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Disclaimer: These are my personal opinions. I emphatically do NOT speak for my employer.

Why Rutan Matters

Peter A. Taylor
February 2005

Several knowledgable people have taken the view that the winning of the X-prize by Burt Rutan's SpaceShipOne (SS1) is really no big deal. The argument states that while there is a superficial similarity between a suborbital flight and an orbital one, the differences are huge. Kinetic energy is proportional to speed squared. Thermal problems during re-entry are related to the maximum heating rate, which is roughly proportional to speed cubed. Propellant requirements for a rocket stage are an exponential function of speed. So if you consider that SS1's top speed of Mach 3.5 was only about one seventh of orbital speed, and try to quantify what percentage of the challenge of orbital speed Rutan has overcome, it's easy to argue that it isn't significant.

The argument continues that many technical details of SS1 are very unlike those that will be required for orbital flight. SS1 used a hybrid rocket with a pressure-fed room-temperature storable oxidizer, and is fully reusable. An orbital vehicle will almost surely use a pump fed liquid fuel with cryogenic oxygen, and some of the tankage at least is likely to be expendable. SS1 doesn't even begin to let us guess at how Rutan might solve the thermal problem of re-entry. Nor is it obvious how he is likely to land an orbital vehicle. Deployable wings or rotors are certainly possible, but they won't look like the tail rotation mechanism on SS1.

Nor is the launch configuration entirely novel. The Orbital Sciences Corp. Pegasus is a winged, solid fuel rocket that is launched from a subsonic aircraft, and it has been flying for years. Decades before that, the B-52/X-15 combination performed similar suborbital manned flights.

It's true that SS1 was privately funded, but US aerospace contractors have been building commercial launchers for commercial customers for decades, and the cost of launching commercial payloads isn't much different from the cost of launching government unmanned payloads. It's also true that there might be a suborbital tourism industry that could bring some money into the suborbital launch business, but it seems like an economic dead end. The size of the suborbital tourism market doesn't seem likely to be all that large, and there is still that vast performance gulf between suborbital and orbital flight. From an economic standpoint, the suborbital and orbital space flight markets seem pretty much irrelevant to one another.

It's true that the development and operating costs of the White Knight (WK) and SS1 are dramatically lower than the cost of any current manned space vehicle (and Rutan managed to do this without getting anyone killed), but its performance isn't comparable, so the cost comparison seems meaningless. Why should anyone expect orbital launch costs to drop dramatically based on the success of SS1? If the end of the orbital launch tunnel is so far away from SS1, how is it that anyone can claim that they see light at the end of it?

The answer, as social psychologist Kurt Lewin put it, is that "Nothing is as practical as a good theory." We know that orbital launches are two orders of magnitude more expensive than a quick back-of-the-envelope calculation would suggest based on the costs of kerosene and oxygen. What we don't really understand is why. Everyone in the industry seems to have a pet theory to explain high launch costs, which they will defend with great heat, but there are literally dozens of these theories, and many of them are quite silly. NASA, for example, wasted hundreds of millions of dollars on the X-33, which was based on the theory that a single-stage to orbit (SSTO) launcher would be dramatically cheaper than a two- or three-stage launcher. Apart from its technical problems, the economic theory behind the X-33 just didn't make sense. What we need are not just theories, but good theories, and we need to test them.

This is what Rutan appears to be doing. While the White Knight may be comparable to the Lockheed L-1011 used to launch the Pegasus, SS1 is different from Pegasus in several critical, related ways. SS1 is reusable, and Pegasus is not. SS1 is capable of incremental testing (because of not being reusable, every flight of a Pegasus is its first and last). Finally, SS1 is capable of safe, intact, low-cost abort at any point in its flight. This intact abort capability is what Maxwell Hunter argued was the key to low cost space launch.

The end of the tunnel is still a long distance away. It remains to be seen if Rutan can build an orbital launcher while retaining essentially intact abort capability (I'll forgive him if he uses low-cost expendable tankage). It may also be that several theories are conflated (I'm not sure if having a human on board is good or bad), or that a theory that is true for a first or second stage may not be true for a third stage. Maybe Rutan is thinking something totally different. But what I see in SS1 is that we have at least a tentative confirmation that Maxwell Hunter was right about intact abort, and someone is planning on testing the theory further. I am quite hopeful.

Again, these are my personal opinions. I emphatically do NOT speak for my employer.

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