Why Doesn't A Spacecraft Also Heat Up During Launch?

Could the heat that a spacecraft has to endure during re-entry also happen during launch?
This astronaut photograph highlights the reentry plasma trail of a deorbiting spacecraft, the ISS Progress 42P supply vehicle (Russian designation M-10M). Acquired October 29, 2011. Image Credit: NASA

This astronaut photograph highlights the reentry plasma trail of a deorbiting spacecraft, the ISS Progress 42P supply vehicle (Russian designation M-10M). Acquired October 29, 2011. Image Credit: NASA

Originally posted at Forbes!

In principle, it’s the same atmosphere, and if the conditions were the same between launch and landing, you’d certainly expect there to be the same heating on the spacecraft in both cases. However, there are a few key differences between landing and launch which means that the spacecraft winds up dealing with much more heat on its way back down to the surface than it does going up.

The biggest difference is due to the fact that when you launch a spacecraft, you start out stationary relative to the ground. The major goal of a launch is to speed yourself up to the point where gravity’s downward force is no longer enough to pull you back down to Earth. Any given orbit is simply a matter of falling while moving around the object you would like to not crash into; you’re still falling, but the sideways motion is enough to keep you at the same distance above the surface. A spacecraft’s launch is the slow, rumbling start to building up this speed.

When you’re in orbit, you’re going much faster relative to the ground. The International Space Station, for instance, orbits the Earth once every 90 minutes or so, which means that it’s going slightly over 17,000 miles per hour up there. Any spacecraft which visits the ISS therefore has to start from 0 miles an hour, stationary on a launch pad, and reach 17,000 miles an hour. If you’re starting in space, and trying to land on earth, you need to reverse this; you need to start from zipping around above the planet, and slow yourself down to 0 miles an hour.

In addition to the difference in starting speed, the atmosphere is not evenly dense from top to bottom. The top (i.e., facing outer space) edge is much less dense than the bottom is. And if you’re heating up the spacecraft due to interactions with the atmosphere, those interactions are going to be dependent on both the speed at which you encounter the atmosphere and the density of the atmosphere.

When you’re leaving the planet, your craft will be moving rather slowly while it’s going through the densest parts of the atmosphere, which means that the heat generated will be fairly small. It’s only when the craft leaves the most dense regions and gets into the very thin upper atmosphere that it starts to get up to the orbital speeds that can produce strong heating, but by that point there’s not enough atmosphere left to produce significant heat.

These four shadowgraph images represent early re-entry vehicle concepts. A shadowgraph is a process that makes visible the disturbances that occur in a fluid flow at high velocity, in which light passing through a flowing fluid is refracted by the density gradients in the fluid resulting in bright and dark areas on a screen placed behind the fluid. Image credit: NASA

These four shadowgraph images represent early re-entry vehicle concepts. A shadowgraph is a process that makes visible the disturbances that occur in a fluid flow at high velocity, in which light passing through a flowing fluid is refracted by the density gradients in the fluid resulting in bright and dark areas on a screen placed behind the fluid. Image credit: NASA

If you’re coming back down from space, you’re hitting an increasingly thick wall of the atmosphere at very high speeds, and the magic combination of dense air and high speeds is present, allowing for the super high temperatures to be produced. The atmosphere is so heated by the re-entering craft that it forms a shock wave of plasma around the spacecraft for at least a few minutes as the craft slows down. This plasma interferes with radio communications between ground and spacecraft, so it’s often an anxious time, as the ground control is unable to communicate with the crew (or the robot) to check on how things are going.

To protect the spacecraft, and its crew if one exists, most crafts are fitted with a heat shield; this is a heat resistant material on the leading edge of the re-entering craft. The heat shield is designed to very slowly vaporize at high temperatures, allowing the hottest parts of the shield to flow away from the craft, and preventing high temperatures from building up near the spacecraft, which could result in the destruction of the entire craft. The partial loss of the integrity of the heat shield on the space shuttle Columbia led to the tragic loss of craft and crew in 2003; the heat shield is a critical component to a safe and successful re-entry.

NASA's Stardust sample return capsule successfully landed at the U.S. Air Force Utah Test and Training Range at 2:10 a.m. Pacific time (3:10 a.m. Mountain time). The capsule contains cometary and interstellar samples gathered by the Stardust spacecraft. Image credit: NASA

NASA's Stardust sample return capsule successfully landed at the U.S. Air Force Utah Test and Training Range at 2:10 a.m. Pacific time (3:10 a.m. Mountain time). The capsule contains cometary and interstellar samples gathered by the Stardust spacecraft. Image credit: NASA

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What would a human need to survive re-entry on their own?

What sort of protection would a human need to survive reentry into the atmosphere without a space ship or other vessel? Preferably with parachute and sans parachute scenarios.

By the time you finish protecting someone from re-entry, you will have basically built a little person-sized spacecraft around them. Re-entry is a technologically challenging thing to survive, and even the smallest problem can escalate quickly, as the Columbia disaster taught us only too well.

