Over at The Space Review today, Stephen Ashworth responds to the idiotic Space News piece by the ESA guys last week. I generally agree with his critique, though he’s far too fond of airbreathers.
20 thoughts on “The Space Tourism Hoax”
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Unless you’re a politician, it’s a lot harder to claim something can’t be done after it’s been done. We just have to wait and see. I’m hopeful.
though he’s far too fond of airbreathers
Having read up on airbreathers a bit I’m starting to like them more and more, provided you start with something simple like a ramjet instead of the holy grail of hypersonics researchers, the scramjet. You are probably familiar with the 1966-1967 Marquardt study. The supercharged ejector ramjet sounds like a very interesting addition on an RLV or partial RLV. I’d like to understand if this could be combined with Truax/Microcosm/Beal cheap expendable pressure-fed stages.
I’d like to understand if this could be combined with Truax/Microcosm/Beal cheap expendable pressure-fed stages.
Air breathing expendable launchers don’t appear to make much sense to me. And for a first stage, I find if very unlikely that adding air breathing would be cheaper or easier than just making the stage larger.
Since ramjets are so simple, it might be worthwhile to add them to a very simple pressure-fed stage. It could reduce GLOW, which is useful since that would reduce the needed pressure in the tanks which determines their mass. It could also reduce delta-v requirements on a crew vehicle, which helps if you need a heavy TPS. Adding a supercharged ejector ramjet to a reusable upper stage would at least give it more cross-range and a useful powered landing capability.
I agree airbreathers are overhyped, but ramjets, not scramjets, seem useful enough in the near term, both for orbital and suborbital applications.
I think I can persuade most people that even an optimistic Supercharged Ejector Ramjet, or similar, can always be beaten by an ensemble of a well-chosen conventional turbine engine and rocket engine for any given thrust level, providing more Isp and a better thrust to weight ratio. My experience in this industry, and as the designer of more than a couple partially airbreathing spaceplanes, suggests strongly to me that the moment you perturb a trajectory in the effort to get more air to breathe, you depart decisively optimum, and if you don’t perturb the trajectory, the thrust of the various airbreathers is insufficient. Put another way, if you’re going to space, air’s just in your way. Go around, don’t go through.
I don’t agree with the talk about ramjets. They apparently are only useful in a narrow window of speeds, from somewhere above Mach 1 to somewhere above Mach 4. You still have to get up to the initial speed to start one and the inefficiency of slowing air down to subsonic speeds caps your highest speed (and thrust/ISP as well) of a ramjet.
In discussion of scramjets, they have claimed ranges that are far wider. For example, I recall one that was, with a combination of methane and hydrogen fuel (yes, that is overly complex design there) thought to have a viable range between Mach 6 and Mach 18 or so. That’s half of your needed delta v (you need to get to Mach 25 or so).
So it ended up having two propulsion systems (rocket and scramjet) and three propellants (methane, LH2, and LOX). You’d fire the rocket to get to Mach 6, light the scramjet which would burn through methane and LH2 in that order, then at Mach 18, you’d switch back to rocket mode and complete the trip.
This idea glosses over many huge, glaring problems of scramjets, including not least that we don’t have a scramjet that has worked under anything resembling real flight conditions for more than a few seconds.
@Mitchell Burnside Clapp:
I’d love to hear more about this, although this may not be the correct place for it.
My wife, children, Mom, Dad – indeed all my family and friends breathe air. I am very fond of airbreathers.
What? He’s talking about space craft. Never mind.
Heheh, good one. In that sense it’s all about getting airbreathers into space.
I have long believed that the breathing of air is a privilege that should be reserved for the pilot.
Martijn, here’s the basic reasoning: Pick a thrust to weight ratio and specific impulse for your supercharged ejector ramjet or whatever cosmic airbreathing technology you like. Alongside of that, choose state of the art values for thrust:weight and Isp for both rockets and conventional turbine engines. Now imagine an ensemble of rocket+turbine that gives the same thrust as your imaginary engine.
