De-Rocket Science

Saturday, February 23, 2008

 

In my Rocket Science article I discussed what it takes to put something into orbit. But what about coming back down to Earth? That is, how do you get out of orbit? To get into orbit the key ingredient is lots of speed horizontal to the Earth. It follows then that to get out of orbit we need to lose that speed. So to lose our speed we just press on the brake petal, right?

Uh-uh. Down here on good old stop-friendly Earth when you press on your car's brake petal you use the friction between your brake pads and the wheel, and between the wheel and the surface of the road, to bleed off your speed. In orbit there's no road. In fact there's nothing at all to create friction with.

So how do we stop in space? Now that shuttle traffic is common (and post-Columbia) most people have some understanding that we use the Earth's atmosphere as a brake. But why do we do this? Why bother with heat shields? Why don't we just use the same rockets that gave us our speed in the first place to slow us down by firing them in reverse?

That in fact can work, but in order to shed 17,000mph of speed you're going to need a lot of rocket fuel. And, in fact, you're going to need more fuel than just what it takes to stop since you have to spend fuel to get the braking fuel into orbit!

If you think about the size of the cargo and crew areas of the shuttle and Apollo rockets compared to the rest of the rocket (on the order of 20 to 1 by weight of fuel to cargo for the Shuttle) you realize that most of the rocket is fuel. If you used only rockets to stop your motion then you'd have to have a shuttle sized rocket up in orbit! That means to put that rocket into orbit you'd need a rocket on Earth on the order of twenty times larger than the shuttle is to begin with.

The trick then is to use a small rocket just to slow you down enough so that your path no longer reaches beyond the edge of the Earth. Orbitting means your path matches the curve of the Earth. If we slow down just a little bit (about 200mph for the Shuttle) then our path is just shy of the surface of the Earth. The change of path is enough to bring the shuttle into the atmosphere, and then we begin to be able to use friction with the atmosphere to further drain the speed. The slower the Shuttle (or any reentry vehicle) goes the deeper it sinks and the thicker the air gets, thus slowing the craft even more.

In the process of shedding 17,000mph of speed we build up a lot of heat. That's why we need heat shields to protect the craft. Re-entry craft are also brought in at a shallow angle to the atmosphere to make the re-entry slower.

A meteor burns up when it enters the Earth's atmosphere since it has so much speed (typically even more than just orbital speed), and because it usually comes into the atmosphere at a steep angle. If the Shuttle came in at such an angle even the heat shields wouldn't be enough to protect it. However, since most of the Shuttle's motion is parallel to the Earth's surface anyway it's easy to come in at a shallow angle. That means it takes longer to slow down, and that means that less heat is applied to the surface of the heat shield for a given amount of time.

In the early manned missions (Mercury, Gemini, and Apollo) we used ablative technology in the heat shields. We purposefully allowed the shields to be eaten away by the heat since we never intended to use them again. On the re-usable shuttle we invented materials which shed heat at an amazing rate.

Here's a picture of a piece of Shuttle tile material which has been heated white hot. It sheds heat so fast that the edges are already cool enough to hold even while the core is still white hot.

There are some counter-intuitive realities to orbital mechanics. First of all, for a given planet the orbital altitude is entirely dependent on speed. The lower you orbit the faster you have to go. Geosynchronous satellites orbit at a much slower speed than satellites in low Earth orbit (such as the International Space Station).

On our current mission to the planet Mercury, being executed by the spacecraft MESSENGER, the spacecraft has to slow down in order to speed up. That is, it has to slow down to allow itself to fall down the Sun's gravity well (about which Mercury orbits in a much tighter orbit than Earth). However, as it falls all that way in towards the Sun and Mercury it picks up so much speed from falling that it again has to shed speed. Since Mercury does not have an atmosphere to brake against NASA created a path of ever tightning circles around the Sun using the effects of the gravity wells of the Earth and Venus (and Mercury) in order to get MESSENGER into orbit around Mercury. Once orbiting Mercury, however, MESSENGER will be going much faster around the Sun than it was when it left the Earth.

Additional sources of information can be found on the Rocket Science page.

Also the following:

Monday, September 4, 2017:

SpaceX has, for a couple years now, been doing exactly what I implied was not feasible in this blog entry. There are some important differences between what I was talking about above, and what SpaceX does. However, I have to say, what SpaceX is doing is little short of miraculous. My discussion above is a back-of-the-napkin calculation. I don't rule out what SpaceX is doing, but at the time I wrote that I never expected that returning a rocket to Earth in 1950's Sci Fi movie style (landing upright again) would ever be done. It's clear that SpaceX took the napkin calculation and squeezed out everything except the most efficient elements. They do use the atmosphere a bit, they return only the main booster (so far), and the booster is dramatically lighter when they're returning as when they're launching. The result is the awesome spectacle of them landing their booster on its tail. Wow!