikely to have better overall economics or operating characteristics, for all of the reasons I've listed previously.
The key goal of an orbital launch vehicle is generating the necessary speed for orbit (over 8,000 m/s, around mach 25). The difficulty of reaching the altitude of low Earth orbit is inconsequential in comparison. A rocket has the advantage that it can do its accelerating wherever it's more convenient, so the typical flight profile is first up and then over, because it's a hell of a lot easier to accelerate and travel at high speeds above most of the atmosphere. For example, the Falcon 9 reaches an altitude of 5km before it even goes supersonic, and will reach an altitude of 30km within the first 2 minutes of launch.
An airbreathing engine however needs to stick around in dense enough atmosphere for its engines to work. And if a vehicle relies on a significant amount of airbreathing then it needs to spend a significant amount of time in that denser atmosphere. And that means that it needs to do more of its accelerating in denser air, which means that it will encounter higher aerodynamic forces, higher drag, more heat issues, a higher max-Q, etc. Those sorts of forces tend to be the "long poles" that aerospace vehicles are designed around, it dictates everything from the materials used to the type of construction to the service life of the vehicle's frame, etc. This is something that positively cannot be avoided for an airbreathing vehicle.
Sure, the SRBs generate a ton of thrust on the Shuttle, but they also help push the Shuttle quickly to higher altitudes and lower air pressure. Before the Shuttle hits mach 2.5 (of 25) it is already at an altitude where atmospheric pressure is 1% of sea level.
As I said before, mass isn't the big driver of cost in orbital launch vehicles, cost comes from complexity which comes from operational complexity (flight profile, staging, etc.) and design complexity (engines, control systems, handling, etc.) A vehicle which saves fuel but increases operational complexity is not a cheap vehicle. Fuel costs around $1,000 a tonne, whereas an engine can easily cost $10,000 / kg.
The biggest win that a vehicle like Skylon would have initially is that it might make it easier to make reusable launchers. If that's the case then even an expensive launcher which can be reused only a handful of times might still be useful in reducing overall launch costs. But if an entirely rocket based vehicle can be made to be reusable then it's very unlikely to have better overall economics or operating characteristics, for all of the reasons I've listed previously.
I think we're mostly in agreement now, just we disagree on our guesses of benefits vs risks.
The key point seems to be the complexity penalty of adding airbreathing to the engine vs the weight savings of less reaction mass. If we're comparing reusable apples to apples, this is really the value proposition. I'm clearly more optimistic on this point.
Also any engine that uses ram effect becomes more efficient at higher speeds. The SR71 uses less fuel per unit of distance the faster it goes, which is a bit counterintuitive. How big a benefit this is for space launch I can't really guesstimate but it's probably minor.
Reaction mass savings means more than just oxidizer material cost though. It ripples through the whole design. There aren't many times when the mass fraction of a rocket is working for you instead of against you.
I think we skipped over that a horizontal takeoff requires a lot less launch infrastructure. But being smart with rockets and launching from a barge in the ocean can equalize things.
As a summary, I think you and Elon may be right about Skylon for space launch. Mass produced rockets can get pretty cheap, and SpaceX does aspire to full reusability.
But space launch is only one of the two applications of a design like Skylon. Nothing SpaceX develops will be used for terrestrial transport. You aren't going to take a rocket to visit your family for the holidays, so Skylon may find a market there.
Skylon also could be used as a WhiteKnight style carrier for a more traditional second stage, which might still be interesting for space launch, but I'm pessimistic on this point because I think if the numbers worked the air force would already be using such systems instead of Deltas.
Skylon can also hedge that their high flow flash chiller is useful in other applications, and apparently they've developed an interesting high temperature composite material.
So on the whole I think it's interesting to watch what happens to them, even if it's not a sure bet.
The key point seems to be the complexity penalty of adding airbreathing to the engine vs the weight savings of less reaction mass. If we're comparing reusable apples to apples, this is really the value proposition. I'm clearly more optimistic on this point.
Also any engine that uses ram effect becomes more efficient at higher speeds. The SR71 uses less fuel per unit of distance the faster it goes, which is a bit counterintuitive. How big a benefit this is for space launch I can't really guesstimate but it's probably minor.
Reaction mass savings means more than just oxidizer material cost though. It ripples through the whole design. There aren't many times when the mass fraction of a rocket is working for you instead of against you.
I think we skipped over that a horizontal takeoff requires a lot less launch infrastructure. But being smart with rockets and launching from a barge in the ocean can equalize things.
As a summary, I think you and Elon may be right about Skylon for space launch. Mass produced rockets can get pretty cheap, and SpaceX does aspire to full reusability.
But space launch is only one of the two applications of a design like Skylon. Nothing SpaceX develops will be used for terrestrial transport. You aren't going to take a rocket to visit your family for the holidays, so Skylon may find a market there.
Skylon also could be used as a WhiteKnight style carrier for a more traditional second stage, which might still be interesting for space launch, but I'm pessimistic on this point because I think if the numbers worked the air force would already be using such systems instead of Deltas.
Skylon can also hedge that their high flow flash chiller is useful in other applications, and apparently they've developed an interesting high temperature composite material.
So on the whole I think it's interesting to watch what happens to them, even if it's not a sure bet.
Elon didn't directly address the advantage of the Skylon (he said he hadn't really looked into it, so fair enough). The main advantage is not that you don't need to bring oxygen for a portion of our trip (which as Mr. Musk points out wouldn't really save very much), but rather that you don't need to bring reaction mass.
Jet engines are just really really fast propellers. They don't need to carry their own reaction mass like a rocket. This gaves them a completely different efficiency profile (no reaction mass, means less weight, which means less fuel, which means less weight, rinse/repeat to the limit). This is the reason that 747s use jet engines rather than rockets. Being able to use both technologies fluidly within their optimal atmospheric envelopes would be a major breakthrough and would completely change access to space.
tl;dr Skylon can use a small, efficient jet engine to get high and fast enough that air resistance stops mattering and while slowly pivoting to a conventional rocket (which is optimal when the air gets thin).
