This has quite a long heritage, with Reaction Engines Limited being co-founded by one of the designers of HOTOL[0], which was a similar design for an air-breathing rocket engine which got a lot of press in the UK in the 1980s.
There's at least one very large drawback with the Skylon design for orbital flight.
You drag those air breathing engines with huge inlets and heat exchangers all the way to orbit, plus all that extra hydrogen tankage, large landing gear etc.
As a first stage it would make a lot more sense. Then launch a much smaller pure rocket second stage from there. Staging at high altitude with low dynamic pressure should be relatively easy, probably easier than high speed atmospheric flight or re-entry from orbital velocities with Skylon.
The inlets are supposed to close before the engine enter pure-rocket mode. So I suppose that's not much more drag, especially since at this point the aircraft is at very high altitude thus with low air pressure.
So the main drawback is not drag but rather weight. Presumably, they've made the necessary calculations and the advantages of breathing air appear to make up for the additional weight.
You always drag engines of the last stage all the way to orbit. More, with a single stage to orbit the last stage is the first stage - and SSTO isn't quite considered an always loosing idea. More - Skylon has advantage of launching only with fuel - and for rockets oxydizer weights several times more than fuel.
Are you sure the drawback you're referring to is actually a drawback in comparison to what the industry have?
Yes, I'm sure. I'll explain a bit more how Skylon is different from staged rockets:
1) Skylon has air breathing engines. Air at sea level is about one thousandth the density of normal rocket propellants and thus the machinery for compressing that air (inlets, spinning compressors, in Skylon's case heat exchangers) is large. Air breathing engines need to be much larger than pure rocket engines of equivalent thrust, and thus heavy.
2) Since it's the first stage engines, they are large because of that. SpaceX for example has nine engines in the first stage while only one engine in the second stage. If we assume 500 kg per engine, 8 engines saves 4 tons. Since their payload is about 10 tons, they would only have about 6 tons of payload if they flew with all engines to orbit. (Not a very precise calculation I admit.) Why would they do that? And that's with rocket engines that have 150:1 thrust to weight ratio. Modern operational air breathing engines have about 10:1 thrust to weight, one fifteenth of Merlin 1D.
3) Carrying oxygen isn't as big a problem as it seems to be. It's denser than water so the tanks are actually small. And rockets already have tremendous thrust to weight as we have demonstrated.
Thank you. Good points, but I think we'll have to do a full blown calculations to decide. In aerospace margins are usually thin and it's easy to have the sign error.
1) I fully agree that air-breathing engines are heavier per unit of thrust. Merlins - pretty good rocket engines by that parameter - lift, as you say, 150 times their weight; for air-breathing engines it's 5-10.
2) Aircraft engines usually don't need to be particularly thrusty; wings generate lift, so you can take off even with T/W < 1.
3) Liquid oxygen has density about 1.1 that of water. Kerosene, in comparison, is about 0.8. Difference less than 1.5 times, but the mass ratio is more like 2.5:1, so ditching oxygen tanks - especially at the start, when the system is relatively heavy - can save quite a bit. And why engines' T/W is relevant here?
2) And now you're carrying wings to orbit too. More dead mass that could be payload...
3) My comment was to compare liquid oxygen with liquid hydrogen which is 0.07 of water density, so you get 16 kg of oxygen for 1 kg of hydrogen for the same tank volume. Skylon uses loads of hydrogen. It's those hydrogen tanks that make it so big. Such tanks are also a lot of dead mass to carry to orbit... Also costs probably scale with dry mass, not wet mass.
I am not a rocket scientist but I think the horizontal take-off is a huge drawback.
* The wings would only be useful at relatively low speeds and then just add lots of mass.
* Wings that are efficient at high mach-numbers would be less effective at take-off. Presumably lots of fuel would be burned at low speed to make up for this.
* The structure would need to be strong enough to function whilst horizontal and filled with fuel.
* It would need a large heavy undercarriage capable of supporting the entire take-off weight.
I am unconvinced that horizontal take-off has much benefit anyway. A flight within the atmosphere to a launch site would inevitably use lots of fuel and eliminate any payload capacity. Also, it is hard to believe that anyone is going to allow a massive hydrogen fuel tank to fly over populated areas.
A more traditional single stage to orbit would make more sense IMHO.
