We Don’t Need No Stinkin’ Air Breathers

Henry Spencer (who I expect will be at Space Access this year, after missing last year for the first time ever) explains (once again) why the future of space launch continues to lie with rockets, despite the superficial appeal of not having to carry oxidizer.

This is an important point that I’d never thought about explicitly:

The pure-rocket design was more than twice as heavy as X-30 at takeoff, because of all that LOX. On the other hand, its empty weight – the part you have to build and maintain – was 40% less than X-30’s. It was about half the size. Its fuel and oxidiser together cost less than half as much per flight as X-30’s fuel. And finally, because it quickly climbed out of the atmosphere and did its accelerating in vacuum, it had to endure rather lower stresses and less than 1% of X-30’s friction heating. Which approach would be easier and cheaper to operate was pretty obvious.

This implies that a rocket powered vehicle will have much better off-design (higher delta V, such as more altitude or higher inclination) performance than the air breather, because its dry mass that has to be given the additional velocity is much less. It also means that it will be cheaper to deorbit, and the thermal load will be less, for a given wing area (assuming that it has wings, which an air breather certainly would). I suspect that no matter what the technology level, air-breathing launchers are doomed to remain the equivalent of flying cars — interesting in theory, but never achieved in practice.

17 thoughts on “We Don’t Need No Stinkin’ Air Breathers”

  1. Another point, made by John Carmack in one of his Armadillo essays, is that rockets can fly up out of the atmosphere at arbitrary speeds. You don’t have to boost at 8 Gs right from the ground; you can lazily (relative to an aircraft that needs to maintain velocity) make your way up for the first 50~100 km until the atmosphere is nice and thin and -then- throttle it up.

    There’s a weight & fuel cost to that, but the advantage is that you don’t have to build rockets to be super-aerodynamic if you don’t want to for some reason. Much less stress on the air frame too, which lowers costs and boosts safety margins.

    Of course that means you’d need powered descent to get the rocket back, but that’ll be a non-problem once we have LEO fuel depots.

  2. I think to most people air breathing sounds like a good idea – until they actually run some sims.

    I was looking at a new rocket type (that I’m going to talk about at Space Access, btw) that essentially gets wings for free. I thought that would be a huge improvement – until I simulated it. Yes, there is a slight improvement – but the most efficient flight plan is still almost vertical until you leave the atmosphere!

    Air breathing is the same. Yes, you get improved performance for the first 30 seconds. But it just isn’t that worth it…

  3. Putting gas stations in orbit would allow airbreathers to use less weight for re-entry shielding. So I wouldn’t give up on them just yet.

    Another advantage of the winged vehicle is that you can get home on fewer engines and not end up a spot on the tarmac should one or more engines die. I expect passengers would appreciate that aspect.

  4. How do you get fewer than zero engines? Atmosphere comes in handing for non-air breathing, wingless vehicles as well.

    You’re certainly right about appreciating a soft landing.

    I wonder why balloons are not usually considered; both for going up and coming back down.

  5. The other problem of air breathers not often mentioned is the air intake.

    To “dip” air out of the atmosphere to get oxygen, you have to slow it down to the speed you are going, and when you reach any reasonable fraction of orbital velocity, the stagnation temperature of gas slowed down from that speed is that of molecular dissociation.

    The LACE solution, I guess, is that you scoop air only at lower speeds and (selectively) liquify the oxygen in it to fill the LOX tank of your rocket. Doing that is close to the air-launched-from-subsonic-mothership concept, and it does not buy you all that much. Also, LACE requires you to vent LH2 to get the cold to liquify air, and while LH2 is light, it is frightfully bulky tank-wise.

    The scramjet solution is to not bother slowing the air down to stagnation. Kind of like trying to drink for a firehose. Or keep a match lit in the stream of a firehose.

  6. LPBarker, I can see a lot of strengths to what JP Aerospace is doing, but what sort of total mass and cost/kg are we looking at with those systems (in the near future)? I don’t see anything on the website regarding that.

    Further, and given the recent comsat collision I think this is a reasonable concern, what’s the risk of puncture for the DSS and Orbital Airship? What would a puncture look like; slow deflation or sudden “Oh, crap!” moment?

  7. Paul, as I understand it, scramjets attempt to get around that problem by not decelerating air so much. Air is slowed and compressed somewhat, the fuel (for example, kerosene or hydrogen) is injected into this hot air and spontaneously ignites, there is an expansion phase and it goes out the end. Problem is that this has to be done in a small fraction of a second.