Jet engines are just really really fast propellers. They don't need to carry their own reaction mass like a rocket. This gaves them a completely different efficiency profile (no reaction mass, means less weight, which means less fuel, which means less weight, rinse/repeat to the limit). This is the reason that 747s use jet engines rather than rockets. Being able to use both technologies fluidly within their optimal atmospheric envelopes would be a major breakthrough and would completely change access to space.
tl;dr Skylon can use a small, efficient jet engine to get high and fast enough that air resistance stops mattering and while slowly pivoting to a conventional rocket (which is optimal when the air gets thin).
Thanks for the description. So for a conventional rocket, like a Flacon 9, how much of the overall energy used is due to air friction? I would have thought, its just a small fraction.
According to this link (http://www.g2mil.com/high.htm), air friction is not completely trivial. Additionally rocket engines are more efficient in less dense atmosphere "because the thinner air allows a better plume". They estimate that launching from 30,000 feet provides 9.3% greater thrust.
Note that much of the energy spent for orbital flight is not spend getting height, but spent getting speed. Efficiency is greatly increased if you can reach high speeds without having to carry the reaction mass to reach those speeds.
LEO requires about 7.8 km/s, Skylon's jet engines can go around 1.7 km/s. You are reaching 20% of your orbital velocity without reaction mass.
http://en.wikipedia.org/wiki/SABRE_(rocket_engine)#Advantage...
The big savings, like spacex's grasshopper project, is of course a reusable space vehicle. The advantage to spacex's approach is that Skylon is a single stage to orbit space plane (you don't need to land a bunch of stages using retro-rockets).
If I was placing a bet, I would bet on spacex since they have a proven track record (rockets are easy, organizing/funding rocket companies is hard). Skylon is a great idea though and it is the general direction that aircraft engine design is headed (a peak at the future).
Note that much of the energy spent for orbital flight is not spend getting height, but spent getting speed. Efficiency is greatly increased if you can reach high speeds without having to carry the reaction mass to reach those speeds.
LEO requires about 7.8 km/s, Skylon's jet engines can go around 1.7 km/s. You are reaching 20% of your orbital velocity without reaction mass.
http://en.wikipedia.org/wiki/SABRE_(rocket_engine)#Advantage...
The big savings, like spacex's grasshopper project, is of course a reusable space vehicle. The advantage to spacex's approach is that Skylon is a single stage to orbit space plane (you don't need to land a bunch of stages using retro-rockets).
If I was placing a bet, I would bet on spacex since they have a proven track record (rockets are easy, organizing/funding rocket companies is hard). Skylon is a great idea though and it is the general direction that aircraft engine design is headed (a peak at the future).
>LEO requires about 7.8 km/s, Skylon's jet engines can go around 1.7 km/s. You are reaching 20% of your orbital velocity without reaction mass.
Unfortunately, kinetic energy scales as speed squared, so 20% of your orbital speed represents less than 5% of your orbital kinetic energy. To put this in perspective, the difference in gravitational potential energy between LEO and the earth's surface represents about 15% of your total on-orbit energy.
Now, it's true that an air-breathing engine doesn't need to carry reaction mass (and, sometimes, oxidizer), but the air engines add considerable complexity (which is always a bad thing) as well as weight (because you still need to carry a conventional rocket to finish orbit insertion). So what you need to do is ask how the weight penalty of the air engine compares to the weight penalty of carrying extra fuel in a conventional rocket (bearing in mind, of course, that there a pernicious positive feedback loops when scaling a booster).
Unfortunately, kinetic energy scales as speed squared, so 20% of your orbital speed represents less than 5% of your orbital kinetic energy. To put this in perspective, the difference in gravitational potential energy between LEO and the earth's surface represents about 15% of your total on-orbit energy.
Now, it's true that an air-breathing engine doesn't need to carry reaction mass (and, sometimes, oxidizer), but the air engines add considerable complexity (which is always a bad thing) as well as weight (because you still need to carry a conventional rocket to finish orbit insertion). So what you need to do is ask how the weight penalty of the air engine compares to the weight penalty of carrying extra fuel in a conventional rocket (bearing in mind, of course, that there a pernicious positive feedback loops when scaling a booster).
>Unfortunately, kinetic energy scales as speed squared, so 20% of your orbital speed represents less than 5% of your orbital kinetic energy.
Fortunately this is counterbalanced by the Oberth effect. Getting to 20% of your orbital velocity requires expending 20% of your rocket's delta-V. And since delta-V is logarithmic in your propellant mass (rocket equation) that could easily translate to needing half as much fuel.
Fortunately this is counterbalanced by the Oberth effect. Getting to 20% of your orbital velocity requires expending 20% of your rocket's delta-V. And since delta-V is logarithmic in your propellant mass (rocket equation) that could easily translate to needing half as much fuel.
>Fortunately this is counterbalanced by the Oberth effect. Getting to 20% of your orbital velocity requires expending 20% of your rocket's delta-V. And since delta-V is logarithmic in your propellant mass (rocket equation) that could easily translate to needing half as much fuel.