The main source of the problems with re-entry is that if you’re orbiting the earth, you’re going extremely fast. The ISS travels at just under 8 km/s, which translates into 17,224 mph, or 27,720 km/h. When landing, generally we want our sideways velocity to be as close to zero as physically possible, so we’re going to have to slow down by more than 17,000 mph.

The atmosphere itself is a pretty good set of brakes- it’s a much thicker medium to go through than space, so it will slow you down, just like walking knee-deep in water is slower going than walking on land. The trouble with using the atmosphere is that you tend to exchange your speed for heat. The force of being dragged through the air is a force of friction, and as all the air particles collide with the re-entering craft, they donate a little bit of heat with each collision. Unfortunately for a poorly protected person, the atmosphere’s friction generates so much heat that the air itself turns into a 3000F (~1650C) plasma. The job of the Space Shuttle’s heat protecting tiles is to resist this intense heat (there’s something similar on the bottom of every object that has re-entered our atmosphere). When this plasma builds up, the craft is effectively cut off of communications with the ground, since all radio waves are blocked - this is known as the plasma blackout period, and lasts for a little over ten minutes. If you’re trying to protect a person from re-entry, step one is going to be to make sure that you have surrounded your person in some seriously intense insulation to keep several thousand degree plasma from roasting them to a powder.

On top of being incredibly heat-resistant, the insulation is going to have to be incredibly resistant to cold, since we’re starting in the frigid temperatures of space. For the survival of our human, it’s not enough to have a plasma-protecting layer that doesn’t crack in the insane cold of space; our person must also be kept in the very narrow range of temperatures in which we humans are comfortable. Fortunately, temperature regulation is usually built into another piece of equipment he’s going to need - a pressurized suit. Since we’re starting in space (zero air pressure), and the descent goes through a lot of extremely thin atmosphere (very little air pressure), we need our person to be cocooned in a pressurized suit to keep the gasses in his blood from boiling from the lack of atmospheric pressure.

Even with these considerations taken care of, we can’t just wrap our unlucky space-jumper in some kind of high test, pressurized, internal temperature-regulating bubble wrap and fling them out of the International Space Station. The human body is a very delicate thing and does not handle large accelerations well, and this includes spinning. If our jumper lost even a little bit of stability as he fell through the atmosphere, he could begin to tumble. The chaotic rotation of a tumble can cause strong forces - several times the force that gravity normally exerts (abbreviated 1G). A force of 6Gs for more than a few seconds can cause even seasoned pilots to black out. At the point when our pilot has blacked out, there’s almost no hope of recovering from the tumble, and if the force on the body is not reduced quickly, your chance of death increases rapidly. Tumbling was one of the major concerns with Felix Baumgartner’s jump from 24 miles up, and he did in fact tumble for some time, but managed to pull out of it - if the tumble had continued or had been harder to escape, he would have been in serious trouble.

So now, in order to be safe, we need an aerodynamically stable pressurized plasma-proof coating for our space jumper, just to survive early re-entry. This is effectively a small craft built around our person, and we’re not even close to the ground yet.

Parachutes aren’t very useful until you get reasonably close to the ground; the air needs to be thick enough to exert a strong drag force to help you slow down when it catches in the chute. You also need to be going sufficiently slowly that the air is not being heated into a several thousand degree plasma around you. But once you get down to this level, parachutes are a fantastically useful and reliable method of slowing yourself to the point where you probably will not die upon impact with the ground. Because they’re so reliable, we have attached several of these to nearly every single object we land on a surface, which includes on Mars. The Space Shuttle had a trio of parachutes deployed upon touchdown to help it slow down, and all the Gemini & Apollo class missions had parachutes deployed before splashdown. The Soyuz capsules still land this way - it’s a tried and true method.

Trying to land without a parachute is a lot harder. The space shuttle did most of its slowing down (once it made it past the plasma stage) by gliding. The shuttle was an impressive feat of engineering; once the guided portion of re-entry was over, the pilot of the shuttle manually landed a completely unpowered craft, slowing it down to a touchdown speed of ~220 mph. The runway for the shuttle is phenomenally long (about 15000 feet) and made of high traction concrete, just to give it enough time to roll to a stop.

Other than gliding, the technology to control a landing is only now being developed. SpaceX is working on what they call the Grasshopper engine, which is meant to be able to do vertical takeoffs and landings, and just a few days ago managed to take off, lean over to one side, hover, and then safely land again. So presumably, if a person’s structured protective re-entry gear came with rockets on the bottom of it, it could control the descent and slow them down automatically to a smooth landing. This is probably a lot kinder of a landing than the Mars Pathfinder method of landing, which is to have downward facing rockets slow you most of the way down, and then surround yourself in airbags and bounce to a stop. (Here again we have the problem of the human body being a lot more delicate than the Mars rovers - the Mars rovers hit the ground going slightly over 50 mph, which caused accelerations on the rovers themselves of over 50Gs - not particularly good for a human body.)

It’s by no means a simple task to make it back down to the ground from Earth orbit. We may someday have the materials and the technology to protect someone from the many forces involved, but at the moment, it would be a plunge to certain death.

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