Comic Discussion > QUESTIONABLE CONTENT

WCDT December 1-5, 2014 (2845-2849)

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Kugai:
They had something like that planned as an escape vehicle from the ISS at one stage I think, but budget cuts killed it and they went with a Soyuz Escape Craft that's docked to the station.

Half Empty Coffee Cup:
Also, that method of reentry (skimming the atmosphere) is called aerobreaking and has been used when sending landers to Mars.

ReindeerFlotilla:
Delta v.

Friction doesn't really work at 5 miles per second. At near terminal velocity (which is comparatively slow) hitting water can be no less dangerous than hitting concrete. The water can't get out of the way fast enough. The effect is similar with air, at high speeds. That's why meteors explode. The stress of slamming into the atmosphere. This is why the eat of reentry is compressive, not friction. Compression makes things hot. Compression while moving at 5 miles per second turns air into plasma.

If you try a shallow decent into the atmosphere, odds are you'll bounce off. Catastrophically. Assuming you survive your close encounter with the atmosphere, you'll be on parabolic trajectory, going back into space, until you run out of energy and start falling again. Speeding up. You'll be going slower, but not that much slower.

Rinse and repeat a couple of dozen times and try to do the math that will tell you what part of the planet you'll be over when you finally come in slowly enough not to bounce.

The combined stress on the space craft may exceed the stresses of an simple direct entry. The pucker factor has to.

Any plan that avoids this is going to need reaction mass. If you had a thruster that could slow you down enough to glide in, you might as well slow all the way down and come in at nice few hundred miles an hour. Sure, the trip down would be longer, but no compression to worry about.

That leaves the question of where you're going to get the reaction mass to pull this stunt off.

Say you leave Earth for Mars. What you do is rob the Earth-Moon system of a bit of momentum--we're going 1112 mile per minute. we can spare it. You give a little of that momentum to Mars. Then you take it back to make the trip home. when you get back, you give some of that Momentum to Earth, slowing down again.

The process of getting to orbit is much the same. The problem is, you can't effectivelly Rob Earth to give you a boost. You do steal some momentum, but you work really hard to do it.

Getting to space isn't really that hard. SpaceShip One/Two does it without going much faster than sound. To quote Randall:

--- Quote ---The reason it's hard to get to orbit isn't that space is high up.

It's hard to get to orbit because you have to go so fast.

--- End quote ---

The Rutan craft basically pull the same stunt the X-15 did. The fancy feathering maneuver at the end allows them to bleed more speed with less "OH BEARD PLEASE HELP US WE'RE GOING TO DIE." That said, they aren't going fast enough to stay in space. Feathering or not, the SpaceShip series is coming back down, and it'll do it shortly after it gets up there.

Rockets use insane amounts of fuel to go insanely fast just to get to LEO. 18,000 miles per hour.

In order to descend like a glider, you first have to slow down from 18000 to a more sedate 300 or so.

And that's what the space shuttle was designed to do. the most efficient way to bleed of that speed was and compressive heat.

The next option was fuel, but that runs into the tyranny of the rocket equation. The fule needed to lift the Shuttle and hurl it at 18000 miles per hour, so that it achieves Arthur Dent's most astounding trick (falling to Earth and missing), isn't really that much. just a fraction of the fuel used. The vast majority of the fule is devoted to lifting the fuel. Lifting enough fuel to slow back down means another, equal, vast majority dedicated to lifting that fuel. (The rocket equation. an easy way to figure out how much fuel you need to lift the fuel you need, to lift the fuel you need, to lift the fuel you need to lift the fuel you need... Er. put it this way. Say you need 1 unit of fuel mass to lift one unit of payload mass.The problem is, the mass of your fully fueled miss is now two units. Which means you need 2 more units of fuel. But then your mass is 4 units. so you need 4 more... You're saved by the fact that your fuel is a propellant. You get lift by throwing it the other way, really hard. So seconds after you fire the engines, you've thrown away a lot of mass. You get less massive as you go up.)

Returning to our 1 unit for 1 unit, assuming an exhaust velocity of six kilometers per second, you need almost 6 units of fuel to lift your payload and fuel to lift that payload. If you add enough fuel to slow back down, your fuel cost jumps to +11 units. Your vehicle mass just doubled. That's off the cuff, but considering the cost to orbit, doubling the payload you're putting up there, doubles your expenditure. Space is expensive.

DSL:
Sure is. We're talking about two sides of the bootstrap situation, though. You the initial side, where the human space program is now and we have to pack.everything we need at the beginning, and I'm thinking ahead to that place I hope we get, when there's enough of a.support infrastructure in place, in orbit or even at destination, that future flights will be able to lean on it and not have to carry everything. It'll.tale a.long time to get to that point but once it's reached, the growth will (be able to be) exponential.
Meanwhile, I'm kicking myself for.not paying more attention to the Dawn mission to Vesta and, soon enough, Ceres. And Pluto Express not long after that.

ReindeerFlotilla:
I'm not sure it will take a long time.

See: Elon Musk.

If anyone had ever told me the answer to the reusable space craft proble was to bring the lower stage down for a controlled  vertical landing, I'd have called them nuts.

Space X roasts yer nuts.

Orion is 8 years from demonstration and proofing flights. Dragon is delivering cargo, and Falcon Heavy is planned to demo next year. Musk started rolling at the same time Constellation did. They are on track to meet the Constellation schedule, while Orion is 7 years behind.

I'm sure there's a physical answer to the deorbit problem. But finding that answer in NASA, getting it funded, and built... That's impossible.

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