I know I posted this question many years ago, but I can’t find it any more. My question is, why do spacecraft (any objects, for that matter) have to endure the “burnup” of re-entry into Earth’s atmosphere?
Why can’t a shuttle, for example, simply slow down to zero mph and then just float down through the upper atmosphere, then deploy parachutes as gravity takes over?
I think this question should also involve the so-called “space elevator” that scientists envision could be a long cable made from carbon nanotubes that extends all the way from the surface of the Earth into space. How would objects travel down the cable from space into the Earth’s atmosphere? Would they have to experience “burnup” as well?
I understand that this question involves escape velocity somewhere but hey, I’m not a rocket scientist.
I am not a rocket scientist, but it seems to me that slowing down from 17000mph or however fast they go would take about as much fuel as it takes to get going that fast. Plus you have to carry all that extra fuel up with you. It seems to me the re-entry is a cheap and easy way to decelerate. Thermal protection weighs less than fuel.
Since most of your take-off weight was fuel, you’ll need a lot less to slow a descent once you begin to fall out of orbit. There have been several plans in the past, and I think in the present, for spacecraft that will return in the manner you mention. Of course they don’t float down to earth, they just slow sufficiently during re-entry so they don’t hit the atmosphere at the extremely high speeds they do now. But air braking is a very effective technique that re-entry craft will probably always take advantage of.
I’m not a physicist, and I’m sure someone who actually knows something will be along, but as I understand it, you need to shed the orbital velocity before you can land. After all, it is the velocity that’s keeping you in orbit. Doing it by burning fuel would require they carry more than twice as much (because they’d have to haul the additional mass into space so the takeoff would require more). The atmosphere provides drag to slow down for free.
People who reach the upper atsmosphere in balloons can parachute down safely without burning up.
Edit: Obviously, I didn’t preview. What they said.
Being in orbit is simply a matter of “falling” around the Earth. The craft is always under the influence of gravity. As soon as they attempt to slow down, the vehicle will not float so much as plummet.
There’s more to orbiting a spacecraft than just going up until there’s no more air. If you fired a rocket 300 miles straight up and turned off the motor, it would fall 300 miles straight back down. (It’s not really quite that simple, the Earth is rotating, etc.)
For something to stay in orbit, it needs to be going very quickly around the Earth. The Shuttle used to go around the Earth about every 90 minutes. If you just let go of something at a certain height, it will fall. Newton’s First Law says that something will move forever in a straight line unless some force acts on it. An orbit is kind of the sweet spot between those two extremes; the gravity pulls the moving object just enough to deflect its path until it circles back around to where it started. (It’s really an ellipse, etc.)
The reason for re-entry is that it’s cheapest way to slow down again. You’re going about 17,000 miles an hour. You could turn around and face backwards and fire the rockets until you slowed down, but that would take a huge amount of fuel. To carry all that fuel into orbit with you, you’d have needed a much, much bigger rocket just to get off the ground in the first place.
For all the difficulty and danger of re-entry, friction is the easiest way to slow down. Take all the energy you want to get rid of and turn it into heat. Then find some way to get rid of the heat.
The ship’s in orbit - moving at a velocity of thousands of mph around the earth. To change its velocity you need to apply a force; so slowing it down to zero mph is as much effort as speeding it up would be.
The shuttle doesn’t have the fuel to do that. It used it all getting up there. Sure, it could just take twice as much fuel, but then you need twice as much effort to get it into orbit in the first place - moving payloads into space is extraordinarily expensive. I think I read somewhere it was millions of dollars per pound of weight.
Alternatively, the atmosphere will do it for you for free at the cost of getting a bit warm.
Yup. Loads of it. Completely escaping Earth’s gravity well is harder than achieving orbit (assuming we’re talking a reasonably close orbit - if it’s already in an orbit far enough away, it’s much easier).
At liftoff, an orbiter and External Tank carry 835,958 gallons of the principle liquid propellants: hydrogen, oxygen, hydrazine, monomethylhydrazine, and nitrogen tetroxide. The total weight is 1,607,185 pounds.The average cost to launch a Space Shuttle is about $450 million per mission.
That was what the third stage was for in the Apollo missions (and, once it was out of fuel, the CM would take over for mid-course corrections and lunar orbital insertions etc.).
Bad science or good? In the film Aliens they use a literal dropship to get to the surface of the planet-the shuttle bay doors open, the latches release, and the dropship falls like a rock. This would mean that the mothership wasn’t in orbit at all, but just kind of hovering above the planet.
The ship has artificial gravity. The initial drop was probably the dropship being accelerated by the gravity. You can see its engine turn on during the sequence. So I don’t think the Nostromo needs to be hovering.
Yeah, so? If you have the energy reserves necessary to travel interstellar distances in any reasonable amount of time, pulling less than 1 G long enough to drop a transport should be a cakewalk.
None of this statement really makes any sense. “Slowing down to zero” can mean many things, but let’s assume that it means the shuttle is traveling the same speed as the earth is rotating so that the shuttle has no speed relative to the atmosphere. If you could do that instantaneously, the shuttle would just fall to earth like a rock. Despite what you hear in the popular press, there is plenty of gravity acting on the shuttle. By the time you reached an altitude at which you could deploy a parachute you would be moving too fast.
How did the Soviets land their spacecraft at Baikonur Cosmodrome since they couldn’t splash down and had to make a soft landing on land? It took a lot of extra fuel, like people have been saying in this thread? How did they manage it?
It takes a bit of extra fuel, but not much, because the Soyuz still uses aerobraking and parachutes for all but the last second of the descent. The heat shield and then the parachutes slow it down to about 24 feet per second, and then in the last second of descent, small engines fire to quickly cut the speed down to five feet per second. Cite.
Yes, I agree that my statement about “floating” doesn’t make much sense, as there are probably dozens of factors going on that I have no idea about, but like I say, I’m no rocket scientist. I just meant that the shuttle would somehow come to a relatively slow speed (relative to what, I’m not sure) enough for it not to have external temperatures rise above what is ambient already.
I also understand that in order to slow down it would need to burn loads of fuel, but in this case I’m being completely hypothetical – I’m just asking that if it COULD somehow go slowly enough, could it just descend until it hit upper atmosphere and then perhaps glide (although I know it would actually drop like a stone) and then come in to land without needing a heat shield etc.
Satellites orbit geosynchronically (?), but doesn’t that mean they’re travelling at the same rate that the earth rotates (ie. about 1,000 mph), in other words, stationary with relation to the spot above the earth that they’re observing? Technically speaking, relative to the Earth, they’re standing still. Now, if they just descended from there, somehow maintaining a sensible speed and not hurtling down like a meteor, couldn’t they just descend through the atmosphere at their leisure and then deploy a parachute or engage landing rockets?
I know this is all very simplistic and childish speculation but all I’m asking is, would it be possible to descend from space to the Earth’s surface without hurtling through the atmosphere at 7,000+ mph? Even more intriguing (maybe off topic), but would a single astronaut somehow be able to do this and then deploy a parachute (like Joseph Kittinger) from about 100,000 feet?
In order to do descend you would slow down (relative to the center of mass of the earth) which would mean you are no longer stationary relative to a fixed position on the earth. The short answers to your questions are “no”.