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Xerus illustration
New commercial suborbital vehicles like the Xerus (above) can take advantage of technological innovations in the four decades since the X-15. (credit: XCOR/Space Adventures)

X-15 and today’s spaceplanes

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TSR: The X-15’s LR-99 engine did not always start. How do the igniters and engines used by today’s programs compare?

Rutan: The hybrid motor developed for the SS1 is simple compared to the liquid motor on the X-15. It has one valve and redundant igniters. It has never failed to start.

Delong: Engine no-lights are neither safety nor mission critical when sitting on a runway with a tourist passenger in the back seat. XCOR designs rocket engines for safety, reliability, and operability, in that order. Of course, ignition reliability is a goal, but it can’t be allowed to degrade safety. Rocket engine startup has historically been among the most hazardous operation, and XCOR is proud that we have never had a hard start. This has led to more frequent no-lights.

Clapp: Well, if our engine doesn’t start we turn around and land. But ignition reliability is much higher than previously. In our case, we have a source of very hot air available to use to get the fire going. Different problem, really.

Clapp: X-15 was, by today’s standards, Pleistocene in its approach to flight controls and avionics.

TSR: X-15 needed challenging pilot control and novel avionics to maintain stability on atmospheric reentry. How have stability and safety improved during reentry?

Rutan: X-15 had to be very carefully flown during entry in order to survive. SS1 will reenter safely even if the pilot is a spectator, i.e., it reenters fine by itself without pilot control. The fatal accident that killed Mike Adams in the X-15 would not have been an accident in SS1.

Delong: What was “novel avionics” back then is now far easier due to the power of microcomputers, but proper attitude on reentry is still critical (I am sure others will disagree). It has to be noted that the only loss of an X-15 vehicle was caused in large part by the flight control computer having a nervous breakdown. (There were of course other contributing causes) Since the X-15 program started, vehicle stability is far better understood.

Clapp: X-15 was, by today’s standards, Pleistocene in its approach to flight controls and avionics. In this day and age, information processing is something we do very well. In our vehicle, the pilot will be operating a rate command attitude hold system throughout the flight envelope, with different control effectors used to control aircraft attitude depending on its position, but selected without the pilot being aware of what control effector was being used.

TSR: How does ammonia compare to fuels used by today’s programs?

Delong: Ammonia is terrible! Yuk! It’s toxic, expensive, and a poor fuel. At that time, they wanted to use hydrazine, but it wasn’t yet available. The technology for cooling engines with kerosene had not yet been developed, and they wanted more performance than they could get from the diluted alcohols then in use. They made it work, after a huge effort, and there are much better fuels available today—nobody is likely to use ammonia again for a chemical fuel.

Clapp: It burns cooler; that is why they picked it. No one uses it nowadays. Most programs it seems to me are using jet fuel and oxygen although there is one team using rubber and laughing gas.

TSR: How have systems evolved from peroxide-fueled APUs?

Delong: Today’s APUs are similar, except they use hydrazine because of its higher performance. There is some change in the common wisdom that changing back to H2O2 will be a good trade between lower performance and lower toxicity.

Clapp: I think the popular opinion favors batteries for most subsystems actuation. Scaled apparently has chosen corned beef sandwiches (i.e. human muscle power). We’re using a chemically-powered APU though not with peroxide.

TSR: How have exterior materials improved from Inconel-X?

Rutan: SS1 has no metal in its structure. It would need additional thermal protection to survive the X-15’s max speed of Mach 6.7.

Delong: Inconel 718 (similar and almost as good as Inconel-X) is now a commercial product that many fabricators routinely work with. This makes commercial uses cheaper and easier to do. Also, there are some modern composites that can take the place of Inconel in many places.

Clapp: I think the popular opinion favors batteries for most subsystems actuation. Scaled apparently has chosen corned beef sandwiches (i.e. human muscle power).

Clapp: Inconel-X is a pretty good alloy although I favor Inconel 718-SPF for similar applications myself. Most of the hot metal research over the last three decades has been focused on turbine metals. In any case, the requirements for our vehicle do not drive the design to the use of exotic metals. I think our peak temperature is under 1000°F [540°C], which means stainless steel will be fine.

TSR: Any other technical innovations to discuss?

Clapp: Don’t denigrate the importance of design. It’s not the instruments in the orchestra; it’s the piece of music they’re playing. And the skill of the musicians.

TSR: Milton Thompson, pilot and author of At the Edge of Space, the X-15 Flight Program liked to buzz ground personnel at the speed of sound. How has tolerance for rowdy behavior at work evolved?

Clapp: Well, that’s just unprofessional.

Delong: Far less tolerance now. Maybe that’s why people want to go on adventure flights?

TSR: “The entire flight control went belly up and the only reaction was to check the circuit breaker, reset, then proceed with the flight.” (Thompson, p. 110). Other times X-15 procedures calling for abort were overridden. Do the dozens of people involved in an X-15 test create an implicit pressure to proceed like NASA is fighting? How is private tolerance of procedures and the procedures themselves different today?

Delong: If a private organization violates a written procedure and somebody is injured (or a desert tortoise is harmed), there will be multiple investigations by government regulators. The X-15 program had neither EPA, nor OSHA, nor the FAA to answer to. In order to get an FAA launch license, written procedures have to be in place, and these evolve until flight. XCOR, with 14 employees, is paying $8000 per year in state required Workers Compensation insurance. And we have never had an accident. If a government program kills somebody, there is a pause and public debate. If a small company kills somebody, they go out of business and the officers go to jail. That’s strong incentive.

Clapp: Plan to fly, fly the plan. That’s what professional flight testers do. In our case, the intent is to have a design where flight can be aborted at any time. One should never be shy about aborting on anomaly.

TSR: The first astronaut wings qualifying flight by X-15 was made by Robert White on 7/17/62. Are we seven years away from a private moon landing? What do the next 7 years hold for private programs?

Delong: If a small company kills somebody, they go out of business and the officers go to jail. That’s strong incentive.

Rutan: White did not fly above 100 kilometers, only above 50 miles [80 km], then considered space by the US government. Only Walker flew above 100 km. Walker was killed when he ran his F-104 into the B-70 in 1966. The X-15 he flew above 100 km was the one that killed Adams. Thus there are no remaining X-15s that have flown to space.

We will have expensive barnstorming with suborbital ships within the next five years. Suborbital flights will become affordable (less than $30,000) within 12 to 15 years.

Delong: It’s like an article I read recently in The Space Review. [“Scenario Planning for Suborbital”, July 19, 2004] There’s good scenarios and bad. My personal opinion is that the best scenario leads to a thousand suborbital flights a year within four years, private orbital tourist flights in seven years, the first Apollo 10 flight in nine years, and the first land-and-return passenger in ten years.

The worst bad scenario is that somebody gets killed within the next few flights and American private flights are shut down. Then, some private efforts fade away and the rest become government contractors. Private space flight comes five years later in Russia, Canada, Argentina, and Romania. Americans travel abroad to fly, just as they are doing now for orbital flights.

Clapp: Seven years to the moon is a little optimistic. We’re not in the moral equivalent of a war here, nor are we funded nearly as intensively. The question is really one of will, and of markets. No one is paying for what we’re doing other than people who have reached into their own pockets to do so.


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