You can create a plot with a point on it for the assumed SERJ engine. The axes are specific impulse and thrust to weight ratio. You can also draw a curve from the rocket and turbine data, with points corresponding to 100% rocket, 90% rocket, 10% airbreather, and so on. In all the studies I did, based on values that seemed realistic to me at the time, the curve for the ensemble always passed above the SERJ point, which indicated to me that you could always beat the SERJ system with a good choice of rocket and airbreathing thrust.
Don Hart, former head of the Rocket Propulsion Lab at Edwards, told me at the beginning of Copper Canyon/NASP that the history of air-breathing propulsion to orbit always begins with a large big air-breathing engine and a tiny little rocket engine used only for that “last bit of delta-v” necessary to get into orbit. As the program progresses, the air-breathing engine provides less and less of the delta-v and the rocket grows bigger. Finally, all the real delta-v comes from the rocket and the program gets cancelled. He was dead on.
For pure performance, a rocket alone is very difficult to beat. Air breathing engines bring some flexibility to some commercial applications. Flying out of Jacksonville would benefit from some form of jet first stage.
@Mitchell Burnside Clapp:
What about cost? The idea behind the pressure-fed boosters is that you make them really dumb, relatively small and hopefully really cheap to produce with mass production. Storable, dense, nontoxic propellants, no turbopumps, no igniters, no gimbals, no gas generators, hardly any avionics. All the expensive bits would go on a reusable spaceplane. A ramjet seems so simple, would it really have a negative impact on the cost of such a cheap expendable booster?
And a SERJ is nice for descent and landing, which might be enough of a reason to add it to a spaceplane. And once you have that, why not use it for ascent too? Every little bit helps. And for suborbital applications it would be nice if a spaceplane could take off from a runway under its own power, without any need for refueling.
Just to be clear: I’m seeking to understand, not to convince.
@Mitchell Burnside Clapp:
Are you just skeptical about combined cycle propulsion or about all airbreathing earth to orbit or suborbit applications? Other than air launch or aerial refueling.
“As the program progresses, the air-breathing engine provides less and less of the delta-v and the rocket grows bigger. Finally, all the real delta-v comes from the rocket and the program gets cancelled. He was dead on.”
This is pretty much why Skylon is the only HTHL SSTO airbreather I consider commercially credible (all-rocket HTHL SSTO doesn’t seem possible without a ground accelerator of some sort [landing gear weight is never low enough for the liftoff weight], eliminating your standard runway launch flexibility…though I’ll definitely entertain aerial refueling).
They realized what you said up front, and never try to operate beyond the limits of turbojets, staying completely away from ramjet (supersonic combustion or otherwise), radical inlet designs and high Mach heat/drag issues on ascent completely.
http://groups.google.com/group/sci.space.tech/browse_thread/thread/7f5612419778472e/a77ae063d7b94fa4?lnk=st&q=%22Skylon%22%2B%22Henry+Spenc%20er%22&rnum=6&hl=en#a77ae063d7b94fa4
For all-rocket SSTO, VTVL is the best way to go, I think. Especially *if* a need for heavy-lift RLVs emerges…
Martijn, it’s not that I’m skeptical about combined cycle propulsion, it’s just that I don’t think it offers any advantage. You want to take off and land on a jet engine? I’m inclined to agree with you. A SERJ is nice for takeoff and landing, as you say, but you know what’s even nicer? A turbofan. And they have the advantage of an R&D cost of zero.
The thrust to weight of even a ramjet is dismal by rocket standards, especially if you attempt to operate it on a rocket-optimal trajectory. It barely blows out its own base drag.
Those of us who have considered an airbreathing component to aerospace craft are drawn to the potential benefits of the higher Isp that such engines offer. But upon closer evaluation and analysis, these Isp benefits slip away, because of the other compromises that must be made to accommodate airbreathing propulsion. John Whitehead’s paper provides an interesting examination of this problem, with a straight-forward, easy-to-follow analysis:
Airbreathing Acceleration Toward Earth Orbit
AIAA 2007-5837
http://pdf.aiaa.org/preview/CDReadyMJPC07_1492/PV2007_5837.pdf
His conclusion: some airbreathing propulsion might provide some net benefits up to about Mach 6. Above that speed, the benefits are rapidly supplanted by other problems and difficulties, and overall performance deficiencies.
