The Space Review

X-15 pioneered some major new design advances, but also had a number of weaknesses. (credit: NASA/DFRC)

X-15 and today’s spaceplanes

Spaceplanes have come a long way from the X-15 to today. (See a backgrounder on the X-15 program.) Sam Dinkin recently interviewed Burt Rutan, Dan Delong, and Mitchell Burnside Clapp to get their impressions on the difference between the X-15 program and the suborbital rocketplanes they’re developing commercially.

Burt Rutan is an aerospace legend who has developed over 40 aircraft including the VariEze and the world-flight Voyager before the White Knight and SpaceShipOne.

Scaled Composites, LLC is an aerospace and specialty composites development comany located in Mojave, California (about 150 kilometers north of Los Angeles). Founded in 1982 by Burt Rutan, Scaled has broad experience in air vehicle design, tooling, and manufacturing, specialty composite structure design, analysis and fabrication, and developmental flight test.

Dan Delong is chief engineer for XCOR Aerospace. He spent his early career in underwater systems, where he was lead or chief engineer on prototype commercial underwater vehicles as well as several “deep black” projects. At XCOR he designed the EZ-Rocket and the Xerus.

XCOR Aerospace is a California corporation located in Mojave, California. The company is in the business of developing and producing safe, reliable and reusable rocket engines and rocket powered vehicles.

Mitchell Burnside Clapp is Senior Designer and Engineer at Rocketplane. He led the design effort at the US Air Force’s Phillips Lab that developed the “Black Horse” rocketplane. At Rocketplane, he designed the XP.

Rocketplane Limited, Inc. is a company dedicated to revolutionizing space travel by building a rocket powered aircraft and applying standard aviation practices to create a vehicle that will fly to space as readily as conventional aircraft fly to places on Earth’s surface. Rocketplanes permit travel to space, travel in space, and travel through space.

Opening thoughts on comparing the X-15 to spaceplanes of today:

Burt Rutan of Scaled Composites: The X-15 cannot be compared to SpaceShipOne directly since its goal was not just to go to 100 kilometers, but to fly high-Mach research out to Mach 8 (actual max Mach was 6.7). The X-15 flew only two flights above 100 km, two consecutive flights by Joe Walker.

Dan Delong of XCOR Aerospace: First I have to point out the obvious: that the X-15 was a research vehicle by a government agency built and flown for the purpose of doing cutting-edge research flights. We are trying to make money, not endanger anybody, and bring costs down to where price is sufficiently greater than cost to make it self-sustaining in the commercial marketplace.

Mitchell Burnside Clapp of Rocketplane: The mission of the X-15 was research flight, whereas our mission is routine operations. What one does in the context of a flight test program is different from what is done with the operational vehicle.

The Space Review: How do private program costs today compare to the X-15 which conducted 200 flights for $300 million in 1969 dollars or about $1.5 billion in 2004 dollars? How does price per flight by the 200th flight compare from an estimated $600,000 in 1969 dollars or $3 million/flight in 2004 dollars?

Rutan: I do not doubt that the price per flight of X-15 was $3 million, since it was a government research program. The price per flight during the research program of SS1 after 4 powered flights is about $300,000, or $100,000 per seat (less than 4% of X-15 costs).

Delong: Program costs are proprietary, but price for an X Prize-type ride is being advertised by Space Adventures et al. for around $100,000. This is significantly more than Xerus flight cost, and a sufficient number of them repay development costs. We do admit publicly that EZ-Rocket marginal flight cost is about $900.

XCOR is confident that the current “market” price of $98,000 is realistic. The number of flights at that price to recoup development costs is on the close order of 200.

Clapp: Well, forgive me if I don’t express my prices in archeological dollars. X-15 pricing was driven by range infrastructure, propellants, support aircraft, and a design created to perform research, not operations. We expect a flight of our vehicle, which to be fair would not be nearly so fast as the hottest X-15 flights, to have a direct cost per flight of under $50,000. How much under I’m not at liberty to go into, but from the business plans of most of these people, charging about $100,000 per seat and making $200,000 per flight gross, you can see a need to keep direct operating costs low enough to make money.

TSR: How do resources for today’s private programs compare to the B-52, three ground stations and up to six chase planes for each flight?

Rutan: We chase with a donated Starship (for launch) and an Extra (small aerobatic aircraft, used to chase landing). The Alpha jet is used by our customer for video documentation. The White Knight is our B-52.

Rutan: The price per flight during the research program of SS1 after 4 powered flights is about $300,000, or $100,000 per seat.

Delong: The B-52 was needed to get the performance for X-15 to explore as far into the unknown as they could. The rocket equation is simple: MR = e(deltaV/Vexh). Translated into English, the velocity needed is a function of only the specific impulse and mass ratio. Using reasonable design margins and non-toxic propellants, the velocity needed for an X Prize trajectory doesn’t need the carrier aircraft.

The three ground stations are an example of modern technology making the mission cheaper. GPS and inertial navigation systems are small, cheap, and light enough to give the pilot all the redundant information he needs. You need six chase planes if the vehicle outruns them and they have to be pre-positioned, but tourism flights do not cover much ground; they are more vertical. Certainly, during early testing (the first 40 flights), chase planes will help a great deal, but later revenue generating flights shouldn’t need them at all.

Clapp: I don’t think anyone is advocating such things… [F]rom my point of view, the vehicle should be self-contained and autonomous. You have to ask yourself, “What are all those people going to be able to do for you in operations?”

Specifically, the vehicle is engineered so that every piece of information that the pilot would use to make a decision is available to the pilot in the cockpit in real time.

TSR: X-15 used 10 dry lakebeds with 2-4 possible lakebeds for each mission as possible landing sites. What sort of landing range do today’s private programs need?

Rutan: Since the X-15 could not survive a steep descent into the atmosphere after flying to 100 kilometers, it had to fly 300 miles [480 km] away to allow a 40-degree climb and descent. SpaceShipOne can survive a vertical plunge, thus its entire flight can be within 25 miles [40 km] of the landing airport.

Delong: X-15 was developed by a highly experienced team… part of that team can be eliminated because we now have the hypersonic flight experience pioneered by that very same vehicle and others.

Delong: This is another artifact of the X-15 research mission that covered a lot of ground. It is also a feature of the power-off glide ability of each particular vehicle. You have to assume engine shut-down at any time and be able to glide to a landing. The Xerus test program has alternate landing sites, but all are paved runways in the local area. Various low-probability emergency aborts may use the lakebeds but they are not primary abort sites.

Clapp: [W]e use a single runway. Our vehicle takes off, flies on its jet engines uprange, turns around, and performs its mission in the direction of the arrival runway. The jet engines are ignited after entry, but left in idle thrust as a safety measure. All approaches are flown as if they are power off approaches, against the day that becomes the actual state of the aircraft.

TSR: Anything to add about the resources of X-15 versus today’s private space plane programs?

Delong: X-15 was developed by a highly experienced team using state-of-the-art design tools and pushing the state-of-the-art in materials and fabrication technology. Part of that team can be eliminated because we now have the hypersonic flight experience pioneered by that very same vehicle and others. Much of that team can be eliminated because of the modern design tools that live in the PC on my desk. Materials available commercially today did not exist then, even to cutting edge government research programs. And some of those new, high performance materials are low cost.

Clapp: Did they have more money to spend? Of course. But their requirements were different, and keeping costs down wasn’t on the list.

page 2: technical innovations >>


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