Why do spacecraft have to go through re-entry?

Factual Questions
21

I am not a rocket engineer but I am an engineer and very much a space enthusiast so to answer your question the best I can:

It’s fuel. You could do a controlled reentry with retro rockets like we have to do on the Moon but here (as with Mars to an extent) the atmosphere is an advantage.

Here is a whimsical but reasonably accurate diagram (click the pic to embiggen) showing the delta-vees needed to move among the inner solar system. Note that the vines represent aero-braker manuvers and are essentially “free” in terms of fuel budget so it makes economic sense to make use of it.

22

If you have enough fuel, you can maintain any position you want. An object descending slowly from geosynchronous orbit (while maintaining position over a fixed point) would need a fairly complex sequence of accelerations, but it could definitely be done.

Getting back to the OP–as the other posters have said, you need lots of fuel to go from orbital speed back to 0. A general rule of thumb is that doubling delta-V means squaring the mass fraction. So if you have a rocket that can take 0.1 of its mass to orbit, to avoid aerobraking your rocket would have a mass fraction of 0.01. In other words, for the same payload you’d need a rocket 10 times as big.

A mass fraction of 0.1 is very good, too–no rocket actually achieves that. A more typical fraction is 0.04.

23

But that defeats the purpose of the question, doesn’t it? If you have enough fuel you can fly around however you like.

24

My reading is that there’s some confusion as to whether reentry heat is entirely due to shedding velocity, or if there’s some other factor. I just wanted to be clear that there’s no other factor, but to maintain position (when descending from GSO) you’d need lots of fuel.

25

If you had enough fuel, you could do just about anything. In theory, you could probably fly a descent profile that would look just like film of the liftoff run backwards, but you’d have to have as much weight in orbit (shuttle, external tank, full fuel, and solid rocket boosters) as the shuttle has when it’s on the launch pad.

But the weights involved are pretty daunting. I remember a class in college where I saw some performance graphs of the Saturn V; weight, speed, altitude graphed over time. It burned off something like 800,000 pounds of fuel in the first 2:30. I don’t know if there’s some exact ratio. The shuttle weights 4,400,000 pounds at liftoff, empty weight of the orbiter is 172,000, payload to low earth orbit is 53,600; so it’s something like 20 pounds at liftoff to get 1 pound into orbit. If you wanted to use thrust for ascent and descent, it’s 400-to-1.

The radius of the Earth is about 4,000 miles, *2 to get the diameter, *pi to get the circumference. It’s ~25,000 miles and you go around once in 24 hours. So at the equator you are going about 1,000 mph to the east. A geostationary satellite is 22,300 miles above the equator. Do the math and it’s going about 7,000 mph. Still a lot of energy you have to get rid of before you land gently on the ground.

26

Neither are we.

Think about it.
It’s in orbit moving at a high speed.
You want to bring it to ground at zero speed.
The speed has to change, right? How do you change the speed? If you don’t change it and simply lower it, when it’s at ground level, it’s still gonna be moving horizontally at the same speed it was when it was in orbit - thousands of miles per hour.
You have to reduce that horizontal (orbital) speed. That requires exactly the sa,e amount of energy as it took to achieve that speed in the first place.

Sure. Maybe he’d come downwards slowly. But he’d have a horizontal speed of thousands of miles per hour. That won’t matter till he reaches the ground, of course, at which point if he can’t run thousands of mile per hour he’s gonna be so much jelly.
It’s all about that speed with which it’s whizzing round the earth. You can’t change speeds - faster or slower - without exerting a force; that’s basic hundreds of years old Newtonian laws of motion. Where does that force come from? Fuel.

27

To be in a geostationary orbit, you have to be at a particular height above the Earth’s equator so that the force of Earth’s gravity is such that you can orbit the Earth in exactly 24 hours. This means that as you’re orbiting the Earth, the Earth is spinning below you, and so you stay at exactly the same spot above the Earth. Note that this is only possible when orbiting exactly above the equator, a tilted orbit means that you’d oscilate above and below the equator and wouldn’t get the advantage of staying at exactly the same spot.

So, you’re in geostationary orbit. Now you want to land. So you just fire your rockets up, and you start drifting down to Earth. Except now you’re lower but have the same speed, this means that the circumference of your circular orbit is smaller, which means you complete an orbit faster. Which means you’re not geostationary anymore, which means the surface of the Earth is now whizzing past you, and as you enter the atmosphere the atmosphere is whizzing past you which means wind at super-high speeds, which heats you up, which is what re-entry is all about.

People have a very hard time understanding exactly what it means to be in orbit. An orbit is a very particular kind of falling. You know how if you throw a rock up in the air it falls back to the ground. Throw it hard enough and it goes up really high, but still falls back down. Throw it even harder and you and throw it right off the Earth, and it will fly away into space.

Now imagine instead of throwing the rock straight up, you’ve got a cannon that shoots the rock at waist level. Obviously, a cannonball fired from a cannon aimed perfectly parallel to the ground hits the ground at the same time as a cannonball that’s just dropped. However, the Earth is curved. What if you could fire a cannonball so fast that by the time it dropped four feet, it was so far away across the horizon that the ground was four feet lower? Then the cannonball would keep going around and around the Earth, every time it dropped a foot the ground would be a foot lower. And the cannonball would be in orbit around the Earth.