Well, that doesn't really address my point, which was that you need to compare the weight of the hybrid engine to the weight of the extra fuel. The first problem is that an air-breathing engine is going to be something like 3 to 5% of the initial mass, and you have to carry it with you to orbit[1]. The second problem is how the fuel scales:
v_hybrid = 0.8 * v_conventional
Assume both have similar engines:
ln(m_hybrid-initial/ m_hybrid-final) = ln((m_conventonal-initial / m_conventional-final)^0.8)
m_hybrid-intial = (m_conventional-intial / m_conventional-final)^0.8 * m_hybrid-final
let delta equal the expression in parenthesis
m_h-i = delta^0.8 * (m_payload + m_engine) = delta^0.8 * (m_payload + m_h-i * 0.05)
m_h-i * (1 - 0.05 * delta^0.8) = delta^0.8 * m_payload
So the fuel load in a hybrid is going to be:
m_final = delta^(1-0.2) / (1 - 0.05delta^(1-0.2)) m_payload,
The factor in the denominator is what really kills you, and the hybrid is only going to give you a net benefit for deltas less than about 15. So, not only is there not a factor of 2 fuel savings, there isn't any fuel savings at all! Even if you assume an engine weight of only 3% of initial mass, the benefit is only for delta < 35, which is better than just about all actual (as opposed to paper) launchers. By the way, the Shuttle had a delta of about 85-90 for LEO precisely because its designers made the decision to bring wings (which we neglected above) along for the ride to orbit. That also contributed to the 1 in 50 accident rate of that launch system.
And none of this addresses the fact that you are optimizing the f*ck out of one of the least expensive components of launch cost by introducing all sorts of unnecessary complexity.
[1] Ok, I suppose you don't, but then you have to have some way of recovering it, and that adds an enormous amount of complexity to the system.
Well, that doesn't really address my point, which was that you need to compare the weight of the hybrid engine to the weight of the extra fuel. The first problem is that an air-breathing engine is going to be something like 3 to 5% of the initial mass, and you have to carry it with you to orbit[1]. The second problem is how the fuel scales:
v_hybrid = 0.8 * v_conventional
Assume both have similar engines:
ln(m_hybrid-initial/ m_hybrid-final) = ln((m_conventonal-initial / m_conventional-final)^0.8)
m_hybrid-intial = (m_conventional-intial / m_conventional-final)^0.8 * m_hybrid-final
let delta equal the expression in parenthesis
m_h-i = delta^0.8 * (m_payload + m_engine) = delta^0.8 * (m_payload + m_h-i * 0.05)
m_h-i * (1 - 0.05 * delta^0.8) = delta^0.8 * m_payload
So the fuel load in a hybrid is going to be:
m_final = delta^(1-0.2) / (1 - 0.05delta^(1-0.2)) m_payload,
The factor in the denominator is what really kills you, and the hybrid is only going to give you a net benefit for deltas less than about 15. So, not only is there not a factor of 2 fuel savings, there isn't any fuel savings at all! Even if you assume an engine weight of only 3% of initial mass, the benefit is only for delta < 35, which is better than just about all actual (as opposed to paper) launchers. By the way, the Shuttle had a delta of about 85-90 for LEO precisely because its designers made the decision to bring wings (which we neglected above) along for the ride to orbit. That also contributed to the 1 in 50 accident rate of that launch system.
And none of this addresses the fact that you are optimizing the f*ck out of one of the least expensive components of launch cost by introducing all sorts of unnecessary complexity.
[1] Ok, I suppose you don't, but then you have to have some way of recovering it, and that adds an enormous amount of complexity to the system.
>And none of this addresses the fact that you are optimizing the fck out of one of the least expensive components of launch cost by introducing all sorts of unnecessary complexity.
As I understand it, construction of the rocket is the most expensive part of a launch system. The point of skylon is to create a reusable single stage to orbit space plane. Shouldn't skylon's reusability make it "optimize the fck" out of one of the most expensive components?
As I understand it, construction of the rocket is the most expensive part of a launch system. The point of skylon is to create a reusable single stage to orbit space plane. Shouldn't skylon's reusability make it "optimize the fck" out of one of the most expensive components?
Funny how this stuff works. The hot whiz-bang kid rewriting all the rules can become tomorrow's rent-seeking cronyistic old guard overnight. I wonder if a rocket vs sabre engine war is about to start. I hope so. Easier access to orbit and a 4 hour trip from NY to Tokyo would be pretty amazing.
I think you're forcing that narrative, the old guard does not tend to start out their statements with "I may have totally incorrect assumptions here, but...".
Maybe, but I imagine someone like Musk would be pretty good with rhetoric and knows if he utters the wrong things at a press conference it could lead to disastrous results for his companies. He's just being diplomatic because it pays to be diplomatic.
Or, you know, he could actually just be telling the truth in saying that he is willing to concede he may be wrong on the issue if his assumptions are wrong.
SpaceX are absolutely right to be using tried-and-tested technology in their rockets, and so Elon Musk's scepticism is sensible in context of what they're trying to achieve. The technology mentioned in this article is more applicable 10, 20 years down the line; the R&D costs would be prohibitive to make it cost-effective to be used by SpaceX now, even if they could get it to work reliably and secure the IP rights.
>tried-and-tested technology
His objection isn't based on technology though. It's based on physically enveloping the problem and comparing the two cases.
His assumptions seem to be: LOX will remain cheap. The atmosphere will still have 21% oxygen. Your technology can't separate the N2 from the O2 before ingestion.
Elon says that the braking effect of ingesting that nitrogen vastly overwhelms the small benefit of having a smaller O2 tank. That's it.
This argument doesn't rely on the air-breathing stuff being more complicated or expensive or heavy (though it will be). Technology might fix those problems, but that simple kinematic fact is enough.
His objection isn't based on technology though. It's based on physically enveloping the problem and comparing the two cases.
His assumptions seem to be: LOX will remain cheap. The atmosphere will still have 21% oxygen. Your technology can't separate the N2 from the O2 before ingestion.
Elon says that the braking effect of ingesting that nitrogen vastly overwhelms the small benefit of having a smaller O2 tank. That's it.
This argument doesn't rely on the air-breathing stuff being more complicated or expensive or heavy (though it will be). Technology might fix those problems, but that simple kinematic fact is enough.
