The Space-X Falcon 9 Revolution

So, last Friday, Space-X managed, after technical and weather delays, to send their Dragon cargo capsule up to the International Space Station. That’s great, but that’s not the story worth telling.

What is worth talking about is what happened to the first stage of the Falcon 9 booster rocket.

We’ve all seen film of the various stages of rockets separating and falling back to earth. And with Friday’s launch of Falcon 9, that’s just what happened. But for the first time, rather than just falling back to earth, the first stage booster executed a controlled descent to a controlled landing in the sea.

Spaceflight is hideously expensive, roughly $10,000 per pound to Low Earth Orbit.  And a large part of that is because the rockets that boost payloads into orbit are expended. Every rocket motor is an incredibly precise, extremely complicated engineering marvel. And yet, they’ve traditionally been used once, and thrown away.

Probably something like 90% of the fuel and thrust of a rocket sending a payload to orbit is spent sending the fuel and rocket up, not the payload itself. As any airline pilot can tell you, it takes fuel to haul fuel. That’s why most rockets are multistage. After burning the first stage, it’s just dead weight, and no sense hauling it any further. A lighter second stage with a smaller motor can take over.

But it is those very same first stage engines that are most expensive.

So Space-X looked at ways to recover those very expensive engines. And decided the best way to save them was to have the first stage make a controlled descent to the earth, eventually with the rocket landing on deployed landing legs.  If that seems pretty incredible to you, well, you’re not alone. When I first heard of the plan, I was skeptical. It’s a difficult flight to control, and the extra weight of fuel needed imposes its own penalty.

But then, unlike my co-author Roamy, I’m not a rocket scientist.

Space-X first decided to see if they could actually control a rocket in low altitude and have it successfully land on its own feet, as it were. To do so, they built a low altitude rocket resembling the Falcon 9 first stage and called it the Grasshopper.

 

Pretty nifty.

As for controlling the first stage after an actual launch, I forgot they would be letting the atmosphere do a lot of the work.  When a rocket first takes off, it’s at its greatest weight, and in the thickest air, and so has the least acceleration. As the weight of fuel burns off, and the air resistance diminishes at altitude, and yet the thrust generated remains the same, the acceleration increases, reaching its maximum at burnout, or “staging” if you will.

So now our first stage, at something like 50 miles altitude, effectively the edge of space, is flying separate from the second stage and the payload. Rather than just tumbling down, it can use only one of its 9 motors to begin a controlled deceleration. And as it encounters ever thickening atmosphere, its speed will decrease at an ever increasing rate. The rocket itself is pretty light. A vast percentage of its takeoff weight is its fuel (and oxidizer, of course). With most of it gone, it takes less thrust (and consequently, even less fuel) to decelerate.

Friday’s launch was a test primarily of the ability of Falcon 9 to handle the high altitude part of a reusable booster, that is, the part from Mach 10 down to low airspeeds. And it was intended to drop the booster in the ocean. If things had gone wrong, slamming a rocket into the roof of granny’s house would be bad. Because it landed in a salt water environment, rebuilding the liquid fuel motors would be quite difficult. In the future Space-X hopes to land first stages on dry land. If they can successfully do so (and it is really starting to look like they can) and they can quickly refurbish the booster for a second flight, they may cut costs of launch, in terms of pounds to LEO in half. That would the most significant decrease in launch costs in the history of spaceflight.

And that’s why I call this the Falcon revolution.

Bulletproofing a space station

Roamy here.  So I’ve talked about bulletproof vests and space debris, it’s finally time to talk about shielding.  A common way of testing different shielding designs is a two-stage light gas gun.

Your gun might be bigger, but mine shoots faster.

A high-speed rifle can shoot around 1.5 km/sec; a light gas gun can shoot up to 8 km/sec.  The “light gas” part comes from using either hydrogen or helium in the first stage.  A really good explanation of how this works can be found here. Basically you blow up some gunpowder, which moves a piston, which compresses the light gas behind an aluminum disk machined to burst at a specific pressure. Disk bursts, moving the projectile down the barrel to the target, past X-rays and/or cameras so you can figure out the speed, then impact.

Shielding for space debris is based on the Whipple shield, where you have a sacrificial plate to break up the debris into smaller particles.  (Dr. Fred Whipple, not the “don’t squeeze the Charmin” guy)  For the Space Station, you also have a thermal insulation blanket made up of thin layers of metallized plastic film and netting spacers.  A lot of thought goes into how thick the sacrificial plate should be, the spacing between all the components, and how to replace it on orbit when it’s done its job.

If you have just a sacrificial plate of aluminum and a thermal blanket, you can still get this kind of damage.

Sorry for the poor quality, but it was the best I could find that was cleared.
Add in a blanket like a bulletproof vest – Kevlar and Nextel or other aramids and ceramic cloth, and you get some dimples and craters instead.
L to R, sacrificial plate, protective blanket, simulated pressure wall

That little marble in front of the protective blanket in the picture above?  One of those fired at 6.7 km/sec is what caused that damage.  Remember from your physics class, kinetic energy equals one-half mass times velocity squared.  Only one gram or so, but that velocity squared is a bitch.

Update by XBradTC: Basically, NASA engineers are facing the same problem that every tank, bullet-proof vest, and warship designer faces- the balance between weight, protection and likely risk.

It would be the simplest thing imaginable to protect an infantryman from head to toe against small arms fire. The problem is, the weight needed to protect him would leave him immobile. An infantryman who can’t move isn’t an infantryman anymore. So risk becomes a factor. For a soldier that will spend most of his time in a Humvee, it’s pretty easy to justify adding a groin protector and deltoid armor. But a troop who has to ascend and descend steep mountains all day in Afghanistan can’t really hump that extra armor. The risk of sustaining a non-lethal wound to extremities becomes acceptable in that case.

Same thing with tanks. Tanks are more heavily armored on the front slope than elsewhere, because that is where they are most likely to be hit by the most dangerous projectiles.  You can’t armor something so heavily that it is invulnerable without sacrificing the mobility of the tank to a degree that renders it useless.

In NASA’s case, it costs a lot of money for every pound lifted into orbit. Without any protection, you face an unacceptable risk from small impacts. But with too much mass devoted to protection, you sacrifice lifting up the resources that are the whole point of the project.