Of course, this isn’t possible because air would slow the cannonball pretty quickly. But if you could fire the cannonball from really high up, where there isn’t any air, you could shoot it sideways such that it constantly falls toward the Earth and constantly misses.

And because the farther away you are from the Earth, the weaker the Earth’s gravitational pull is, the higher you are the slower you need to fire the cannon. For example, the Moon is like a giant cannonball that takes a whole month to fall all the way around the Earth. And the Earth is a giant cannonball that takes a whole year to fall all the way around the Sun.

So as you can see, any time you’re in orbit around the Earth, you’re not just floating. You’re falling around the Earth very quickly. So if you want to get back to the surface of the Earth, you need to be traveling at very close to the same speed as the surface of the Earth, otherwise you’ll smash into it.

28

That was a very good explanation. I’m in a Larry Niven mood (thanks to another thread) so I would only add the laws of motion mantra in The Smoke Ring: “East takes you out, out takes you west, west takes you in, and in takes you east.” Of course, those people were living in a gas torus around a neutron star but those laws apply to any gravitational field provided east and west are defined similarly. I suppose if you had a source of near limitless energy, a torchship perhaps, you could continuously burn your engines to the east accelerating your craft to the west and, varying the thrust so as to maintain your position over the same point on the equator, lower your ship until you enter the atmosphere at which point you begin to angle your thrusters more and more straight down until eventually they are straight down as you gracefully control your decent until touchdown but the energy expenditure would be enormous.

The bad guys in the last Star Trek movie had to have been doing the same thing though it wasn’t discussed explicitly. They were hovering their drilling machine in an orbit much lower than GEO, just outside the atmosphere in fact, in order to drill a hole in the planet from space to drop their <mumble> magic planet killing stuff into the core. At least that’s how I mentally hand-waved it to myself. Maybe one percent of the audience probably even cared about such a thing but I needed it in order to suspend my disbelief.

So yeah, you could do it, if you had the energy budget. I’m an RF engineer by trade so an analog would be another law/mantra: You can solve almost every radio problem by just pumping up the power. Everything becomes simpler with the application of more power. For just one example, you can pump through increasingly more data per unit of time by using more sophisticated modulation schemes. Of course, with mobile devices (cell phones) so small that they really should carry a choking hazard warning, more power isn’t always an option.

The trick in engineering is to do it efficiently within acceptable margin. So why not use aerobraking when it’s available? I should also add that even if you were landing on an airless world (e.g. the Moon) you don’t just lower yourself. You briefly (on the order of seconds or minutes) burn your engines such that it changes your orbit so that it now intersects the surface of the world. Remember that you are technically orbiting the center of the body. Then you coast/fall until you’re close but you don’t want to land at that speed so in the last few minutes you start burning your engines so as to reduce that velocity and find a landing zone. And then a final series of burns that reduce your velocity to zero with respect to the body at the same time your encounter the surface of the body. Better than Neil Armstrong; way better than Pete Conrad. :slight_smile:

29

It depends on the elements (parameters) of the orbit, but if you are willing to take enough time by thrusting at every perigee until you achieve escape (low energy maneuver) it can take a very modest amount of extra fuel. On the other hand, if you are doing a direct injection maneuver as the Apollo missions did, you need a more powerful propulsion stage with a lot of propellant like the Saturn S-IVB.

The American manned space projects using capsules (Mercury, Gemini, and Apollo) didn’t elect for a water landing because it was any softer or safer, and in fact, water landing creates some additional hazards and equipment required as Gus Grissom discovered during Mercury-Redstone 4. The American programs selected spashdown because of the large uncertainty in re-entry trajectory and the lack of open space on a suitable track to be designated for the landing zone. Splashdown in broad ocean area was the only practical choice, albeit one that the nascent Air Force manned space program balked at as it would require support from the Navy.

The Soviet space capsule spacecraft (Vostok, Voskhod, and Soyuz) launched from Tyuratam (Baikonur) but typically landed in the spasely populated areas well to the north and west of the cosmodrome. The early landings were anything but soft, and in fact the Vostok required the cosmonaut to climb out of the capsule during the final stage of landing and parachute down separate from the craft. The capsules used a series of staged and reefed large parachute canopies for final deceleration and controlled landing, just as American systems did.

It would not be practicable to launch a vehicle that was capable of landing in that manner. For reasons already stated up thread, a vehicle capable of negating the orbital speed of the spacecraft would be roughly the size of a launch vehicle. In other words, to slow an Apollo spacecraft from Low Earth Orbit would require a vehicle the size of a Saturn IB, and to launch that combined system would require a launch vehicle twenty to thirty times that size. So no, it isn’t possible using anything like extant technology.

Stranger

30

What I don’t understand is, why do so many people think that the heat of reentry is something terrible—to be “endured”?

