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Let's start out with an air breathing SSTO, like Skylon. These things have pretty bad mass ratios and pretty razor thin margins. Given that the engines react with the oxygen in the air during the first stage, you'll have an unequal ratio of fuel to oxidizer. The limit of the air breathing stage is dictated by limits of the mach number with the ramjet operation.

But what if you had a crazy scheme to extend this air breathing mode? It's a shorter trip to move air from the upper atmosphere to space than it is from ground to space. Additionally, your fuel tank will be contracting during the air breathing mode. There's extra empty volume in your craft being freed up while at the same time you're swimming in Oxidizer. What if your engines siphoned some extra air into an expanding 3rd tank near the end of your air breathing stage, which is then reacted with the fuel during the first part of your airless flight?

Would you have to use a gaseous form? Or given the cryo temperatures that Skylon has in some parts, could it be possible to liquify some amount of air? Perhaps the heat exchangers can't handle this, so it's completely ridiculous. Nonetheless, if they could, Oxygen liquifies before Nitrogen if I understand correctly, so this could tip the ratio in your favor. If you did use a gaseous form, perhaps this would require hardening the fuel tank, which would kill whatever mass ratio benefit you were hoping for.

Can we manage to rule this out categorically (as in there's no realistic scenario where it helps), or would it only be a matter of vast additional complexity?

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  • $\begingroup$ I don't understand what performance increase you are referring to. $\endgroup$
    – GdD
    Commented Oct 23, 2014 at 12:25
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    $\begingroup$ I fear that at razor thin margins of Skylon / SABRE it's the fact that you're losing mass faster than you're losing thrust that gets you into orbit in the first place. OK so you could launch a bit lighter, but you also increase air resistance for the mass that you store, increase complexity (and mass) of already complex system, and lower performance of your oxidizer. It would take a lot longer to accelerate and increase altitude then, which is also a problem because of thermal expansion. Here's a good recent lecture on Skylon / SABRE by Alan Bond: youtube.com/watch?v=jzoP-bXMJjM $\endgroup$
    – TildalWave
    Commented Oct 23, 2014 at 14:10
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    $\begingroup$ @DeerHunter It didn't sound right to me either, and I had checked it multiple times. I know we're adding huge complications due to the pressure (who knows) and whatnot, but confining this to standard conditions, boiling point of O2 is 90 K and N2 is 77 K. Since we're describing a process of lowering the temperature, liquid Oxygen condensate is the first thing to come out. From my own memory this seemed wrong, but the numbers are what they are. $\endgroup$
    – AlanSE
    Commented Oct 23, 2014 at 14:49
  • $\begingroup$ @AlanSE - I should have known better. Thanks for the reminder. $\endgroup$
    – Deer Hunter
    Commented Oct 23, 2014 at 14:53
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    $\begingroup$ If you're scooping up air into a tank as you go, by definition you have to accelerate that mass of air, from roughly stationary to the speed your rocket is going at the point you collect it. $\endgroup$
    – Russell Borogove
    Commented Oct 23, 2014 at 16:11

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Compression will by it's nature heat things up. Storing air while in flight would tend to heat the system up. Keeping rockets cool is a major challenge, so an increase of heat would likely impact things even further. Yes, you could get some improvements in the air breathing portion of the flight. But you gain a significant amount by launching outside of the atmosphere for a SSTO, and you can get easily to 50% of orbital speed using a SCRAMJET system. It seems unlikely that the extra weight and heat that would result from compressing and storing air in flight would be of any value at all.

The 50% orbital speed comes from the fastest air-breathing aircraft, which was the X-43A, which set the current record of 7,000 mph. This is approximately half of the orbital speed, which is between 15,430 mph to 17,450 mph (Wikipedia). I'm assuming that we can improve the technology a bit to get the rest of the way to half-way there.

In reference to the SABRE engine of Skylon, there's this quote:

While this sounds simple, the problem is that in air-breathing mode, the air must be compressed to around 140 atmospheres before injection into the combustion chambers which raises its temperature so high that it would melt any known material.

They can get around that to an extent, but the heat is quite dramatic.

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    $\begingroup$ Re You can get easily to 50% of orbital speed using a SCRAMJET system -- citation needed! Easily on paper, perhaps, with a sufficiently spherical scramjet cow. Easily in practice? I don't think so. $\endgroup$
    – David Hammen
    Commented Oct 23, 2014 at 17:00
  • $\begingroup$ @DavidHammen I put some references together, the fastest air breathing aircraft went about 7000 mph. I'm assuming something like that is achievable. $\endgroup$
    – PearsonArtPhoto ♦
    Commented Oct 23, 2014 at 18:01
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    $\begingroup$ That 7000 mph (actually, 6600 mph) is 38% of 17,465 mph, which is the speed a vehicle 100 miles above the surface needs to be "orbiting". Anything slower and that 100 miles is apogee, not perigee. The "X" in X-43A means "experimental", which in turn means not yet reliable, not at all cheap, and the opposite of "easily". The follow-on to X-43, the X-51, went slower than the X-43 because the Air Force isn't all that interested in SSTO. They want a very fast suborbital vehicle. $\endgroup$
    – David Hammen
    Commented Oct 23, 2014 at 18:25
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    $\begingroup$ It's not a linear improvement from 38% to 100%. It is so very non-linear. Leading edges want to melt above Mach 5, and everything wants to melt when the speed gets above Mach 7. $\endgroup$
    – David Hammen
    Commented Oct 23, 2014 at 18:28
  • $\begingroup$ Most SSTOs assume some sort of a Scramjet technology, which is estimated to go at around Mach 11- 12, theoretically. Skylon looks to be at mach 5 via the air breathing engine, but the article mentions the difficulties. $\endgroup$
    – PearsonArtPhoto ♦
    Commented Oct 24, 2014 at 2:04
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Yes, "Liquid Air Cycle Engine" (LACE) is the proper nomenclature for this "camel" concept, and it has been studied before. Some incarnations have gone into early design by space agencies.

NASA GTX Reference

Apparently, a LACE had been seriously considered as something which would be the next design step after the X-33. I still find this a little confusing, but it would have been the 2nd out of 3 technology phases toward a 40% mass fraction (!!) SSTO craft. It appears to be differentiated from the 3rd phase by vertical takeoff and some initial stored Oxygen, whereas the 3rd phase would be a runway takeoff and have no Oxygen stored before takeoff. It is different from the X-33 obviously because the X-33 uses no propellant taken from the atmosphere.

Since the X-33 program was halted, it's not surprising this had little mention.

http://web.archive.org/web/20030827015859/http://www.grc.nasa.gov/WWW/RT2002/5000/5880trefny.html

Indian DRDO/ISRO's Avatar

They had a concept for a SSTO where the Oxygen was liquified by some modifications to an airbreathing ramjet stage, which is closer to what I envisioning with Skylon as the baseline.

http://www.webcitation.org/query?url=http://www.geocities.com/spacetransport/spacecraft-avatar.html&date=2009-10-25+22:15:24

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