In our previous post, we discussed why a space plane is not well suited to missions in which there is no reason to bring something from orbit back to Earth.
In this post we ask: Why would you want to bring something in orbit back to Earth and land on a runway?
There are two parts to this question:
(1) Why would you want to bring something back to Earth?
(2) Why would you want to land on a runway, since there are other ways to return something to Earth in a recoverable way?
We look at this in more detail below, but the only thing we have found that looks like it might make a compelling case to return to Earth to be recovered is possibly a maneuvering bus with a robotic arm, assuming the robotic arm is sophisticated and therefore unique and expensive. And even in this case there is no good reason why would need it to land like an airplane, instead of landing on ground or water with parachutes.
As noted previously, the idea that landing with parachutes is seen as preferable to landing with wings is strongly indicated by the fact that the spacecraft that will replace the Space Shuttle—called Orion—is being designed to return the crew to Earth in a reusable capsule that uses parachutes, like all non-Shuttle space flights have done. Moreover, it is designed to land on the ground, as Russian and Chinese space capsules do.
Reasons that have been discussed for bringing objects back to Earth include these:
-Space-testing components and bringing them back for examination.
Doing so might be nice, all else being equal, but how useful is it really to bring things back and how much extra expense is that worth, particularly given the accumulated experience of many years of developing space-rated components and the ability to build monitoring systems into the components that can telemeter information back to the ground? Moreover, even if it were worth bringing some items back after space testing, they could be returned by parachute in a small return capsule.
-Recovering a satellite and bringing it back to repair it or see why it failed.
It makes little sense to send up a space plane to bring back a satellite from orbit. First, most of the high-value satellites—including communication satellites in GEO and GPS satellites in semi-synchronous orbit—are at altitudes too high to be recovered with a space plane. Second, the highest value objects in low Earth orbit—such as U.S. reconnaissance satellites—are too large to recover. U.S. Keyhole satellites, for example, are too large to fit in even the Space Shuttle bay with their solar panels deployed.
Third, it is unlikely to make economic sense to recover small satellites, which are the only ones that can be recovered. For example, a Globalstar satellite is too large to fit in the X-37B cargo bay even without its solar panels. Moreover, the cost of a Globalstar satellite is less than $20 million, which is much less than the cost of launching a space plane into orbit to recover the satellite, which is estimated in excess of $100 million. So it would make much more sense simply to launch a new satellite.
Some have suggested that the U.S. might want to capture an adversary’s satellite and bring it back to examine it or interfere with it. If the goal was to interfere with a satellite but not recover it, many simpler ways of doing so that don’t require a space plane, are available. Capturing a satellite for examination would be observable and would likely be perceived as an act of war. And, as noted above, this could only be done for small satellites in low Earth orbit. It seems unlikely that any advantage gained from examining an adversary’s satellite would be worth the provocation of taking it from orbit.
-On-orbit servicing
While on-orbit servicing of a satellite would not require bringing a satellite back to Earth, it might require a sophisticated robotic arm on the servicing spacecraft. If there were specialized uses for such an arm and the arm was sufficiently expensive, it might make sense to recover the arm and reuse it. But as noted above, it would not need to be returned by a space plane.
-Active space debris removal
One idea being explored for reducing space debris is actively removing large pieces of space debris, such as dead satellites or used rocket stages. It might be tempting to think about sending a space plane on such a clean-up mission, to rendezvous with large debris and pull them out of orbit. But the way to remove debris would be to maneuver them so they will de-orbit and re-enter the atmosphere where they will burn up, not to return them to Earth. And for maneuvering debris to de-orbit, you would want to maximize the ability of the spacecraft to maneuver, which we showed in our previous post argues against a space plane.
-Returning people to Earth.
While the Space Shuttle was used for this, we note above that the follow-on to the Shuttle will use parachutes rather than wings for this mission.
One idea that has been proposed, admittedly with little traction, for years is delivering small numbers (a dozen or so) of soldiers quickly around the world. While a sub-orbital ride would have a somewhat shorter transit time (two hours as compared to 12 hours on an airplane to get a quarter of the way around the world), to keep the total delivery time short, you would also have to ensure a very short time for preparation. This would be difficult when launching a vehicle intended to carry a considerably larger crew than the Space Shuttle along with its equipment and supplies. And unless you had a job for only this small number of troops, in which case you could presumably find an alternate and stealthy way to insert them, you would need tens of such missions, all launched at the same time if the mission was time sensitive enough to make such delivery attractive. If you had more time, either for preparing multiple heavy launchers or for launching a series of missions, you would presumably have time to transport people and supplies by somewhat slower but more traditional means, like military aircraft.
Aside from its speed, this method is sometimes promoted as a way of avoiding the need to get permission to fly through the airspace of countries neighboring a hot spot. But since a country’s airspace extends to an altitude of 80 km, it’s not clear how you could land without violating surrounding airspace, even if you managed to overfly countries in the middle of your trajectory. And the last part of the journey would necessarily be slow enough to permit a runway landing, and would therefore be comparable to an aircraft. So it is unlikely you could use the space plane to avoid the risk of anti-aircraft fire.
Conclusion
As we show in these two posts, we are unable to find a compelling mission for a space plane, and are therefore skeptical about the value of developing one. We don’t disagree that there are things a space plane might be able to do, but using it for these missions appears to be much more expensive and operationally more difficult than alternatives. We would like to see the discussion of a space plane get beyond general visions of roles and into a more detailed discussions that compare alternatives.
We hope these posts help move such a discussion forward.