Yet interest in high-speed airbreathing propulsion ebbs and flows on about 10 year cycles (by my reckoning). We see the benefits of airbreathing propulsion, we begin engine development and testing programs, but then we encounter the problems and difficulties inherent in these engine cycles. We come to perceive the mountain of difficulties we face in bringing these systems to fruition, and when compared with flat, even path offered by rocket propulsion, we chose that path every time.
So why do we keep coming back? Most of us don’t. For many within the “New Space” community, airbreathing propulsion is considered worse than useless, because it diverts resources and distracts investors which would be better spent on developing multi-stage rocket vehicles, like we’ve been doing all along. We keep taking the path plowed for us by our NASA ancestors. It’s easier, less expensive to develop, and we know it works. And it may even have superior performance, depending on what assumptions you make.
Yet there are still a few of us, lured irresistibly by that siren-like song of reduced propellant fractions, who still work on high-speed airbreathing engines, somehow playing the long-odds that maybe there’s a subtle, not-so-obvious path using airbreathing engines that will provide substantial performance improvements. I’m one of those workers. I still believe that there are under-explored technologies and approaches that might still offer great potential. Some examples: The Air Turbo Rocket (ATR) Engine Cycle. Lower T/W than a rocket, Lower Isp than a turbojet, yet possessing the highest thrust-per-unit frontal area than any other airbreather. another example: Multi-Dimensional Optimization (MDO) of ascent trajectories, like the work Frank Chavez does:
Minimum-Fuel Trajectories for hypersonic Vehicles with Aeropropulsive Interactions
AIAA 1993-3660
Fuel-Optimal SSTO Mission Analysis for a generic Hypersonic Vehicle
AIAA 1995-3372
I think its fair to say that many believe it’ll be a cold day in hell before airbreathing propulsion becomes a viable launch vehicle propulsion option. Me? Still looking for that cold day. But here’s a few things to think about:
1. We have an existence proof of the utility of an airbreathing component to a suborbital launch vehicle system: WhiteKnightOne and SS1
2. When you put wings on a rocket-powered vehicle, you in essence become an airbreather, because you’re generating a force (lift in this case) from the wings, which exchange momentum with the rocket motor through the L/D ratio.
The promise of airbreathing propulsion may be a quest for the holy grail, or the city of Troy. But we certainly won’t find it if we don’t look.
Leave the atmosphere to the air breathers (subsonic – supersonic gets expensive) and the vacuum to the rockets. This allows each stage to be appropriately specialized – uncompromised.
Proponents of combined cycle systems claim there are advantages if you use integrated systems instead of separate ones. IIRC a SERJ is believed to be lighter than a combined rocket + turbofan and offers better performance overall. I cannot verify these claims, but I’d like to know more. Were the Marquardt guys simply wrong or have their conclusions been superseded because of new developments in turbomachinery, material etc?
Martijn, I didn’t see any advantage to a SERJ engine over a combined rocket and turbofan. Indeed, quite the opposite. While a SERJ engine might offer an impressive thrust to weight ratio at a specific operating condition, it isn’t a condition you want to spend any time at. In general engines of this type only work well at dynamic pressures that are bad for structures, aerodynamics, and heat transfer characteristics. Pick any number you like for a SERJ engine – then compare to an ensemble of a turbofan with a T:W of 8 and a rocket with a T:W of, say 80 or so. I couldn’t make it compute.
Philosophically, I’m increasingly of the opinion that new aerospace vehicles will be excellent not because of new technologies but because of really good design. I sometimes make an analogy to music: We’ve been playing the same musical instruments for a really long time, but new music comes from new arrangements of the sounds of existing instruments. And even a room full of virtuoso musicians isn’t an orchestra without something to play.