> while this can only be useful where you still have atmosphere and oxygen
The planned SABRE engine can act as a rocket as well closing the intake and using stored liquid oxygen so it would be capable of operating without an atmosphere and oxygen and landing on Mars.
The planned SABRE engine can act as a rocket as well closing the intake and using stored liquid oxygen so it would be capable of operating without an atmosphere and oxygen and landing on Mars.
Can it take off from Mars? In another HN thread a month ago, we discussed how Mars' atmosphere is so thin that a Cessna would need 747 wings and near Mach 1 velocity to take off.
Rockets are propelled by their own exhaust.
You might be able to use this engine to get in and out of orbit, and than use a traditional rocket engine for the space travel itself.
More importantly (for intra-planetary travel) if you can get close to escape velocity then you spend most of the flight time in a near zero resistance environment which dramatically improves fuel efficiency as well as significantly reducing flight time.
The only problem with Elon's response is that rockets will never be used for terrestrial transportation. SABRE might.
This is just rocket-based combined-cycle propulsion that has been proposed and studied by various organizations over the years.
http://en.wikipedia.org/wiki/Rocket-based_combined_cycle
http://investor.gencorp.com/releasedetail.cfm?ReleaseID=7085...
http://www.isset.org/nasa/nano/www.grc.nasa.gov/WWW/AERO/bas...
The concept is not new, but Reaction is probably the closest to a commercial implementation. Also, it's not fair to compare it to the jet when RBCC isn't a fundamentally novel form of propulsion, just a combination of existing ones (like a car that is both gas and electric propelled). This is not something that could supplant the jet.
Some people are comparing this to SpaceX's pure-rocket approach. Up until yesterday, SpaceX and Scaled were working on their own type of combined-cycle launch with Stratolaunch: http://www.flightglobal.com/news/articles/stratolaunch-and-s.... The difference is that the jet and rocket would be separate vehicles instead of fitting into one nacelle. It'd have some of the advantages of using air as propellant as with a RBCC engine, but easier to design I think.
http://en.wikipedia.org/wiki/Rocket-based_combined_cycle
http://investor.gencorp.com/releasedetail.cfm?ReleaseID=7085...
http://www.isset.org/nasa/nano/www.grc.nasa.gov/WWW/AERO/bas...
The concept is not new, but Reaction is probably the closest to a commercial implementation. Also, it's not fair to compare it to the jet when RBCC isn't a fundamentally novel form of propulsion, just a combination of existing ones (like a car that is both gas and electric propelled). This is not something that could supplant the jet.
Some people are comparing this to SpaceX's pure-rocket approach. Up until yesterday, SpaceX and Scaled were working on their own type of combined-cycle launch with Stratolaunch: http://www.flightglobal.com/news/articles/stratolaunch-and-s.... The difference is that the jet and rocket would be separate vehicles instead of fitting into one nacelle. It'd have some of the advantages of using air as propellant as with a RBCC engine, but easier to design I think.
The Stratolaunch craft could be used to launch suborbital rocketplanes. You could have the 21st century counterpart to the Concorde, going over twice as fast, combined with space tourism.
The precooler / heat exchanger is the SABRE's distinction. Ramjets and scramjets don't have such. It enables low internal gas velocity and small compressors and more thrust from a smaller engine and higher top speed than jets.
It's like saying when the Wankel was invented that "it's just another internal combustion engine". Sure, it is, but it's quite different from most of the others.
It's like saying when the Wankel was invented that "it's just another internal combustion engine". Sure, it is, but it's quite different from most of the others.
[deleted]
It is combined cycle, but that does not mean it's equivalent to the RBCC design. There are several variations of combined cycle engines. RBCC is not a catchall. This is a refinement of the Liquid Air Cycle Engine concept that was researched by the US military in the 60's and the british in the 80's. The principal british investigator was Alan Bond, who is also the principal in Reaction.
It is appropriate to distinguish it from both jet engines and rocket engines because in the Brayton Cycle compression is adiabatic rather than diabatic in SABRE. I don't believe there is a formal term for this cycle, but the closest existing would be the Ericcson Cycle.
I do not understand your preoccupation with whether to categorize something as novel or not. The have a chiller that drops the temperature of a near supersonic air flow over 1000C in a fraction of a second. If that doesn't meet your standards of novel I don't know what would.
It is appropriate to distinguish it from both jet engines and rocket engines because in the Brayton Cycle compression is adiabatic rather than diabatic in SABRE. I don't believe there is a formal term for this cycle, but the closest existing would be the Ericcson Cycle.
I do not understand your preoccupation with whether to categorize something as novel or not. The have a chiller that drops the temperature of a near supersonic air flow over 1000C in a fraction of a second. If that doesn't meet your standards of novel I don't know what would.
The MAGLEV Launcher.
http://en.wikipedia.org/wiki/Rocket_sled_launch
http://www.reallyrocketscience.com/node/2393
Using MAGLEV as a launch assist technology to offset the amount of energy derived from burning tons and tons of fuel. Some of the articles estimate a potential to increase payload by 80% compared to a conventionally launched rocket.
If you want to get a little deeper into it, this is a good read:
http://upload.wikimedia.org/wikipedia/commons/5/58/Maglifter...
Some fun data points:
http://en.wikipedia.org/wiki/Rocket_sled_launch
http://www.reallyrocketscience.com/node/2393
Using MAGLEV as a launch assist technology to offset the amount of energy derived from burning tons and tons of fuel. Some of the articles estimate a potential to increase payload by 80% compared to a conventionally launched rocket.
If you want to get a little deeper into it, this is a good read:
http://upload.wikimedia.org/wikipedia/commons/5/58/Maglifter...
Some fun data points:
Power for large scale system: 10GW for 20 seconds.
Thermal Management system capable of dissipating 40GJ.
The payoff would be immense though. At datacenter electricity prices, the fundamental energy cost to LEO is about 50¢/kg.
