The X-33, which failed to take flight even once after spending more than $1 billion on it, is one of the most infamous examples of failed spaceplane projects. (credit: NASA)
In recent issues of The Space Review, Mike Snead has surveyed the history of, and options for, commercial passenger spaceflight (see “Solving the commercial passenger spaceflight puzzle”, Part 1, Part 2, Part 3; September 3, 9, and 16, 2019). This valuable document is nearly 16,000 words long, with excellent illustrations, and is probably the most useful record of the fraught history of this topic for many years. He makes this key point: despite what some would have us believe, outer space will not be permanently opened to commercial development and settlement without first establishing acceptably safe commercial passenger spaceflight.
He concludes that the only practical way forward is to revitalize a plan to develop a reusable, two-stage-to-orbit, vertically-launched, horizontal-landing system, abbreviated to TSTO VTHL. One scheme originated in 2002, when both the Air Force and NASA were asked to propose a joint program for such a vehicle. However, NASA decided to go it alone, daringly without a preliminary prototype program. Perhaps luckily this decision was sidelined by the debate arising from the loss of the shuttle Columbia and its crew in 2003. Then, as he notes, rather than seeking a reusable system to replace the shuttle, NASA reverted to expendable rockets and capsules. The outcome has been the Orion spacecraft, the SLS heavy-lift rocket, and the separate commercial crew program, none of which move the industry significantly forward towards his goal of routine and safe passenger spaceflight.
Before going further, a disclosure is necessary. I hold the patents for the Swala reusable launch vehicle and, perhaps uniquely amongst spaceplane designers, I am not a rocket scientist or engineer. However, my own engineering expertise includes over 400 assignments in 35 countries.
Snead’s TSTO VTHL concept is a straightforward extension of existing ideas and technology, centering around one big thing: a conventional rocket using liquid oxygen (LOX) and a fuel. The horizontal landing may be that of a spaceplane, but a moment’s thought shows that it cannot be otherwise. To return boosters vertically can be done and is a marvelous development by SpaceX, but Tsiolkovsky’s rocket equation—the one that dominates rocket engineer’s professional lives—prevents reusability with a full TSTO vertical take-off, vertical landing configuration, even if there is no payload to speak of.
The fundamental problem is the weight of oxygen that has to be hauled up in a conventional rocket. To understand this, it is useful to look at the energy requirements of Snead’s TSTO VTHL. Starting from the basics, to put a kilogram into low earth orbit requires, in theory at least, only around 33 megajoules (MJ), or about the energy in a liter of RP-1, the usual kerosene-derived fuel for this task. However, unless you can make use of the oxygen in the atmosphere, every kilogram of this fuel requires about 2.4 kilograms of oxygen to be carried up along with it, so that instead of 33 megajoules per kilogram, the total power requirement just to carry the propellant itself up is now 112 megajoules per kilogram.
This is before the structure and the payload—its deadweight—is factored in. Here the Tsiolkovsky dictatorship is absolute. The 112 megajoules needed could be provided by about 2.5 kilograms of JP-1, which has an exhaust velocity of around 2,800 meters per second. The equation says that for the rocket to get to the approximately 8,000 meters per second needed to stay in orbit, just 10% of the mass of propellant can be deadweight. In this case it will be just 250 grams: say, half a pound. But this deadweight itself needs in turn another 3.4 times its weight of propellant.
So conventional rocketry is a dismal case of diminishing returns. This is why Snead’s proposal is for a two-stage vehicle that relieves itself of the weight of the first, booster, stage by discarding it, perhaps to a SpaceX-inspired soft landing. A single-stage-to-orbit (SSTO) spaceplane, as always, seems an impossible dream. Rocket scientists are certain that staging is necessary if a useful payload is to be placed in orbit.
Perhaps the saddest outcome of an attempt to create a SSTO was with the Lockheed Martin X-33. This was a one third-scale version of its VentureStar SSTO proposal, and it missed getting flown by a whisker, or rather by a sudden unilateral decision by NASA just as it was reaching completion.
It will be understood that a spaceplane that eschews staging must be very, very light. The VentureStar would save the weight of wings by using lifting body aerodynamics. It would also use liquid hydrogen, the most powerful of fuels, fed from unique lightweight cryogenic fuel tanks. It would employ a special lightweight metallic thermal protection system and use an aerospike combustion configuration said to be 15 percent more efficient that one using a conventional nozzle. Together, these innovations would surely defeat the evil rocket equation, and at a relatively low cost.
No. The failure of its innovative fuel tank during testing caused much rancor, and ultimately led to the withdrawal of NASA support after five years of work and the expenditure of $1.5 billion on a spaceplane that, let’s face it, had no payload capacity to speak of.
Such outcomes explain why a fully reusable spaceplane is considered an impossibility. Snead, in his masterly survey, drew this lesson: “We must be careful not to fall, once again, into the trap of putting all of our efforts into achieving a single-stage to orbit capability as was attempted with the failed National Aerospace Plane/X-30 and NASA’s X-33 programs. Some, especially in the research community, will strongly urge that this be done. Such recommendations must be rejected!”
I think this is wrong. There is a classical antecedent. The archaic Greek poet Archilochus spoke of mankind as being of two classes, foxes and hedgehogs. In his words “a fox knows many things, but a hedgehog one important thing.” Most rocket scientists are hedgehogs, which is fortunate given the immensity of the challenges they need to focus on. However, the greatest challenge—a reusable SSTO vehicle—remains. It is sad to see it rejected as totally impossible.
Look at it this way. If I had told you in 2004 that the founder of PayPal had put $30 million into an electric automobile—an idea that had been superseded a hundred years before—would you have bought the company’s stock? But Elon Musk is a fox, and the space launch industry is already experiencing the disruption that such foxes can cause. The SSTO challenge won’t go away; it is waiting for its fox.
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