Why does the military prefer space planes such as the X37?

July 28, 2016 10:55 AM

I've been playing Kerbal Space Program and now I'm like a kid about space again. How come commercial ventures are opting for Mercury style capsules and the military is wanting a space plane? I don't understand the advantage, I assume you can drop a capsule anywhere with the same precision as a glider.
posted by geoff. to Science & Nature (14 answers total)

The military is using unmanned spacecraft, while commercial ventures are doing manned. That's the first big difference

The military is also learning from the problematic elements of the Space Shuttle. Because the Space Shuttle was expensive and had several built in faults. Much safer to do a capsule. Plus, in the case of Space X, the same model of capsule is more suited to be landing on Earth, the Moon and Mars. A glider/plane wouldn't be capable of doing that.

Keep in mind that the original idea of the Space Shuttle was different and wound up getting wittled down by politics and money.

Also, a military space plane can land at one number of military air bases, which is cheaper than sending ships out to get it from an ocean landing.
posted by Brandon Blatcher at 11:38 AM on July 28, 2016

Reusable payload bays = maximum versatility for transporting gear, and serving as a temporary lab/observation/recon platform.
posted by BrandonW at 11:38 AM on July 28, 2016

I don't think it's safe to say the "military prefers" winged vehicles because the USAF flies the X-37. They adopted the vehicle from NASA for reasons they're keeping to themselves.

You can't fly back from orbit. Before you reach the portion of the atmosphere where the air is dense enough to support flight, you have to first pass through the area that cannot support flight but is dense enough to generate shock waves in front of the descending vehicle. It's those shock waves, not friction, that produce the energy translated into heat.

The Shuttle's entire underside was covered in tiles, essentially thousands of small heat shields. It reentered at an angle, tiles down, until it had reached the altitude to support flight. At that point it flipped over and flew the last small distance to the landing facility.

The capsule design, with a large heat shield, turns out to be a reasonable approach to getting back from orbit. Arguably, the design does not lend itself to the fragility that the Shuttle's exposed. Capsules are also designed with a degree of aerodynamic capability -- they have some lift -- so do not follow a straight ballistic path back.

We depend on the interaction with the atmosphere to slow a returning vehicle. It's impractical to build a vehicle that could use onboard fuel to reduce its speed in a controlled manner. The volume of fuel needed would, practically, be as much as needed to get it into orbit in the first place.
posted by justcorbly at 11:40 AM on July 28, 2016

>It's those shock waves, not friction, that produce the energy translated into heat.

That's at odds with everything I've ever read about aerodynamic heating. Bow shock waves are generally claimed to be one of the tricks that minimizes re-entry heating by maximizing drag and allowing heat to dissipate in air, instead on of the vehicle surface.
posted by BrandonW at 11:44 AM on July 28, 2016

It's also worth noting that Sierra Nevada is working on the Dream Chaser, which is a winged spacecraft. Originally it was to be manned, but lost out to Space X and Boeing for a contract on sending astronauts to the ISS. It's currently being retooled to be an unmanned spacecraft doing supply runs to the ISS.
posted by Brandon Blatcher at 11:51 AM on July 28, 2016

I'm pretty sure the military wants to quickly fly halfway around the world, land and be able to takeoff again before pulling maintainence. Capsules look cheaper, simpler and more reliable for LEO and back.
posted by ridgerunner at 11:52 AM on July 28, 2016

Seconding ridgerunner's comment-- the Military does want things in orbit, such as spy and communications satellites, radar platforms, and odd things like the big needles it has in space that act as radar targets for ground-based radar. But they also want space as a high-speed shortcut to get to places on earth, either to bomb them, or insert soldiers and marines. Space is also immune to large classes of defense systems based on the ground, which is why the U-2 and the SR-71 were designed-- they got up high and could take photos with a quick turnaround. (The U-2 grew up in an era where the orbiting spy satellites had to send a film canister back to Earth, no shit.) All that those high-altitude planes do, a space-plane would do even faster.

It comes down to whether being in space (for some period of time) is the goal, or using space to get someplace on Earth. What you plan to do when you get there adds to things. Just keep in mind that a capsule can get you someplace, but if that place is unfriendly, you'll probably prefer to have a plane instead.

Welcome to KSP! You're in for some fun. May I recommend the subreddits /r/KerbalSpaceProgram and /r/KerbalAcademy for some interesting reading and great accomplishments in the game. It's also a terrific community that is kind of rare in games: they celebrate the accomplishments of newbies as much as the astonishing feats by the older hands.
posted by Sunburnt at 12:21 PM on July 28, 2016

I should add that the U-2 and SR-71 are basically defenseless-- their main defenses were altitude and speed. The U-2, we all know, was shot down from time to time, at least once to the particular humiliation of the US. The SR-71, on the other hand, is a very challenging thing to shoot at, and if you can't predict its course, it'll be gone before your missile, however fast it can fly, will get high enough to engage.