Weight inefficiency basically doesn't matter if you can achieve reuse.
On an expendable rocket, every bit of inefficiency is more engine, tank, and structure that you have to rebuild for each launch. You want to minimize that. It's also extra fuel, but relative to the total cost of a launch, fuel is essentially free.
If you can reuse the rocket, then that becomes much less important. Maybe you spend 10-100x more building engines and tanks and structures than you would if you had a more efficient design, but you get to amortize that cost across many launches, so it's much less important.
Reuse is, of course, a big if. A gigantic if. The gigantic-est of all ifs. The big question is, "will it work?" If the technology simply doesn't work, or if it "works" to get to orbit but requires so much refurbishment that it's basically like building a whole new rocket each time (see: Space Shuttle), then it's not practical. But if it does work, then who cares if you drag 10x more mass to orbit than you strictly need to?
I am very much in a "I'll believe it when I see it" mode with Skylon, but to me the objections center around the the hard work of going from theory to practice, and the lack of visible progress with same.
But all that extra mass does eat into the payload capacity.
A craft that is 100 tonnes dry mass and delivers 20 tonnes payload needs enough fuel to boost 120 tonnes in the last part of the flight. A centaur upper stage by comparison has a mass of just 2.2 tonnes. So for most of the flight you need much more fuel to boost the same payload.
Who cares about needing more fuel? As I said in the original comment, fuel is basically free. If you manage to build a craft where the cost of extra fuel (even using an order of magnitude more) even becomes noticeable, you've already managed to make launches vastly cheaper than anything out there today.
Well if you have to scale bigger and bigger for the payload, the cost of personnel that you need to have with a bigger craft starts featuring very negatively. Also if you can't operate from regular runways etc.
At some point on a degrading performance curve, you don't reach orbit at all even with zero payload.
The company, which has 75 employees, is valued at about 100 millions.
For software company in California, this level of valuation aproximately corresponds to "aqui-hire" - when the technology is uninteresting, but people can be used.
For aerospace company like Reaction Engines with track record of their founders and results which are being discussed, with all potential opportunities...
It's the Telegraph, this is just a "jolly good show chaps, we Englishmen can really achieve marvellous things can't we, it's not all about those chaps across The Pond, tallyho tallyho" puff piece. Skylon PR bits come out all the time.
Not to belittle their accomplishments, but the press releases after they successfully put some serious flows through the pre-chiller made it seem like there was a guaranteed outcome of a working powerplant within a decade, and a vehicle within 15 years. Despite the exotic fuels, powerplant, and mission profiles.
The F-22 was conceived in the middle of the 80s, had an effectively bottomless pit of funding from DoD after winning the contract in the early 90s, and look when those first came online (2005). And that was just burning JP-8.
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[ 0.19 ms ] story [ 52.8 ms ] thread[0] https://en.wikipedia.org/wiki/HOTOL
As a first stage it would make a lot more sense. Then launch a much smaller pure rocket second stage from there. Staging at high altitude with low dynamic pressure should be relatively easy, probably easier than high speed atmospheric flight or re-entry from orbital velocities with Skylon.
So the main drawback is not drag but rather weight. Presumably, they've made the necessary calculations and the advantages of breathing air appear to make up for the additional weight.
You always drag engines of the last stage all the way to orbit. More, with a single stage to orbit the last stage is the first stage - and SSTO isn't quite considered an always loosing idea. More - Skylon has advantage of launching only with fuel - and for rockets oxydizer weights several times more than fuel.
Are you sure the drawback you're referring to is actually a drawback in comparison to what the industry have?
1) Skylon has air breathing engines. Air at sea level is about one thousandth the density of normal rocket propellants and thus the machinery for compressing that air (inlets, spinning compressors, in Skylon's case heat exchangers) is large. Air breathing engines need to be much larger than pure rocket engines of equivalent thrust, and thus heavy.
2) Since it's the first stage engines, they are large because of that. SpaceX for example has nine engines in the first stage while only one engine in the second stage. If we assume 500 kg per engine, 8 engines saves 4 tons. Since their payload is about 10 tons, they would only have about 6 tons of payload if they flew with all engines to orbit. (Not a very precise calculation I admit.) Why would they do that? And that's with rocket engines that have 150:1 thrust to weight ratio. Modern operational air breathing engines have about 10:1 thrust to weight, one fifteenth of Merlin 1D.
