Why can’t the space shuttle just go slow enough to not be heated up by friction with earths atmosphere on re-entry?
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A space shuttle absolutely could slow down so that it doesn’t heat up on re-entry. However, this would require a huge amount of energy to do, which would require a lot of fuel.
In order to get into orbit, a spacecraft has to accelerate to at least around 18000 mph. That’s how orbits work, if you went any slower, you’d fall back to Earth. Look at how much energy (and fuel) it took for the space shuttle to go from 0 to 18000 mph. It required giant fuel tanks and a huge fireball shooting out the bottom for like 10-15 minutes straight. If you wanted to slow down from 18000 mph to 0 mph without using friction, you’d need to use the same amount of energy.
You might think that it would cost 2x the fuel to do a powered deorbit. The real problem is that if you wanted to use your engines to slow down, you’d have to carry all that fuel up into space. And fuel is heavy. So, now in order to launch the space shuttle along with all that extra fuel, you need even more fuel for the initial launch.
I haven’t done the math, but I wouldn’t be surprised if it would require 5-10x as much fuel to launch a spacecraft+fuel that is capable of a powered descent. The launch cost would be significantly higher. Even with the most efficient spacecraft, it still costs well over $1000 to lift a pound of payload into orbit. Imagine the extra cost of bringing 20000 pounds of fuel into orbit just to avoid a traditional re-entry.
Basically, we use atmospheric friction as a “free” energy source to slow the spacecraft down from 18000 mph to 0 mph, so that we don’t have to haul up tons of extra fuel for that purpose.
Real life orbit is just falling fast enough in a sideways direction that your altitude is maintained above the friction of earth’s atmosphere, so you dont slow down, (but not *too* fast or you’ll escape orbit). This is very different from most examples of future spaceships we see in sci-fi, which have seemingly limitless energy reserves to magically overcome the incredibly strong and constant pull of a planet’s Gravity.
Irl, Without a very high sideways speed relative to earth you need to spend constant fuel to accellerate 9.8 m/s² moving away from Earth to counteract the 9.8m/s² of gravity pulling on you, as you float nearby in space.
But if you start out with your speed moving sideways fast enough, with the direction pointed in a tangent direction to the earth (i.e. above the atmosphere tracing the equator), then you can stop spending fuel once you’re in outer space. The sideways momentum doesn’t need fuel to maintain it once there’s no more air molecules to slow you down. The constant pull of gravity keeps your ship curving around in orbit at a stable altitude so you wont fly straight off into the sun or out of the solar system. The pull is constant so by speeding up or slowing down sideways, you orbit higher or lower.
It would be mathematically impractical or impossible to carry enough fuel to descend straight down into atmosphere from above. The gravitational pull simply too constant and atmosphere is too thin to go slow enough at a direct angle down (like a helicopter / elevator). The gradual descent at a slight angle is the only way to reliably not become a meteorite at the bottom. It gives you time to release all the momentum stored up when you first launched into orbit, as friction as you decelerate.
So basically we turn the energy from launch and getting into orbit as a reserve of momentum that keeps the ship above the planet for free, but we have to eventually lose the momentum to descend. It wouldn’t make sense to spend fuel slowing down from 8000 m/s sideways to 0 sideways while also spending fuel thrusting up to keep from dropping like a rock. Instead we use the atmosphere as a aerobrake so that the only fuel needed to decend is the fuel needed to move into some atmospheric air molecules from the zone above them.
If you’re interested in learning this stuff about orbital mechanics intuitively I have to recommend the sim game Kerbal Space Program, which is like a combination spaceship builder, launch & orbit simulator, and space explorer management game.
not enough fuel. the spaceshuttle has to go a certain speed to stay up there, and this is fast. if you want to slow it down, you need a lot of fuel that you would have to bring it with you which also costs extra fuel. friction reentry is much cheaper.