Fuel (energy) cost is less than 1% of the cost to launch. The big costs are manpower, maintenance, and refurbishment.
That was the interesting takeaway from the Elon Musk talk about how cheap the fuel was compared to everything else. Having the Skylon be reusable then shifts some of the operational costs back to fuel.
Of course a plane that can fly a Mach 5 and 100,000' has other uses, the most obvious one being the SR-71 replacement. Nothing like a bit of high speed aerial surveillance "right now" to help folks on the ground make better decisions. I'm sure someone would pony up the $400M they need.
Of course a plane that can fly a Mach 5 and 100,000' has other uses, the most obvious one being the SR-71 replacement. Nothing like a bit of high speed aerial surveillance "right now" to help folks on the ground make better decisions. I'm sure someone would pony up the $400M they need.
Yes, but having anything be reusable does that. If you can make up what you would have gained with Skylon by making the fuel tank 5% larger, then the opportunity cost of developing a radically new engine isn't worth it. Mach 5 does have other uses, though.
I don't know the first thing about orbital-scale rocket economics, so, please, regard this as complete and utter speculative nonsense.
My thinking is that, if there's a future where we might have regular launches to orbit all of those costs will have to drop. At that point fuel might just start to become far more significant.
Let's imagine weekly "Orbit The Earth Adventure"(TM) launches in 25 or 50 years. The spacecraft, crews and maintenance would have to be optimized in order to enable the business model. I think it could very well be in that context that a MAGLEV launch-assist vehicle might make sense.
Again, I don't know what I am talking about and don't really have the time to research the subject and learn about the economics of low-earth-orbit manned space flight. I'll just have to leave it at that.
My thinking is that, if there's a future where we might have regular launches to orbit all of those costs will have to drop. At that point fuel might just start to become far more significant.
Let's imagine weekly "Orbit The Earth Adventure"(TM) launches in 25 or 50 years. The spacecraft, crews and maintenance would have to be optimized in order to enable the business model. I think it could very well be in that context that a MAGLEV launch-assist vehicle might make sense.
Again, I don't know what I am talking about and don't really have the time to research the subject and learn about the economics of low-earth-orbit manned space flight. I'll just have to leave it at that.
> My thinking is that, if there's a future where we might have regular launches to orbit all of those costs will have to drop.
http://news.ycombinator.com/item?id=4844581
> I think it could very well be in that context that a MAGLEV launch-assist vehicle might make sense.
Maglev would indeed allow smaller, less complex vehicles built with less stringent margins to be launched.
http://news.ycombinator.com/item?id=4844581
> I think it could very well be in that context that a MAGLEV launch-assist vehicle might make sense.
Maglev would indeed allow smaller, less complex vehicles built with less stringent margins to be launched.
[deleted]
Elon might build it for the Hyperloop, if you believe the speculation on the hyperloop blog posts last week.
From the article: "The company has deliberately avoided filing patents on its heat exchanger technology to avoid details of how it works - particularly the method for preventing the build-up of frost - becoming public."
I would think that for something like this, the patent system would actually work fine. Anybody who tries to make this is going to be located in the US, Canada, Western Europe, or Japan. It's not like they're going to make cheap ones in China and India, and if they did it would be easily correctable with the WTO. What am I missing?
I would think that for something like this, the patent system would actually work fine. Anybody who tries to make this is going to be located in the US, Canada, Western Europe, or Japan. It's not like they're going to make cheap ones in China and India, and if they did it would be easily correctable with the WTO. What am I missing?
SpaceX doesn't have any patents either. Musk has specifically said this is because in the long term their main competitors are the chinese, who treat patents as recipe books.
I appreciate the argument about SpaceX (and the other thoughtful replies to my question). I just think that this case is fundamentally different from a rocket company. This is something that, to make profit, would need to be used by global airlines, where patent laws are much more enforceable.
My thinking boils down to this: if SpaceX comes up with a novel, patentable way to improve rocketry, China can steal it.
If these guys come up with a much faster aircraft engine, even if China made them for cheap, they could and would prevent United, Lufthansa, BA, and the other big airlines from using it.
China can steal IP when it is used internally, or sold as consumer goods to other countries that violate patent laws. They can't steal IP and sell it to large multinationals.
My thinking boils down to this: if SpaceX comes up with a novel, patentable way to improve rocketry, China can steal it.
If these guys come up with a much faster aircraft engine, even if China made them for cheap, they could and would prevent United, Lufthansa, BA, and the other big airlines from using it.
China can steal IP when it is used internally, or sold as consumer goods to other countries that violate patent laws. They can't steal IP and sell it to large multinationals.
You're missing the point that a competitor could build it in direct violation of the patent, go into space, get all of the glory, while they tie this company up in court for tens of years and cost them millions of dollars. This company might eventually win but by then the other company has become massively successful.
This has happened before. It will happen again.
This has happened before. It will happen again.
Maybe the principals are easily applied in a just different enough to not violate patent way.
Maybe they don't want to spend the time or money in court instead of on getting to market.
Maybe they know big aerospace companies will be able to out spend them on lawyering/lobying and so will loose.
Maybe they believe their time to make profit > than the duration of patent protection.
Maybe they don't want to spend the time or money in court instead of on getting to market.
Maybe they know big aerospace companies will be able to out spend them on lawyering/lobying and so will loose.
Maybe they believe their time to make profit > than the duration of patent protection.
According to this BBC Four documentary (found in another comment)
https://www.youtube.com/watch?v=vZ_a21fPkYM
the engineers wanted to apply for a patent once the British government stopped funding their work, but the government classified their work as secret, preventing them from doing so.