    Brock, I volunteer with JP Aerospace. Large balloons like that do not deflate suddenly unless there is a lot of tension on the skin of the balloon, usually a combination of considerable overpressure in the balloon and a skin that has reached a point where it pretty much has reached its stretching limit (like rubber balloons at full inflation, try poking partly inflated rubber balloons at various degrees of inflation to see),

    Some balloons (the “zero pressure” balloons) are just oversized bags with a hole in the bottom and a lump of helium in the top. As the balloon rises, the helium expands, pushing any regular air out the hole in the bottom. Weather balloons have a very slight overpressure at low altitudes and will slowly deflate if you poke a hole in one. But once you reach the maximum stretching limit, they’ll pop like a fully inflated party balloon.

    The Deep Sky Station (or DSS, which is sort of an asterisk shaped structure) is in the safest outdoors place on Earth for outside objects poking holes in balloons (at 100k to 400k feet, forget what the planned altitude range is). It’s well above planes (and all the sharp things on the ground), and has plenty of atmosphere above it to shield against meteor strikes.

    The Orbital Airship will, of course, enter orbit. I can’t say much about the JP Aerospace design (both non disclosure and not knowing the full details of the planned vehicle), but it’s not that hard to create self-sealing systems for small punctures. To insure against large punctures, I’d just segment the airship so that only a portion of the buoyancy and/or propellant are lost.

  8. Careful here.

    Having wings and breathing air are two very different things.
    Wings in no way imply air breathing.

    It has been known for fifty years that ballistic launchers are more mass efficient than horizontal ones.

    On the other hand, because winged launchers can have all altitude abort capability, they may well be more economically efficient. And that’s what counts.

  9. Putting gas stations in orbit would allow airbreathers to use less weight for re-entry shielding. So I wouldn’t give up on them just yet.

    I don’t see how “gas stations” relate to TPS.

    Besides, for airbreathers, the thermal regime is generally more challenging on the way up than it is during reentry. Rockets can get out of the atmosphere before most of the acceleration takes place but airbreathers can’t.

  10. “… because winged launchers can have all altitude abort capability…”

    Assuming perhaps horizontal takeoff.

    Something like the VentureStar would still be in a dead man zone before it was high/fast enough to pitch down and glide…

    As we saw with the DC-X\A, as long as you have good engines, VTVLs can still recover from events that compromise the ship’s aerodynamics, fairly early in flight.

  11. Karl,

    Yeah, I saw on the blog that you’re involved. Congrats on your Ph.D., by the way.

    No word on mass to orbit or costs though? I’m just curious is we’re talking pounds, tons or multi-ton, and what the $/kg range is.

  12. Thanks for the link. With the DSS at 140k ft. altitude, I wonder how big an issue remaining on station is?

    I had been thinking of launching a rocket in the thin atmosphere. Taking a second airship all the way to orbit is a mind blower.

  13. No word on mass to orbit or costs though? I’m just curious is we’re talking pounds, tons or multi-ton, and what the $/kg range is.

    My take is that nobody in the world knows enough about the technology to make predictions on cost per kilogram. I believe the Orbital Airship is intended to carry at least a few people plus some payload, at most a few tons.

  14. I can see airbreathers being useful for point-to-point spaceflight.

    Suborbital trajectories should require less fuel and speed. This may allow some wiggle for certain hybrid qualities.

    Point-to-point could in fact be one of the first lucrative space businesses. I hear the military is interested.

    A learjet sized corp jet that could take someone from the U.S. to Japan and back in one day would be awfully sexy.

  15. Brock & Ken, what Karl said. 🙂

    Also, John Powell’s book “Floating to Space” is a great, informative read. I highly recommend it.

  16. “A recent flight mishap delivered a powerful example of how invulnerable to puncture these balloons are. Canadian scientists lost control of a 100-meter-diameter weather balloon in August 1998. Fighter jets from three nations were scrambled to shoot it down as it first flew across Canada, then the North Atlantic, Norway, Russia, and into the Arctic Ocean. Canadian F-18 fighters put an estimated 1000 20-mm cannon shells into the balloon, which obstinately continued flying for another six days.”
    – The Paradigm Shift to Effects-Based Space:
    Near-Space as a Combat Space Effects Enabler

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