It’s only going so fast because it had to to get up there. While it could slow down on it’s own out would require a huge amount of fule. Lots of fule to slow down and a more to carry the weight of the added fule. It’s just far cheaper to use the free energy of the atmosphere as the breaks.
The other answers do a great job covering the fuel aspects, but even if fuel was not a problem, it would still not be an ideal way to do this. Here is another point to consider.
If a spacecraft is “in orbit”, that means a number of things about its flight path. In order to maintain that orbit, it has to go a certain speed. Slow down, and it drops to a lower orbit. As it is hurtling through space, the Earth’s gravity is constantly pulling it down. It is only by outrunning gravity does it stay in that orbit.
If you try to go slow enough to make a colder reentry, then the spacecraft is giving gravity a chance to pull it down even faster. This will result in a plummeting trajectory unless the spacecraft starts activating some sort of lift-providing device (like a rocket engine) to counteract the pull of gravity. Wings don’t work for this as the atmosphere is too thin and the shuttle too heavy. Otherwise, dropping like this could result in a spin or an unrecoverable dive.
Theoretically, with some sort of futuristic technology that provides nearly unlimited thrust without requiring significant additional fuel, a “cold” reentry could be possible.
Rocket fuel is very heavy. In order to slow down enough to prevent that heating, you need a lot of extra fuel to slow yourself through the descent through the atmosphere until the atmosphere is thick enough to keep you from moving fast enough that heat becomes a problem. Every pound of fuel a ship takes into orbit, it needs about 10x that much fuel to get it there. Using the atmosphere to slow the descent means we aren’t using fuel to do it, so it save a lot of cost and gives us more space to work with payloads. Dealing with the heat of reentry isn’t that big of a deal by comparison
Going slower drops you into a lower orbit. Going faster pushes you into a higher orbit.
by slowing down it has to go into a lower orbit, which drops it lower into the atmosphere, which introduces friction/compression heat issues, which further slows it down, repeat
As I understand it, you COULD, but as others are saying, it would take a LOT of time, a LOT of fuel, or both, and most likely fuel, since the space shuttles aren’t really built for gliding a long way.
There is always going to be a lot of friction when an object going 16,000-20,000 miles per hour hits the atmosphere. You can’t get away from that, and even at military aircraft speeds, the shuttle would have a bad lift-to-glide ratio.
The balance is having it descend fast enough that the heat doesn’t have TIME to work its way though the protective tiles and other heat shields, but NOT descend SO fast that the heat becomes so INTENSE that it OVERCOMES the heat shielding.
Unless you slow it down to a crawl first, that’s the job, more or less. But, if you slow it down TOO much it completely loses the ability to glide, and it would “stall” and then fall straight down. You wouldn’t have any lift, and you’d have very poor control over the ability to steer, level, climb, or drop.
If they redesigned it to glide better at slower speeds, it would be even harder to launch into orbit, use LOTS more fuel, and even more vulnerable to damage, like having the wings ripped off, upon re-entry.
Putting some numbers to other peoples posts – Orbital Velocity in Low Earth Orbit (LEO), is ~9 kilometers per sec which is around the equivalent of Mach 25. The space shuttle orbiter had a maximum mass w/payload of around 110,000kg, roughly the same mass as a small american house. This is a an extremely heavy object moving at 10 times the speed of a Barrett .50 Caliber bullet, it is impossible to “just slow down” without external help.
At launch fully fulled, it weighed 2,030,000 kg of which ~90% was just fuel to get it up to Mach 25 so it can stay in orbit. There’s no physical way that the shuttle could carry an appreciable fraction of that to orbit. It’s much easier to use the “free” drag from slamming into the atmosphere to slow down – thus why every spacecraft that has landed on a body with an atmosphere has done that.
OP HERE: These comments have all been very useful. Thanks to everyone! I think I get it now, more or less. To summarize, it’s all about fuel weight. The shuttle couldn’t carry enough fuel to slow down the amount it’d need to, right?
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