EDITED TO ADD: Apparently the original secret classification expired in 1993, at which point they filed a patent which ended up being owned by Rolls Royce. Their current work circumvents that patent.
https://www.youtube.com/watch?v=vZ_a21fPkYM
the engineers wanted to apply for a patent once the British government stopped funding their work, but the government classified their work as secret, preventing them from doing so.
EDITED TO ADD: Apparently the original secret classification expired in 1993, at which point they filed a patent which ended up being owned by Rolls Royce. Their current work circumvents that patent.
I think "prior art" lends these companies some protection while not having detailed patents filed.
Unfortunate title here - British Aerospace was involved in the project some years ago, but Reaction Engines is a distinct company and I believe they exist despite BAe's "assistance"
As an American, I parsed the title as "a British aerospace company..." not "the British Aerospace company..."
Indeed. The capitalisation was what threw me originally, but I understand that the title isn't wrong - hence the use of "unfortunate" rather than "incorrect".
As an Englishman, I parsed the title the same way as you did, though I did wonder if it meant BA.
BA is British Airways. BAE Systems is the new name for what used to be British Aerospace, which used BAe as its abbreviation, to avoid confusion with BA.
I don't know why it bothers me, but I really dislike the used-to-be-an-acronym-but-not-any-more names. For example SRI (not affiliated with Stanford so the S doesn't mean that any more), and my least favourite "HRL Laboratories" which we have to pretend does not expand to "Hughes Research Laboratories Laboratories". Yuk.
I don't know why it bothers me, but I really dislike the used-to-be-an-acronym-but-not-any-more names. For example SRI (not affiliated with Stanford so the S doesn't mean that any more), and my least favourite "HRL Laboratories" which we have to pretend does not expand to "Hughes Research Laboratories Laboratories". Yuk.
The problem with these advanced technologies is two fold
1. it requires a large amount of hydrogen - which is not heavy so it looks good on paper but is very cold and voluminous, meaning you need huge insulated tanks. They are expensive to build, have bad mass fraction and are aerodynamically problematic. Hydrogen is also expensive to handle in systems and infrastructure.
2. the dry mass of an air breathing engine sucks because they process gases (rockets process liquids with 1000x density), though SABRE is better than stuff like scramjets. This is especially bad for an SSTO like they are proposing since you carry the inlets and precoolers and all that all the way to orbit.
My bet for cheap spaceflight would be a two stage kerosene-oxygen rocket. A SABRE engine might make a great first stage but I doubt if you would want to carry it to orbit. I haven't performed much calculations though.
1. it requires a large amount of hydrogen - which is not heavy so it looks good on paper but is very cold and voluminous, meaning you need huge insulated tanks. They are expensive to build, have bad mass fraction and are aerodynamically problematic. Hydrogen is also expensive to handle in systems and infrastructure.
2. the dry mass of an air breathing engine sucks because they process gases (rockets process liquids with 1000x density), though SABRE is better than stuff like scramjets. This is especially bad for an SSTO like they are proposing since you carry the inlets and precoolers and all that all the way to orbit.
My bet for cheap spaceflight would be a two stage kerosene-oxygen rocket. A SABRE engine might make a great first stage but I doubt if you would want to carry it to orbit. I haven't performed much calculations though.
That was my first thought - Hydrogen has low energy density, even in frozen form. That is the thing that holds Hydrogen cars back more than anything else - tiny range for massive (and costly, and complex) fuel tanks. The same would have to apply to both airplane and rocket designs.
I think the first article I read about Sabre was on HN so it's nice to see it getting followed up.
The technology sounds like it's progressing, even if parts of it are still heavily conceptual, and getting a sign off from the ESA is quite a big step (even if certain space company founders have dissed them recently). The last I read was they were struggling to find funding, which is on one hand utterly surprising as if it works properly then it's got the potential to revolutionise travel and who wouldn't like to say "yeah, I put money in before they were famous", and on the other isn't a surprise at all as the returns are probably a decade out.
Still, I wait patiently and optimistically for when Britain is showing everyone that whilst we were slow off the mark on the space race we aren't out of it yet.
The technology sounds like it's progressing, even if parts of it are still heavily conceptual, and getting a sign off from the ESA is quite a big step (even if certain space company founders have dissed them recently). The last I read was they were struggling to find funding, which is on one hand utterly surprising as if it works properly then it's got the potential to revolutionise travel and who wouldn't like to say "yeah, I put money in before they were famous", and on the other isn't a surprise at all as the returns are probably a decade out.
Still, I wait patiently and optimistically for when Britain is showing everyone that whilst we were slow off the mark on the space race we aren't out of it yet.
Here's a documentary that was broadcast on BBC4 a couple of months ago about Bond and his quixotic journey to get HOTOL/Skylon into the sky:
http://www.youtube.com/watch?v=vZ_a21fPkYM
It includes a test-firing of the new heat-exchanger at the end.
http://www.youtube.com/watch?v=vZ_a21fPkYM
It includes a test-firing of the new heat-exchanger at the end.
One of those truly inspirational stories, where people dedicate their lives over decades to an idea and see it through. Granted, their aircraft isn't yet built, but they're well on the way.
Article from the BCC at http://www.bbc.co.uk/news/science-environment-20510112
Someone asked on another list for an explanation of the press release. This is my try.
Hypersonic engines are up against hard physics. The ram air heats so much in the inlet that it's hard for combustion to add much energy to make it go faster out the back.
The idea behind the SABRE engines is to cool the ram air before it is compressed. The heat exchanger to do this is what the press release is all about. With not much more than a ton of mass, it sucks 400 MW of heat out of the incoming air, dropping the temperature from 1500 C to -150 C in a few inches of heat exchanger that looks much like fabric because the tubes are so tiny.
The engine cycle also uses the temperature difference between the ram air and the LH2 to run the compressor. It takes close to 2/5th of the energy from burning hydrogen to liquefy it. The engines recover much of this by running a helium turbine on the temperature difference between the ram air and the liquid hydrogen flow to the engines. The turbine powers the compressor stage that raises the pressure of the -150 C air to rocket chamber pressure.