Any man-made orbiting object got there by the application of enormous energies. Did you ever notice the huge flames and thundering roar of a shuttle lift-off? or, more so, an Apollo launch? Those rocket engines are producing millions of pounds of thrust to accelerate the payload (Shuttle Orbiter + contents) up to about 17,000 miles per hour to take up station in Low Earth Orbit (LEO) at about 200-300 miles above the Earth’s surface. To bring the object back to the Earth’s surface, intact, requires dissipating that exact same energy, in some form or other.

Since all energy is eventually converted to heat, the most efficient way to reduce the incredible momentum of the orbiting object is to let its orbit slowly decay until it reaches the upper fringes of the atmosphere. There, friction with the thin wisps of gasses at that altitude cause heat to be generated as the orbiting object is slowed, by the force of drag. The air is still way too thin to support any kind of “gliding”, and, unless your parachute had an area of a few square miles, it would not slow you appreciably.

At the altitude of the International Space Station (ISS) (about 240 miles/390 km), the air is thick enough that, without periodically re-boostingthe Space Station, it, too, would de-orbit and burn up on reentry. The Space Shuttle (and the other nations’ orbital vehicles) intentionally used rocket engines fired in their direction of forward motion to adjust their orbital trajectory to one with a perigee (near point to the Earth) deeper in the atmosphere.

As you fell deeper into the atmosphere, eventually the air is thick enough to “glide” the rest of the way down, but unless you have been using the atmosphere to slow down through “aerobraking”, you’ll still be traveling too fast. The shuttle orbiter had the aerodynamic characteristics of a brick, and it reached speeds in excess of Mach 25 (18,000 mph) as it entered the denser air at about 75 miles altitude (400,000 feet). There, it performed a series of sharply-banked (70 degrees or so) S-turns to shed airspeed and altitude, before lining up on the runway and deploying landing gear.

I have a hard time wrapping my head around the idea that slowing the space vehicle down using atmospheric friction, essentially for “free”, is somehow worse than expending an absolutely insane amount of fuel to “simply slow down to zero mph and then just float down through the upper atmosphere, then deploy parachutes as gravity takes over”.

31

I think this perception (that I admittedly shared before reading this thread) stems from the Columbia disaster.

32

I am one of those people. It just dawned on me: Is that the reason the shuttle did not go straight up all the way, but eventually leveled off - to line up with the orbit they wanted to achieve, so to speak?

33

I’m a bit of a space nut going back to the Mercury/Gemini/Apollo days so for me it seems as natural as throwing a ball as far as you can. The only reason the shuttle (or anything bound for space) has to up at all is to get outside the atmosphere. But just as importantly importantly, It has to build up enough horizontal velocity so that it can “fall” around the Earth. For Low Earth Orbit that works out to be about 18,000 miles per hour. If there was no atmosphere, a craft could orbit at virtually any altitude that would clear any mountains that might be in the way. This horizontal velocity is why it’s easier to launch from a location as close to the equator as possible so that you are starting off already moving to the east at close to a 1000 miles per hour thanks to the Earth’s rotation. For the U.S. that meant Florida and California.

Merry Christmas to you and yours!

34

I read somewhere that the launch site being in Florida was kind of a happy accident. The U.S. military was looking for a place to test ballistic missiles. The site criteria called for launching over water, but with land-based tracking stations not too far away. The choices came down to Western Washington (with tracking stations in the Aleutians), Southern California (Baja Peninsula), or Florida (Bahamas). Florida won out at least in part because of the weather, but turned out to have the advantage you cite once we started launching to orbit.

I’m not sure that story totally holds up; the downrange distance to the tracking stations seems to vary by quite a bit. I could probably find where I read it, though.

35

It is a good thought, but slightly distorted: 90% or so of the shock and awe of a shuttle launch was accelerating stuff like fuel to be burned later, external tanks, boosters, payload, etc that never makes it into orbit…so while it is true that that approximately the same energy that inserts the orbiter and payload into orbit needs to be used to get it out of orbit, most of the huge flames were not doing that.
Another worthwhile point is that most of the analysis in this thread has assumed a return-from-earth-orbit re-entry, which is the usual case. The exceptions are worth note: The Apollo moon missions. The command modules returning the crews from the moon were coming in at well above orbital velocity…in fact they had enough velocity to escape earth’s gravity completely if they came in a bit too high (and burn up if a bit too low, it was a pretty narrow window) The Apollo missions needed to use aero-braking even more than orbital missions. They would have either needed to haul their re-entry fuel to the moon and back, or park it in earth orbit on the way out, and collect it on the way home.

36

Here’s a simple way to think about it. Imagine the smallest rocket you can use to place your probe or say a manned capsule into orbit. Roughly speaking you now need another rocket big enough to put THAT whole first rocket into orbit if you want to come down without “rentering” but hovering down instead.

Here’s an off the cuff WAG on the difference. The 1 manned Mercury rockets put one man in orbit then deorbited and rentered. If you didnt’ want to “reenter”, you’d probably need some on the order of the size of the Apollo rockets (and maybe even way bigger than that) to put a Mercury rocket into orbit.