Stealth Spaceships

When I started my original series of posts on space battles, I speculated about what a combat-spacecraft designer might want to do in order to make a vehicle that could avoid enemy detection:

It would make sense to build their outer hulls in a faceted manner, to reduce their radar cross-section. Basically, picture a bigger, armored version of the lunar module.

Almost immediately, I got some feedback pointing me to the “Project Rho” website, which declares quite bluntly that “there ain’t no stealth in space.” The argument goes basically like this: any device you put on a spacecraft has to obey the second law of thermodynamics, which means that it generates waste heat. This heat will raise the temperature of your spacecraft well above the background temperature of ambient space (about 2.7 Kelvin). Therefore, the spacecraft will radiate and will be visible to infrared sensors, no matter what. Therefore, stealth combat spacecraft are impossible.

This argument is fundamentally sound. The principles are correct: you can build a detector that could locate any spacecraft. What I don’t like about this argument is its implied definition of the word “stealth” as “total invisibility.” Yes, it is possible that the detector you build will locate a stealthy space-fighter eventually. That clock is always ticking. But your adversary’s stealthiness can still pay off – if they get to launch their missiles before you spot them!

Later on, when I revisited space-battle physics, I went into a little more detail about possible stealthing technologies for spacecraft. In another post, I thought about some of the thermal concerns our hypothetical space-fighter designer would run into in trying to make the fighter hard to detect.

But the proof, as they say, is in the pudding.

There’s a military aphorism (Wikipedia tells me that Helmuth von Moltke is responsible) that battle plans never survive contact with the enemy. I suspect that, for all anyone’s speculations about what can, cannot, will, will not, or might happen in space combat, if we ever did find ourselves in a space war we would very quickly learn an entirely different set of guiding principles. Whether or not stealth spacecraft are possible will be apparent then, after the fact, no matter what arguments we make today.

However, we can get some insight by asking the question: do any stealth spacecraft exist today?

The answer, as it turns out, is “yes.”

Weather permitting, we are coming up on the launch of a Delta IV Heavy – a gargantuan behemoth leviathan giant of a rocket – carrying a National Reconnaissance Office “spy” satellite with the cryptic designation of NROL-15. Quoting a civilian military space analyst, AmericaSpace reports that  the vehicle

is likely the No. 3 Misty stealth version of the Advanced KH-11 digital imaging reconnaissance satellite. It is designed to operate totally undetected in about a 435 mi. high orbit.

The article includes some description (or speculation?) about the physical appearance of the stealth spacecraft, too:

Looking somewhat like a stubby Hubble space telescope stuffed in an giant F-117 stealth fighter with diverse angles to reflect radar signals in directions other than back to receivers on the ground,  Misty 3 is also  covered in deep black materials designed to absorb so much light that it can not be tracked optically from the ground.

These design aspects are a huge challenge for a satellite that must also deploy solar arrays to generate electrical power and have reflective surfaces to reject heat.  … The satellite may actually change shape to reflect heat when not over hostile countries trying to break its cover.

Apparently, there may also be some tricky maneuvering by the launch vehicle – to disguise the final orbit trajectory of the satellite. There is some speculation at the end of the article about the various options the vehicle might take to pull off that feat of obfuscation.

The bottom line for science fiction: cloaking devices are probably not going to work. But are stealth spacecraft possible or not? Well…we’re already doing it.

People’s Reactions to the MSL Landing System Bother Me

On 5 August, the Mars Science Laboratory Curiosity will attempt its landing on the Red Planet.

MSL is an exciting mission, the biggest rover we’ve ever sent to Mars, packed full of science experiments and capabilities, and it’s going to start things off with a daring landing detailed in this NASA PR video:

I highly recommend fullscreen...

For more information about MSL, I strongly suggest these blogs.

Something that bugs me about MSL, though, is how every time the Internet hears about it, there’s a slew of commentary about how terrible an idea the landing system is. (For a good example, look at the comments on Gizmodo’s blurb about the above video.) People wonder why the system has to be so complex, sometimes asking what happened to the “KISS” (“Keep It Simple, Stupid!”) philosophy of engineering. Others lament how risky the landing system seems. Still more wonder why Curiosity can’t bounce down like the Sojourner or MER rovers did. I’ve even heard some of the mission scientists express reservations about the “skycrane” part of the landing process.

This thing is, each stage of this landing system was driven by engineering requirements. The guys at JPL didn’t just think one day, “hey, you know what would be cool? Landing by rappelling from a jetpack!” This is, in fact, the best solution that the engineers came up with for landing something as massive as the Curiosity rover on Mars.