The design is extremely clever thermodynamics which also avoids most of the metallurgical problems of high temperature. Fabricating the air to helium heat exchanger was a very hard task. They have miles of tiny tubing, tens of thousands of brazed joints and they don't leak!
Using these engines and breathing air, the vehicle reaches 26 km and about a quarter of the velocity to orbit giving an equivalent exhaust velocity (back calculate from hydrogen consumption) of 9 km/s. That's twice as good as the space shuttle main engines. It is expected to go into orbit with 15 tons of payload out of 300 or 5% even though the rest of the acceleration is on internal oxygen that only gives 4.5 km/s exhaust velocity.
Leaving out the oxygen and using big propulsion lasers to heat hydrogen reaction mass, such a vehicle would get 25% of takeoff mass to LEO, reducing the already low cost by a factor of 5. That's enough to change the economics of power satellites from being too expensive to consider to a cost substantially less expensive than any fossil fuel.
But try explaining any of this in a press release.
Hypersonic engines are up against hard physics. The ram air heats so much in the inlet that it's hard for combustion to add much energy to make it go faster out the back.
The idea behind the SABRE engines is to cool the ram air before it is compressed. The heat exchanger to do this is what the press release is all about. With not much more than a ton of mass, it sucks 400 MW of heat out of the incoming air, dropping the temperature from 1500 C to -150 C in a few inches of heat exchanger that looks much like fabric because the tubes are so tiny.
The engine cycle also uses the temperature difference between the ram air and the LH2 to run the compressor. It takes close to 2/5th of the energy from burning hydrogen to liquefy it. The engines recover much of this by running a helium turbine on the temperature difference between the ram air and the liquid hydrogen flow to the engines. The turbine powers the compressor stage that raises the pressure of the -150 C air to rocket chamber pressure.
The design is extremely clever thermodynamics which also avoids most of the metallurgical problems of high temperature. Fabricating the air to helium heat exchanger was a very hard task. They have miles of tiny tubing, tens of thousands of brazed joints and they don't leak!
Using these engines and breathing air, the vehicle reaches 26 km and about a quarter of the velocity to orbit giving an equivalent exhaust velocity (back calculate from hydrogen consumption) of 9 km/s. That's twice as good as the space shuttle main engines. It is expected to go into orbit with 15 tons of payload out of 300 or 5% even though the rest of the acceleration is on internal oxygen that only gives 4.5 km/s exhaust velocity.
Leaving out the oxygen and using big propulsion lasers to heat hydrogen reaction mass, such a vehicle would get 25% of takeoff mass to LEO, reducing the already low cost by a factor of 5. That's enough to change the economics of power satellites from being too expensive to consider to a cost substantially less expensive than any fossil fuel.
But try explaining any of this in a press release.
I wonder how this will impact air travel. It is a real shame that after Concord retired there is no supersonic jetliner in service, which at least
kept the dream alive even if Concord is expensive and short legged. I believe no viable program exists right now for supersonic jetliners. The problem, if I recall correctly from my aerospace professor, is not the jet engine's thrust but heat generated from friction with air. If this Sabre engine can get the airplane mostly out of atmosphere cheaply, heat problem can be alleviated. I don't know how practical my guess is, but
if it works, the market potential is immense.
This guy has been at this in various forms for 30 years. Amazing story and for once not completely fucked up by the British Govt, unlike our aerospace industry after WWII that was world leading.
> This core piece of technology solves one of the constraints that limit jet engines to a top speed of about 2.5 times the speed of sound, which Reaction Engines believes it could double
Is that 2.5 times the speed of sound limit just for jets at low altitude? At high altitude it is certainly not true. Both the SR-71 and MiG 25 did well over Mach 3.
Is that 2.5 times the speed of sound limit just for jets at low altitude? At high altitude it is certainly not true. Both the SR-71 and MiG 25 did well over Mach 3.
They mean turbojets ("conventional" jet engines). The SR-71 doesn't use a conventional jet engine.
http://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#Engine...
>A unique hybrid, the engine can be thought of as a turbojet inside a ramjet. At lower speeds, the turbojet provided most of the compression and most of the energy from fuel combustion. At higher speeds, the turbojet largely ceased to provide thrust; instead, air was compressed by the shock cones and fuel burned in the afterburner.
http://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#Engine...
>A unique hybrid, the engine can be thought of as a turbojet inside a ramjet. At lower speeds, the turbojet provided most of the compression and most of the energy from fuel combustion. At higher speeds, the turbojet largely ceased to provide thrust; instead, air was compressed by the shock cones and fuel burned in the afterburner.
Does anyone have guesses on how the heat exchanger might work to prevent frost?
Perhaps ultrasound?
Perhaps ultrasound?
My guess is that they inject liquid hydrogen into the hot air, so the cooling is actually done in free "air" rather than on the surface of a traditional metal heat exchanger - that would explain how they cool it so fast and help explain how they prevent frost in the heat exchanger during the cooling.
This reformulates the problem into "how do we prevent the injectors from..."
a) Melting (liquid hydrogen inside would help a lot).
b) Preventing the liquid hydrogen inside the injectors from boiling and rupturing the piping and injectors.
c) Avoid all the standard hydrogen problems (e.g. embrittlement).
In this guess, (a) and (b) would be a delicate engineering balance to keep the injectors cold enough to retain their strength yet warm enough to not ice up. (c) has known solutions.
This reformulates the problem into "how do we prevent the injectors from..."
a) Melting (liquid hydrogen inside would help a lot).
b) Preventing the liquid hydrogen inside the injectors from boiling and rupturing the piping and injectors.
c) Avoid all the standard hydrogen problems (e.g. embrittlement).
