Chair Force Engineer

Thursday, April 05, 2007

Spaceplanes of Gossamer Wings?

About a month ago, beloved space-pundit Jeff Bell predicted disastrous safety problems for upcoming space tourist craft like SpaceShipTwo and Rocketplane XP. I will be the first to admit that rocket powered aircraft traveling into the exosphere will almost certainly be more dangerous than conventional aircraft. That being said, I also think that Jeff Bell overestimates the dangers of suborbital spaceplanes.

Most of the problems noted in Dr. Bell's analysis of the data show that engine problems accounted for a vast majority of catastrophic rocketplane incidents. Four of these were attributed to the use of ullmer leather, incompatible with liquid oxygen. The second X-15 had an in-flight engine failure, and the third X-15 had an engine explode on the ground. While this is a problem inherent in all rocketplane projects, it must be noted that the X-15 program was conducted at a time when large liquid rockets, especially complex ones like the XLR-99, were in their infancy. As time has progressed, rocket engines have become more reliable. Spaceplane engines need not be any different.

It should also be noted that all of the projects cited by Dr. Bell were research programs that "pushed the leading edge" of flight research. The X-15A-2 suffered structural damage during a high-speed run, the X-2 was lost due to inertial coupling, and the third X-15A spun out of control during re-entry. Flights of commercial spaceplanes will avoid these problems by flying well within the type's flight envelope instead of shooting for extreme speeds and maneuvers that are the norm in a flight research program. I especially think highly of SpaceShipTwo's "feather" maneuver, which takes much of the risk out of reentry.

Dr. Bell mentions the grounding of three X-1 series aircraft due to fatigue in the tankage. This is a problem that cannot be dismissed, but it's not a show-stopper either. Propellant tanks have to be designed to withstand a certain number of pressurization cycles, and they have to be inspected thoroughly between flights. Fortunately, our ability to inspect for microcracks in these tanks is much better today than it was in the 1950's.

I share Dr. Bell's concern with the SpaceShipTwo propulsion system. I studied hybrids for a lengthy period in college. In my mind, the supposed safety benefits never seemed that great, and they never seemed to outweigh their poor performance when compared with liquid systems (or even all-solid rockets, which usually have better specific impulse than N2O-HTPB hybrids.)

Noticeably absent from Jeff Bell's data is any mention of the Douglas D-558-II Skyrocket. This research aircraft flew 313 times using three airframes. None of the aircraft were lost during the highly-successful program. The program even racked up a remarkable aviation achievement: the first Mach 2 flight in history was achieved by Scott Crossfield in the D-558-II on November 20, 1953.

Of the 313 flights, three different propulsion schemes were employed: turbojet-only, turbojet + rocket, and rocket-only with launch from a B-29 mothership. Admittedly, the turbojet-only flights should be tossed out of this safety record, because they're no different from conventional aircraft. Nonetheless, the Skyrocket demonstrated that there's nothing inherently unsafe about mounting a rocket engine with a winged airframe.

As space tourism reaches the suborbital frontier, rocket-powered aircraft will lead the way. Flying in such craft will undoubtedly carry risks, but safe practices and smart engineering can bring risks down to acceptable levels. The passengers of such spaceplanes should go into their adventures with full awareness of those risks, and a belief that their space experience will outweigh any potential hazards.

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