Let’s look for a moment each successive step in the process:

  1. The heat shield. A lander screams in towards Mars at several kilometers per second – more than orbital velocity. Then we want to get it through an atmosphere, and, really, there’s no choice in the matter: as soon as we hit the atmosphere, we get friction with air molecules. A lot of friction. Friction that superheats our spacecraft. So, we’d better put a heat shield on our vehicle!
  2. The parachute. The heat shield gets our spacecraft down to about Mach 2, but if we were to rely on it the whole time we wouldn’t slow down enough before smacking into the Martian surface. We’ve got to get the speed of our vehicle down, and one of the obvious (and lightweight!) ways to do this is by deploying a parachute. (This is actually the part of the process that boggles my mind the most. Deploying a parachute at Mach 2! Yikes! Yet this is what our last three Martian rovers have all done, successfully.)
  3. Jettisoning things. After we deploy the parachute, the heat shield is just dead weight pulling us down. We want to get the most out of our parachute that we can, so we drop the heat shield away with some pyrotechnic charges. When we don’t need the parachute any more, we’ll similarly cut it loose.
  4. Retro-rockets. Mars’ atmosphere is so thin that even the combination of a capsule heat shield and a parachute doesn’t slow the probe down enough to land safely! Earth’s atmosphere – about a hundred times thicker than Mars’ – is fine for this. We can stuff astronauts in a capsule that rides the parachute all the way down, and doesn’t even need to drop its heat shield. But on Mars, even after the parachute gets our falling vehicle to terminal velocity, we still need to do something to slow it down! So we fire some rockets downward, killing off the rest of our speed. And the rover hangs in midair, about twenty meters above the planet surface. Up until this point, the MSL and MER landing sequences are basically the same.
  5. Rappelling. Finally, we need a way to get down that last few meters to the surface. On the Pathfinder, Spirit, and Opportunity vehicles, we popped airbags out on all sides of the lander and just let them go, inspiring egg-drop competition participants everywhere. But Curiosity is simply too big for this to work: it would be like taking our egg drop and substituting a paperweight for the egg. The rover would squish the balloons, still smashing itself against the hard ground. Another option might have been to have MSL sitting on a platform which descends on rockets all the way to the surface, like Phoenix or the Viking landers did. But the platform you would need to do that properly would end up being big enough that you’d have to go tell the JPL robot-builders to make a smaller rover. So instead, we just lower the rover down on a rope, and as soon as the rover registers touchdown, we fly the rocket platform away.

The controllers we will need to get the skycrane to work are really nothing to fear. They are not fundamentally different from the controllers that keep launch rockets pointing up when our probes leave Earth in the first place. But beyond the general terms, analogous robotic piloting happens all over on Earth – from military drones to quadrotors in research labs. As a dynamics and control engineer, I think this design would have been a challenge – but easily within our capabilities. And in terms of overall complexity, this isn’t any worse than, say, a Space Shuttle launch, or the entirely robotic X37-B.

More fundamentally, though, what bothers me about all the criticism and concern about the MSL landing system is one of philosophy. We should be giving wild ideas a shot – experimental technologies, unconventional science experiments, risky missions. That is how we advance the state of the art: by pushing the envelope. If that means that once in a while our rockets explodes or our space probe smashes into a planet, then so be it. I have no problem with seeing NASA try something innovative a fail once in a while!

You see, we didn’t ever start with the Right Stuff. We learn the Right Stuff. And this is how we learn. We simply need to be willing to accept that fact if we want to go forwards.

Why to be Skeptical of Mars One

Dutch company Mars One offers a plan to start colonizing the Red Planet by, ostensibly, 2023 – starting with a “colony” of four and growing the base every year.

Stephen Colbert's take on Mars One

There are a lot of reasons to be skeptical of this plan. Don’t get me wrong: I would love for these guys to succeed, and I think that – with concerted effort – their timeline is achievable. But there are a few technological red flags. Going from what I see as least to most severe:

  1. Mars One gives a rover top billing in their plan, saying that the rover will scout out the best location for the planetary base. The concept of having a robot autonomously assemble a base before humans ever arrive has a great deal of merit; however, a rover is not going to scout out the prime real estate on Mars. I once asked this guy if, since the MSL Curiosity has a much higher power budget than the MER Spirit or Opportunity, it would be able to drive at a higher speed and really cover Martian distance, to get to different science targets. It turns out that, even with more power at its disposal, there are thermal constraints on how fast motors can drive the rover’s wheels. If Mars One sends a rover, it’s not going to be scouting colony locations. It will be going to the colony’s location.
  2. Mars One wants to use the SpaceX Dragon capsule as a Mars lander. I’m a big fan of SpaceX, and I’m sure that they are thrilled that somebody is looking at Dragons as a Mars vehicle. However, one of the things I learned during my time at NASA is that the MSL is about at the upper size limit for things we can land on Mars using current techniques (aerobraking, parachutes, airbags, etc). Dragon is going to take a lot of development to land on the Martian surface. And it’s going to need a lot of fuel to do so.
  3. I’m not sure there’s enough room in their proposed colony for four people plus the equipment necessary to provide food for those four people. I think they need more inflatable greenhouses, at the least. But this is an point about which I’m not the expert.
  4. Mars One claims that no new technology is necessary to achieve their goals. This statement, I have to say, is bogus. They rightly identify in-situ resource utilization as the best way to provide air, water, and food for their colonists. We need to develop the technology to do that. The colonists need to be shielded from radiation while in transit. We know solutions that might work, but we need to develop and implement the technology. Furthermore, the colonists are going to need products that go beyond the most basic: How will they produce any medicines they require? How will they conduct surgeries with such a small staff? How will they maintain their colony? This project will need a very high level of automation and/or telepresence support from Earth – involving technologies that exist only theoretically today.

Tour de Cure

Hello, everyone!

I’m back from my vacation, and my next project is to spend the next couple weeks getting myself in biking shape for the 2012 Tour de Cure, a fundraising event to support the American Diabetes Association. I am personally pretty #$*@ sick of diabetes – If you would like to help support me, please visit my fundraising page!