A spaceplane will be a super-delicate plane because of the hostility of the space environment and re-entry, at least in any current technological forecast, and so it'll really need to be able to exploit the speed/altitude advantage.

Finally, a military spacecraft that lands somewhere hostile or maybe just technologically unsophisticated will need to take off and return to home base, and a capsule can't do that without a spare rocket. One proposed role for military spaceplanes is to deliver a dozen marines within 4 hours, anywhere on earth, where they can pull out, for example, a US Ambassador and family from a threatened Embassy. The complicated part about having a friendly airfield that's capable of turning around a spaceplane a problem that also needs solving. It can't need a 3-mile runway (ot 7 miles, like at "Area 51"), and it needs to use normal fuels for some of the time, etc.
posted by Sunburnt at 12:48 PM on July 28, 2016

I'm pretty sure the military wants to quickly fly halfway around the world, land and be able to takeoff again before pulling maintainence.

Yeah, this goes back to the two historical strands of astronautical engineering: "air force, but faster" and "rockets, but with a payload other than explosives".
posted by holgate at 1:28 PM on July 28, 2016

Agree the military wants fast global access. But, the year+ X-37 missions, whatever they're doing, serve another purpose.
posted by justcorbly at 4:22 PM on July 28, 2016

Agree the military wants fast global access. But, the year+ X-37 missions, whatever they're doing, serve another purpose.

But I guess my point is I don't see anything that this would do that a traditional sat could do cheaper. They appear to be using it as a short-term sat solution based on the length of the missions. So I can't imagine anything collected is actually on-board unless they wanted year old data.

Reusable payload bays = maximum versatility for transporting gear, and serving as a temporary lab/observation/recon platform.

Right but I doubt they turn this around fast enough for reusability to matter. I'm assuming that it sees a major retrofit everytime it comes down. Might as well develop a Soyuz type craft that you can keep reusing over again and keep a couple in a hanger, same size and dimensions.

Sorry for asking a question that was not answerable I thought there might have been some physics based answer about changing orbits or something that made more sense to me.

But after thinking about it, the only thing that makes remote sense to me is that they would want to switchout the payload, but didn't want to have an entirely new satellite in orbit. Plus they had experience with doing that in a shuttle type environment, so why not just keep doing that. Swap out the payload twice in a mission life span, have it change orbit or whatever, then added benefit of being able to land without building out a new spaceport in case they wanted it to run along a really weird orbit.

Who knows, but now that I think about it more, more I can see there being use cases I would never dream of that makes sense to large government agencies.
posted by geoff. at 5:10 PM on July 28, 2016

But I guess my point is I don't see anything that this would do that a traditional sat could do cheaper.

It's a lot cheaper to bring the X37 back to the base (all three have landed at Vadenburg base in California), rather than dropping it in the ocean and working with the Navy (the X-37 is an Air Force project) to fish it out and ship it back to base.

Might as well develop a Soyuz type craft that you can keep reusing over again and keep a couple in a hanger, same size and dimensions.

1. There are two X37B spacecraft and there have been four missions. The first and third used the one, the second and fourth used the other. The fourth mission is currently in progress.

2. Soyuz spacecraft are not reused.
posted by Brandon Blatcher at 5:37 PM on July 28, 2016

There's a lot to be said for reusability of the lower stages; that's why SpaceX's self-landing first-stage ship is so exciting-- by not throwing away engines every launch, they have enormous cost savings. A quick google indicates that SpaceX has an estimated per-launch cost of under $37 million (est. cost to SpaceX, not cost to the customer, who pays 40% more). Probably a good bit more for a manned flight. By comparison, the United Launch Alliance's launches currently average about $225M per launch, with Atlas V costing $164M at one extreme, and Delta IV Heavy costing $350M at the other.

NASA pays Russia $70 million for a Soyuz seat (which launches 3 people to ISS or other orbit), which is actually pretty comparable to SpaceX, but certainly the US military has no interest in being dependent on a Russia space-launch system. The X-37B cost is classified.

A spaceplane is essentially 100% reusable; you don't count fuel and other life-support expendables in that equation. There probably are some parts that wear out enough after each flight that service or replacement is required, but even something that's above 90% is still a huge cost savings. Also, military can afford to fly missions that're a little riskier to personnel and spacecraft, because they don't need good PR to keep their budget up as much as mission success-- more avoiding bad PR, like a catastrophic aircraft crash into a crowd on the ground and whatnot. (Not that astronaut-pilots are super-cheap to replace-- they're still high-tier personnel, but as spacecraft parts go, pilots are easy for the Air Force to source.)
posted by Sunburnt at 7:02 PM on July 28, 2016

Worth noting that the vast majority of military space launches have not been space planes.
posted by Brandon Blatcher at 6:07 AM on July 29, 2016