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Germany To Test Actively-Cooled Spacecraft
FleaPlus writes "The German Aerospace Center is planning to launch a novel reusable spacecraft in 2011, incorporating flat, damage-resistant tiles. Nitrogen will be pumped through the porous tiles, creating a protective gas layer that actively cools and shields the hottest parts of the spacecraft from the searing heat of reentry. The €12.5M unmanned 'SHEFEX II' project is a major technological step toward the team's eventual goal of a reusable space glider, which will be cheaper and easier to build than NASA's space shuttle."
"...will be cheaper and easier-to-build than NASA's space shuttle." I would hope they could build something cheaper and easier than the 30-plus-year-old shuttle.
Remember, German technology put the first man on the moon.
There is no need for glider-based spacecraft. Wings are useless in space. "man-rated" launch vehicles cost something like $10k per pound to go to orbit. The extra pounds for wings are a massive waste of money and resources.
The original design--The Capsule--was the right idea! Why not build a re-usable capsule?
I saw Gary Hudson present a similar proposal at a members-only conference some years before Rotary Rocket.
Bruce Perens.
Yeah, I'm sure that is one of the reasons people are so concerned about global warming.
Yeah, and if there's even a slight problem with the coolant system -- the liquid turns to gas, expands 1,500x its original size... and is surrounded by ceramic, metal, plasma, and several thousand degree temperatures at a critical point on the airframe.
What could possibly go wrong?
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In short, no.
Nitroglycerin is formed by mixing nitric acid and sulfuric acid (both highly concentrated, purified forms).
Atmospheric nitrogen, on the other hand is remarkably stable. At very high temperatures, (such as you might find at the leading edges of a reentry vehicle) nitrogen can be oxidized to to various forms of NOx. These can form acids in solution, but not in concentrations high enough to worry about.
And when you consider that there is plenty of naturally-available nitrogen in the atmosphere, this small addition probably isn't enough to worry about.
I've thought of active cooling myself. I always wondered, if you used an active cooling system, where would you radiate the heat? In other words, you can carry heat away from the underside of the ship by pumping a fluid through the tiles or whatever, but then you still have to re-radiate that heat someplace. OK, you might be able to transform some of the heat into useful work too; but we're talking about a lot of heat, and even if you got right to the Carnot efficiency the waste still has to go someplace.
I never got as far as doing the "back of the envelope" calculations on some substance with a heat capacity to absorb re-entry heat (and light enough to carry onboard) or the more tricky calculation of how you would conduct the heat from the underside and radiate it topside. I kind of assumed that actual aerospace engineers had done the calcs, and decided it just wouldn't work.
Weight kills in space, so I'd be curious to know how much the system weighs vs tiles or Russian-style ablative coatings. I'm assuming the Russians still use ablatives. I'm sure somebody will correct me if I'm wrong.
For all intensive purposes, "whom" is no longer a word. That begs the question, "who cares"?
I've just looked up the latent heat of vaporization of nitrogen and it's 200 kJ/kg [wikipedia]; its specific heat as a gas is around 1.1 kJ/kg/K, so to boil it and heat it to 1000K takes roughly 1.2 MJ/kg. The kinetic energy of an orbiting spacecraft is roughly 30 Mj/kg and even a spacecraft in a vertical trajectory that reaches 200 km has an energy of roughly 2 MJ/kg. So unless the spacecraft consists almost entirely of nitrogen tank, most of the heat of re-entry will have to go elsewhere. I propose that a better way to think about this cooling scheme is that the nitrogen is being ablated as a way to protect the ceramic tiles.
Does this mean it's a bad idea? Noooo! Replacing the ablated nitrogen is as simply as putting a hose in the tank after the craft lands, while inspecting and replacing ablated ceramic is one of the reasons why the Shuttle takes months to turn around (true fact: the most Shuttle missions NASA ever flew in one year was 10, in a year when they had four birds to fly, i.e. 48 bird-months, or 4.8 months per flight). Also, it seems likely that you can adjust the flow of nitrogen to get the temperature you want (within limits) instead of having to design tiles that can take whatever temperature Nature hands you. I wish these guys the best of German luck.
The problem with the shuttle program was really all the things that had to be done between flights. It was originally supposed to have a two week turnaround, something that turned out to be a pipe dream because of all the things that needed to be inspected and refurbished. If the Germans can make a ship that needs less inspection and maintenance, they can fly it more often. That will bring down the $/kg-to-orbit cost, which I think ought to be the goal of any serious space program at this point.
This technique doesn't cool anything, it prevents the tile from ever heating up in the first place. It has been in use for decades in gas turbines and rocket nozzles. http://en.wikipedia.org/wiki/Turbine_blade#Cooling
Uh, perhaps they'll start before the tiles get that hot? i.e. before it even starts to heat up, so that it never gets that hot?
They sentenced me to twenty years of boredom
I am glad we have you to make these observations, I am sure the scientist and engineers working on this project have not thought about such issues. I urge you to email them right away with your insight into their project.
Isn't the hot air around the returning vehicle a plasma? If it is, can you repel it with proper use of electromagnetism?
Jon Goff, an aerospace engineer whose blog you should probably read in general because it's awesome and chock full of great aerospace analysis/ideas, had a rather intriguing discussion a few months ago about doing pretty much what you describe, applying magnetohydrodynamics to the problem of thermal protection during atmospheric reentry:
http://selenianboondocks.com/category/mhd-aerobraking-and-tps/
(Jon Goff's an engineer with Masten Space Systems, the company that won the most recent Lunar Lander Challenge)
Not that this isn't a great tech demonstrator but why build a capsule that has a reusable heat shield? So you go through all of this trouble to build a beautiful reusable heat shield than slam it into an ocean or desert? Seems like you will be picking salt and sand out of it for a long time. I've seen many Space Shuttle Landing in person and we are going to miss the ability to land a couple of miles away from the hangar.
I love Jesus, except for his foreign policy.