In this guess, (a) and (b) would be a delicate engineering balance to keep the injectors cold enough to retain their strength yet warm enough to not ice up. (c) has known solutions.
Can't say anything about the plausibility of the scheme you mention, but the diagram in the BBC article contradicts it. There you can see that the primary cooling medium is liquid helium, in a closed circuit and a liquid nitrogen heat exchanger. Obviously the diagram can be just flawed, either as a disguise or unintentionally.
My bad, I did not read the much more informative BBC article before speculating.
Having a closed circuit nitrogen heat exchanger is interesting because it is a heat exchanger... you still need to get rid of the heat somewhere. For the demo (per the BBC), they evaporated some of the nitrogen to get rid of the heat, but that means you need excess nitrogen to boil off enough to reject the heat. I wonder how they plan to get rid of the heat in a real aircraft.
It also makes me wonder how they can have enough surface area to absorb the heat in the precooler but maintain enough airflow to generate the thrust they would need.
Having a closed circuit nitrogen heat exchanger is interesting because it is a heat exchanger... you still need to get rid of the heat somewhere. For the demo (per the BBC), they evaporated some of the nitrogen to get rid of the heat, but that means you need excess nitrogen to boil off enough to reject the heat. I wonder how they plan to get rid of the heat in a real aircraft.
It also makes me wonder how they can have enough surface area to absorb the heat in the precooler but maintain enough airflow to generate the thrust they would need.
I wonder how they plan to get rid of the heat in a real aircraft.
Boil hydrogen. (All there for reading or watching.)
Boil hydrogen. (All there for reading or watching.)
How about dividing the heat exchanger into two sections? One section cools the air while the other section defrosts, and then alternate.
playing with pressure differences, thermodynamics, sections and annealing, you could think about freeze-drying while drinking your lyo coffee.
That was my first guess too. With that many pipes, they could have many sections which migrate the frosty sections down into the engine in waves.
My first thought was a metamaterial.
Something like this, I suspect: http://www.sharklet.com/technology/
I think you are right, its probably some metamaterial like shark skin, where the surface of the exchanger itself is designed to prevent frost crystallization from occurring .. i.e. the metamaterial is geared towards the properties of water at the stage where it goes from fluidic to crystal .. I forget the term .. so the structure of the surface of the exchanger is formed such as to demote crystallization long enough for the pressure forces to remove the liquid hydrogen ..
Sounds like quite a disruptive innovation in the aerospace industry. Always wondered how much it cost to get innovations like this off the ground. $400m funding round sounds pretty serious...
That's the blind, the bet required to play, in aerospace, it's not a lot of money at all by their standards.
Too true. The old joke about rocket companies is "If you want to end up with a billion dollars, start a rocket company with ten billion."
Or by social media standards. I know it's a rather over-used meme now, but I'm again reminded of Instagram's $1b buyout vs. this.
This seems like a good blue-sky venture for a scientific philanthropist. There's a lot that could go wrong, it might not work (though as pointed out elsewhere, this isn't all theory - they have tested the heat exchanger in real life on a proper jet engine), but it could be a big win. If only someone like Gates could put down $400m for a chance of a big pay off later.
This seems like a good blue-sky venture for a scientific philanthropist. There's a lot that could go wrong, it might not work (though as pointed out elsewhere, this isn't all theory - they have tested the heat exchanger in real life on a proper jet engine), but it could be a big win. If only someone like Gates could put down $400m for a chance of a big pay off later.
That would be the ante - a forced bet affecting all players. Blinds are typically only posted by one or two players.
Enough to start a rocket company.
(Maybe not a hybrid air-breathing SSTO company.)
(Maybe not a hybrid air-breathing SSTO company.)
I don't see where this technology "fits". When you're going to orbit, 90% of the flight takes place outside the atmosphere, so an air-breathing engine doesn't do much for you.
For commercial sub-orbital hops, they'll have to bring the cost down considerably to compete with existing air service. Sure, we'd all love to be able to go from NYC to Frankfurt in an hour. But how many people will pay ten thousand dollars for the privilege?
For commercial sub-orbital hops, they'll have to bring the cost down considerably to compete with existing air service. Sure, we'd all love to be able to go from NYC to Frankfurt in an hour. But how many people will pay ten thousand dollars for the privilege?
I'll maybe believe it when I learn more. That it's an idea on paper only means it's way too early to tell. I'm also wondering if they are relying on exotic building materials being developed in the future for this to work, like a previous expensive but failed US space-plane program.
The critical part (the cooling mechanism) has been fabricated, and validated in tests.
This is great news, but it also makes me a bit sad - because all this innovation died down in the 80's when everyone lost interest in the space race. We could be 30 years further down the road already and many of us probably won't live to see people setting foot on Mars.
I think many of us will.
Another discussion of this has broken out here:
http://news.ycombinator.com/item?id=4856986
http://news.ycombinator.com/item?id=4856986
"The company has deliberately avoided filing patents on its heat exchanger technology to avoid details of how it works - particularly the method for preventing the build-up of frost - becoming public."
Evidence that the main use case for patents - protecting true innovation and development - does not even need the patent system.
Evidence that the main use case for patents - protecting true innovation and development - does not even need the patent system.
I was skeptic about them so far, I would love to be proven wrong here.
I'm sure some of you chuckleheads are going to figure out how to use this to OC some poor unassuming Ivy Bridge CPU...
It appears they are using a liquid nitrogen boiler to chill a helium loop through their heat exchanger and in turn chill the air.
So, perhaps someone better at these calculations can help out, cooling incoming air by 160°C by moving nitrogen from -195°C to -15°C is going to require them to haul up liquid nitrogen and cool a bunch of atmospheric nitrogen that they don't really need. The heat of vaporization is the key that will make their solution win, but by what factor? How many grams of nitrogen must they haul to chill a gram of atmospheric oxygen?