3) Carrying oxygen isn't as big a problem as it seems to be. It's denser than water so the tanks are actually small. And rockets already have tremendous thrust to weight as we have demonstrated.
1) I fully agree that air-breathing engines are heavier per unit of thrust. Merlins - pretty good rocket engines by that parameter - lift, as you say, 150 times their weight; for air-breathing engines it's 5-10.
2) Aircraft engines usually don't need to be particularly thrusty; wings generate lift, so you can take off even with T/W < 1.
3) Liquid oxygen has density about 1.1 that of water. Kerosene, in comparison, is about 0.8. Difference less than 1.5 times, but the mass ratio is more like 2.5:1, so ditching oxygen tanks - especially at the start, when the system is relatively heavy - can save quite a bit. And why engines' T/W is relevant here?
3) My comment was to compare liquid oxygen with liquid hydrogen which is 0.07 of water density, so you get 16 kg of oxygen for 1 kg of hydrogen for the same tank volume. Skylon uses loads of hydrogen. It's those hydrogen tanks that make it so big. Such tanks are also a lot of dead mass to carry to orbit... Also costs probably scale with dry mass, not wet mass.
* The wings would only be useful at relatively low speeds and then just add lots of mass.
* Wings that are efficient at high mach-numbers would be less effective at take-off. Presumably lots of fuel would be burned at low speed to make up for this.
* The structure would need to be strong enough to function whilst horizontal and filled with fuel.
* It would need a large heavy undercarriage capable of supporting the entire take-off weight.
I am unconvinced that horizontal take-off has much benefit anyway. A flight within the atmosphere to a launch site would inevitably use lots of fuel and eliminate any payload capacity. Also, it is hard to believe that anyone is going to allow a massive hydrogen fuel tank to fly over populated areas.
A more traditional single stage to orbit would make more sense IMHO.
You do still need hydrogen etc, but maybe you can bring that in with a normal jetliner.
On an expendable rocket, every bit of inefficiency is more engine, tank, and structure that you have to rebuild for each launch. You want to minimize that. It's also extra fuel, but relative to the total cost of a launch, fuel is essentially free.
If you can reuse the rocket, then that becomes much less important. Maybe you spend 10-100x more building engines and tanks and structures than you would if you had a more efficient design, but you get to amortize that cost across many launches, so it's much less important.
Reuse is, of course, a big if. A gigantic if. The gigantic-est of all ifs. The big question is, "will it work?" If the technology simply doesn't work, or if it "works" to get to orbit but requires so much refurbishment that it's basically like building a whole new rocket each time (see: Space Shuttle), then it's not practical. But if it does work, then who cares if you drag 10x more mass to orbit than you strictly need to?
I am very much in a "I'll believe it when I see it" mode with Skylon, but to me the objections center around the the hard work of going from theory to practice, and the lack of visible progress with same.
A craft that is 100 tonnes dry mass and delivers 20 tonnes payload needs enough fuel to boost 120 tonnes in the last part of the flight. A centaur upper stage by comparison has a mass of just 2.2 tonnes. So for most of the flight you need much more fuel to boost the same payload.
At some point on a degrading performance curve, you don't reach orbit at all even with zero payload.
For software company in California, this level of valuation aproximately corresponds to "aqui-hire" - when the technology is uninteresting, but people can be used.
For aerospace company like Reaction Engines with track record of their founders and results which are being discussed, with all potential opportunities...
It looks strange.
And 10-15 years? Cessna would be lucky to be able to certify a new single engine piston airplane in that amount of time.
> "An artist's impression of how the new engine would look in ground tests"
A render of a test stand firing? Now that's a bit much. Now that they have $21 million, they had better do that for real on a test stand.
They've been working on this for over a decade. I'd expect at least a small scale demo by now.
Not to belittle their accomplishments, but the press releases after they successfully put some serious flows through the pre-chiller made it seem like there was a guaranteed outcome of a working powerplant within a decade, and a vehicle within 15 years. Despite the exotic fuels, powerplant, and mission profiles.
The F-22 was conceived in the middle of the 80s, had an effectively bottomless pit of funding from DoD after winning the contract in the early 90s, and look when those first came online (2005). And that was just burning JP-8.