What’s going on with the X-37B?

I read today that the USAF has launched one again, on, I think, the 3rd flight.

What is unique about this vehicle compared to most other space systems is:

• It’s reusable and returns to Earth (lands on a runway, ala the Space Shuttle)
• It has a long on-orbit dwell time (270 days was supposedly the spec, with the second flight lasting 469 days)
• Some significant amount of on-orbit delta-v for orbital maneuvering
• Payload capacity “similar to a pick-up truck”

What is this thing for? The government has been strangely silent about its purpose, leading to a lot of random speculation, none of which makes any sense to me. The only thing they’ve said is:

“The X-37B is a risk reduction vehicle for space experimentation and to explore concepts of operation for a long duration, reusable space vehicle.”

I’m sure that’s true to some extent, but I’m sure that’s not all that is going on; otherwise why all the hush-hush?

Some of the theories I’ve seen claim that it’s:

1. For spying on the Chinese manned space program
2. For spying on random spacecraft in orbit
3. Some sort of on-orbit anti-satellite weapon
4. For repairing satellites in orbit
5. A ground-attack (or ICBM interception) weapon (rods-from-god or similar)
6. An orbital bomber
7. For stealing satellites, like in You Only Live Twice
8. Some professor at the Naval War College said “the Air Force has always wanted a crewed space plane, and this was the closest they could get”.

None of those make sense. One at a time:

1 and 2 – Spying on other spacecraft

The DoD is already pretty good at pointing ground telescopes at spacecraft in orbit. Remember they offered to check out Space Shuttle Columbia’s missing tiles back in 2003? Matching orbits and a close approach to an opponent’s spacecraft would be tremendously provocative (it’s not as if you can do it secretly), and how much more can you really learn by looking at a spacecraft up-close vs. from a distance anyway? Not enough to justify this doubtless hugely expensive program, I’m sure. If despite that, you really want to do this, you don’t need a vehicle that can land. As for spying on the Chinese manned space program, why would that be of any interest at all? (Besides, it’s been in the wrong orbit for that.)

So I don’t buy those explanations.

3 – It’s an ASAT weapon

So why does an ASAT weapon need to land? No matter how much the thing costs, it’s got to be cheaper to just build and launch a new one every so often than land it, refurbish it, and re-fly it.

4 – Repairing satellites in orbit

That’s crazy. Maybe if it were manned; if that’s what you want to do it’d be way cheaper to fly repair techs on a SpaceX Dragon. And what possible purpose would there be in having the repair-bot sit on orbit for a year, or have the capability to land?

5 – Rods from God

Again, no need for such a thing to land. If you really want to build that, sure build it, but why complicate things by having it land? Makes no sense.

6 – Orbital bomber

First, that would violate the Outer Space Treaty. I can see people wanting to get out of that treaty, but I think the US would do so explicitly rather than in this not-very-sneaky way. But, again, why does it need to land? Maybe you can make a case that the USAF doesn’t want to leave nukes on orbit forever – they want a way to get them back eventually. But there are many simpler ways to accomplish that – it can de-orbit the warhead with a parachute (ala Corona, as well as the whole Apollo program), they could plan on a future OTV or manned spacecraft to collect warheads in 15 years, etc. Plus, who needs another way to deliver nukes anyway? The Soviets are not coming back, and the Chinese only want to sell us stuff.

7 – Pac-Man (waka waka)

Snatching someone else’s satellite out of orbit would be an act of war. And difficult, because you don’t know where it’s CG is or how much damage it’ll take by being bounced around on re-entry. And why do you need your snatch machine to sit on orbit for a year? You can always just launch it when you need it.

8 – Manned spacecraft wannabe

Except they already built the full-size version (that flying pink elephant, the Space Shuttle), and this one doesn’t carry anybody. And a Dragon will be way cheaper, and NASA is already funding that. Not to mention Orion (oops… too late, I mentioned it. It’ll probably never fly anyway.) And, of course, they’ll have trouble getting even one brave soul to sit in a space the size of a pickup truck bed for a year (let alone finding room for oxygen, water, and food for that duration).

So what’s it for, then?

OK, here’s my theory. It’s a spy satellite full of disk drives.

The payload bay has some sensors, probably high-resolution cameras; maybe other things too. The rest of the payload is disk drives.

The X-37B collects data (pictures, maybe sigint, maybe other things), and stores it on the disk drives. Every so often it changes orbits in order to be able to look at some particular thing at particular times (or just to keep the other guy guessing). Once the disk drives are full (a year or so), or sooner if the data is needed on the ground sooner, it lands.

It lands so that the data on the disk drives can be read off.

Why not just radio the data down? Because there is way, way, too much of it.

Suppose the payload bay is 4 x 8 x 3 meters (roughly a full-size pickup truck bed). That’s 96 cubic meters (96 million cc). A 3.5″ hard drive is 101.6 x 25.4 x 146 millimeters, that’s 377 cc. So there’s enough room for about 250,000 drives. Figure a tenth of that after allowing room for the sensors, power supplies, and cooling (cooling is a big deal in space). 25,000 drives at 3 TBytes each is 73 petabytes. (BTW, that’s about $2.5M worth of disk drives; peanuts for the DoD.)

73 petabytes over a year is 25 gigabits/second. That’s 24×7, including when not over a convenient ground station.

How does that compare with the bandwidth available for a satellite downlink? I don’t know exactly, but the whole X-band is only 500 MHz, as is the Ku band. The Ka band is 2.5 GHz. That’s the whole band. You do the numbers.

[Edit, March 2013: There’s a mistake in the numbers above – a pickup bed is about 4 x 8 x 3 feet, not meters. But on further thought the X-37B is almost certainly using SSDs instead of rotating media, and the density of that is a lot higher. So I think the two mistakes roughly cancel out, without changing the conclusion.]

What can you do with that?

The Earth’s surface area is 510 million square kilometers (about a third of that is land). Let’s assume 24 bits/pixel (you can divide that up into bands as you like). Unless I made a mistake in the math, 73 petabytes is enough for pixels 40 centimeters on a side, of the whole planet.

Or, somewhat bigger pixels, but multiple copies in the disk drives, so hardware on-board the spacecraft can compare old images against new ones, and identify differences. You get the idea.

Makes Google Earth look kinda…lame.

[Disclaimer: This is just a guess; I have no inside info. Just imagination and a calculator.]