Two spaceplanes

After years of struggle and achievement, Secretary of Defense Robert McNamara cancelled Dyna Soar on Dec. 10, 1963. You can’t deny that the Dyna Soar program was an expensive one, nor can you argue that there were missions that Dyna Soar, and only Dyna Soar, could fulfill.

For every task anyone proposed for Dyna Soar, there was another, cheaper way of doing it. All of them could be made smaller, lighter, cheaper and at lower technical risk than Dyna Soar, and launched on smaller boosters.

While a vast amount of work was carried out during the existence of the Dyna Soar program—11 million man-hours of engineering, 14,000 hours of wind-tunnel testing, 9,000 hours in simulators—a definite mission for which the Dyna Soar proved demonstrably superior failed to materialize.

As of the stop-work order, construction was well underway on the first Dyna Soar. At that time, the Air Force had released 49 percent of the require production orders—14,660 of them. The first spaceframe was 40-percent complete.

Fifty years after Dyna Soar ended, the X-37B was orbiting Earth on its third mystery mission.

The X-37B is itself the end result of a development program longer than that of the Dyna Soar. The earliest recognizable antecedent of the X-37B was the Rockwell International “REFLY,” a small REusable FLYback satellite.

REFLY would go into orbit atop a Pegasus booster and would, as with the X-37B, provide power, maneuver and return capability for a payload meant to spend a short time in space. Rockwell submitted a patent application for REFLY in 1993, making the concept at least 21 years old at this point.

After Boeing acquired Rockwell, work on the REFLY continued, leading through the Military Space Plane and Space Maneuver Vehicle programs to the X-40 and finally the X-37B Orbital Test Vehicles. The configuration changed surprisingly little.

NASA chose Boeing to develop a reusable spaceplane in 1999. However, in 2004 NASA transferred the program to the Defense Advanced Research Projects Agency, which not only continued development but also clapped the cloak of classification over it.

While the configuration of the X-37B bears virtually no resemblance to that of the X-20, the cargo bay on the X-37B is larger than that of the X-20. It’s roughly the same cross section, and perhaps half again as long.

Fifty years of advancement in automation means that many missions like those planned for the Dyna Soar would now not need a crewman on-hand. The X-37B also benefits from advances in materials, both structural and thermal.

Where the Dyna Soar was built out of very dense nickel superalloys, the X-37B has a graphite/polymer composite main structure. The Dyna Soar had heat shielding that permitted massive heat loads to penetrate into the vehicle; the X-37B has a Shuttle-like cladding of silica ceramic tiles, effectively keeping the heat out.

The X-37B has a much lighter internal structure, with less need for the cooling systems needed around the Dyna Soar’s cockpit, cargo and equipment bays.

One important difference between the Dyna Soar and the X-37B is the inclusion of primary propulsion in the more recent vehicle. The Dyna Soar glider relied upon the Transtage for orbital maneuvers. The X-37B uses storable bipropellants for both reaction control thrusters and a single main engine in the tail.

The X-37B also has reaction wheels which use rotational momentum to provide attitude control without the expenditure of propellant.

The Dyna Soar used hydrogen/oxygen fuel cells to provide on-board electrical power. While this has proven a successful system on Apollo and the Shuttles, it does limit total energy. Once the cryogenic liquid hydrogen and liquid oxygen are consumed, the ship is out of power.

This was not a major problem for the Dyna Soar, as it was practical to provide enough consumables to outlast the crew who could hardly be expected to remain in their small craft for more than a few weeks. But for the robotic X-37B, far longer missions are—obviously—possible.

Thus a power source that won’t quickly deplete was provided in the form of a solar cell array that deploys from the cargo bay. Radiators are integrated into the payload bay doors, similar to the Space Shuttle.

The X-37B provides similar mission capabilities to the Dyna Soar, but seems to be a substantial improvement upon the earlier design in nearly all respects.

While the X-37B improves on Dyna Soar capabilities in most respects, there is one area in which it has not—what is its actual mission? The Dyna Soar died not because the technology wasn’t there, but because the mission wasn’t.

The one advantage that Dyna Soar offered was the ability to transport crew up and down, but crew are no longer necessary for the bulk of the missions that the military assigned to Dyna Soar.

And the question remains for the X-37B. What payload can it take up and provide for that would make sense to return safely to Earth? It’s generally assumed that the X-37B carries reconnaissance equipment. But all data collected by modern recon systems is transmitted digitally; so what actually needs to come back?

The X-37B could certainly carry out the offensive missions the military once planned for the Dyna Soar. The sensors and weapons needed would be much smaller today than in Dyna Soar’s day.

Additionally, various sources have suggested that the X-37B could carry weapons such as nuclear-armed re-entry vehicles and kinetic energy rods. The weapons could remain on orbit for an extended period and, if not used, return to the surface. This would, of course, be a treaty violation.

The X-20 prototype was left unfinished on the factory floor while at least two X-37Bs have not only been built but launched, even though the same questions of military utility remain unanswered. At least publicly.

Clearly the X-37B program succeeded on the political battleground where Dyna Soar ultimately failed.

Scott Lowther is the publisher of Aerospace Projects Review. The Model 844–2050E is the major feature of the latest issue of APR.