Upcoming X-37B Test: What Can a “Space Plane” Do?

April 15, 2010 | 3:49 pm
Laura Grego
Research Director, Senior Scientist

The U.S. Air Force is preparing to launch a “military space plane” this month. It will be the first flight for the Air Force’s X-37B Orbital Test Vehicle (OTV), and it has been a long time coming. The OTV began as a NASA program in the late 1990s, was transferred to DARPA in 2004, and then finally found its home at the Air Force in 2006. What makes something a “space plane” isn’t entirely clear: concepts have included the ability to transport humans, to launch quickly and on-demand, to be reused, and to make extensive on-orbit maneuvers.

Ideas also abound for what a space plane might be used for, though sometimes these ideas are limited only by imagination and aren’t physically realistic. Missions often discussed are ferrying troops to a military hot spot, rapidly replenishing a satellite constellation, dropping off hypersonic vehicles containing weapons targeted at the ground, performing quick-response unpredictable reconnaissance, and zooming around space to inspect or interfere with satellites.

The two “space plane” attributes of the soon-to-be-launched OTV are maneuverability in space, and the ability to return safely to the ground and to be reused.

Return is crucial if the goal is to transport people or things over long distances. A case for reuse can sometimes be made, but it may come at the cost of additional launch weight. Whether it is sensible depends on the particular use of the plane and economic and logistical details.

While the Air Force is not saying much about the goals of this flight, a likely central purpose is to test the autonomous re-entry technology: the OTV is designed to land autonomously back on Earth.

While “space plane” may call up the idea of the Shuttle, the X-37B is not like the Space Shuttle. Besides being 5 tons and therefore only about 1/20th the mass of the Shuttle, the OTV carries no people, and is launched as a payload on an expendable rocket, rather than with its own engine and reusable boosters as is the Shuttle.

Maneuverability is key, and crucial to many of the imagined uses for a space plane. It is also, however, severely limited because maneuvering in space can require very large amounts of fuel. All maneuvering fuel has to be launched into orbit along with the OTV, and the mass of fuel increases exponentially with the desired amount of maneuvering (see Section 6 and Section 7 of The Physics of Space Security).

The OTV also isn’t the answer to quick, on-demand launch to replace a failed satellite, for example. The goal for on-demand “responsive” launch is to develop small space launchers that can be quickly readied for launch and that don’t require scheduling a date in a large launch facility. However, because the OTV does not have the capability to launch itself and is massive, it would not be an efficient way to put a small satellite in orbit. For example, using the OTV to place in orbit a 500 kg satellite—which by itself could be orbited on a small launcher—would require a large launch vehicle to put the total 5.5 tons (OTV + satellite) in space.

The OTV will also probably not be demonstrating a capability of providing rapid, unpredictable imaging of the earth on this launch. Launches from Cape Canaveral are restricted to orbits with relatively low inclinations, not the polar orbits reconnaissance satellites favor, and the OTV would not be able to move into a polar orbit once it’s launched—more about this below. (Satellites can only observe the Earth at latitudes up to the inclination angle of their orbits.) We will look at whether reconnaissance is a reasonable mission for a space plane in a future post.

This flight is also most certainly not one step closer to a reality of putting weapons, like “rods from God,” in space. Using space-based weapons to hit time-sensitive targets on the ground is many tens of times more costly (and probably less secure and reliable) than using ground-based options, because it requires getting the weapons up into orbit and then back down (see Section 9 of The Physics of Space Security). Moving ahead on space plane technology doesn’t help this case.

Two often-discussed missions for a space plane are 1) visiting satellites already on orbit to inspect, repair, or interfere with them, and 2) launching several small satellites into different orbits. The physics of these two missions are similar, and we’ll take a look at how suited the OTV might be for these (more analytic detail can be found in Section 9 of The Physics of Space Security).

Launching Multiple Satellites

To deploy multiple satellites into different orbits, a space plane would place itself in the first orbit, release the first satellite, maneuver into the next orbit, release the next satellite, and so on. This approach can reduce the number of space launches required to put satellites in orbit, and in fact launches routinely use a non-reusable maneuverable “bus” to do just this. Similarly, rendezvousing with different satellites to either inspect, repair, or interfere with them also requires moving the space plane into different orbits.

The amount of propellant a space plane carries places a fundamental limit to how much it can maneuver in space. While the OTV reportedly has a large engine for maneuvering, the details have not been released; earlier concepts for the X-37 called for a maneuvering capability with a total velocity change of around 3 km/s. (The ability to make orbital changes can be measured in units of velocity, ΔV.)

Some of that maneuvering capability must be reserved for de-orbiting the Space Plane, about ΔV = 0.5 km/s. The remaining fuel—2.5 km/s—would only allow the OTV to change its orbital plane less than 20 degrees at an orbital altitude of 500 km. This would not be enough to move it into a polar orbit from the maximum inclination orbit from Cape Canaveral (57 degrees).

While the OTV would have a very limited ability to put satellites into orbital planes different from the one it was originally launched into, or to rendezvous with satellites in other orbital planes, it could perform more significant maneuvers if it remained in its original orbital plane The OTV could deploy small satellites into different orbits within one plane; for example, ΔV = 0.3 km/s is sufficient for the OTV to move from a 400 km altitude to 1000 km altitude in the same orbital plane and deposit satellites in each of these orbits, and it requires ΔV of about half that to place a second satellite at the same altitude but halfway around the orbit from the first one.

Increasing the maneuverability of a space plane would increase the launch mass rapidly. For example, if the OTV was to deliver three 300 kg satellites to three different orbits, each that required an expenditure of ΔV = 2.5 km/s to get to from the previous one (for a total expenditure of 5 km/s), and assuming the OTV with its de-orbiting fuel together weigh 4 tons, the total launch mass would be 20 tons—the capacity of the Atlas V rocket.

At that point, it may be more useful to launch the small satellites on smaller ground or air-based rockets instead of sending them up together on a space plane. A space plane like the OTV will not be a game-changer in the rapid replenishment of satellite constellations.

For a more in depth analysis of the space plane, click here.