I know then when entering earth, the spacecraft will heat up due to various forces like gravity and drag and friction acting upon it, thus causing it to heat up. On reentry the flight profile is optimized to experience increased drag while maintaining a survivable level of deceleration and thermal load. They do this because the vehicle needs to shed orbital velocity on the order of 16, mph and the cheapest way to do this is to let atmospheric drag slow you down.
The technique is called aerobraking. Because they have designed the flight profile to generate increased drag as compared to launch and because the velocity with which it penetrates the atmosphere, it experiences much greater heat build up than on launch.
The generated heat simply comes from the conservation of energy. The vehicle's velocity is shed as heat via ablation of the reentry shield , adiabatic air compression, and other effects. The kinetic energy of the vehicle is transformed into thermal energy, resulting in the loss of velocity. Just like in your car, when it comes to a stop, the brakes will have become very hot because they have converted the KE of the vehicle into thermal energy.
Now look at the reentry profiles below. You notice that they have a near level part in the middle. That is where the aerobraking maneuver is performed. So this method of landing, without aerobraking is possible its how we land on airless moons , but extremely inefficient. An object trying to get into orbit will travel in a pretty steep parabola. The longer you spend in the atmosphere the more energy you lose to drag, and the more you lose to drag the more fuel you need.
So a solid strategy for achieving orbit is to get to your target orbit with a minimal curve and then burn until you have the right lateral velocity. Part of the reason for this is that increasing your orbital velocity affects your altitude degrees away, on the opposite side of your orbit.
An object that is deorbiting will be losing velocity urg, see edit note 1 and you generally want to use the atmosphere to help you brake, since fuel for braking is the most expensive fuel on the trip. When you're travelling that fast the air simply can't get out of your way quickly enough, and any time you compress something you also heat it up. Or if you want a simpler answer: Heating up due to atmosphere costs you energy, you want to avoid that as much as possible when going up and take advantage of it when coming back down.
Sorry if this answer sounds disjointed. Edit Note 1: I suppose I should be clearer on this During the first part of a deorbit the object is decreasing its acceleration while its velocity is increasing, it doesn't start properly decelerating until it's fairly suborbital. That's probably going to be around the point where aerobraking is doing its job though, somewhere in the area of km up. Exactly where the peak velocity is depends on a lot of things, including the object's terminal velocity and how much fuel you have to use up.
The point I was trying, badly, to make is that an object that wants to deorbit also wants to lose velocity to make that happen in a less destructive way. On launch, the change in velocity is provided by the rocket engines. The acceleration to orbital speed occurs over a fairly long period of time - typically 10 to 15 minutes depending on the design of the launcher. On reentry, the change is velocity is provided by air resistance; this obviously can't occur until the re-entering spacecraft is in relatively dense atmosphere.
Because of this, the vast majority of the deceleration occurs over a very short period of time, about two minutes. All the kinetic energy associated with orbital velocity gets converted to heat in that period. Essentially we can move the spacecraft like a feather into orbit, vertically up and down The other answers do not say this explicitly. But there is a very ugly problem for engineers, the Tsiolkovsky rocket equation and the very deep gravity well of the earth. So we need several stages to achieve orbit.
So we can get finally out of the Earth, but We would need fuel to slow us down again, but we haven't really fuel to spare. So the engineers decided to use atmospheric entry to slow down the spaceship with a heat shield. A softer method is aerobraking to reduce the speed with several passes through the atmosphere. If we would have a torchship that does not work with the rocket limitations, that would be a real nice thing because we wouldn't need the dangerous and unnecessary reentry phase.
While it's already been correctly answered, a suggestion to get a better picture of it: The game Kerbal Space Program. The same technology was later applied to the space program, Anderson says. When it gets hot enough, the material on the shield burns up and causes a chemical reaction that pushes the hot gas away from the spacecraft. The two principle factors that ensure a spacecraft can safely traverse the reentry corridor are the shape of the vehicle and its angle of reentry.
Research conducted by the National Advisory Committee for Aeronautics in showed that a blunt shape lowered the heat load. Also, spacecraft must hit the reentry corridor at a fairly precise angle. Who knows, it might become the standard way to get into space some time in the future, but it is a long way from achieving that potential right now.
If you can send a spacecraft into orbit that way, you can also return it from orbit the same way if you want to. This is the way it is done today: to use the upper atmosphere as a brake, then slowly parachute to the surface or glide down in the lower atmosphere. How easy that is to do depends on the spacecraft. If it is a heavy one like the Space Shuttle now retired of course then it can only slow down deep in the upper atmosphere, where it is dense.
So it gets very hot. It will be able to fly to orbit from a conventional runway though reinforced to carry the extra weight of all the fuel , return back to Earth, and then take off again within a couple of days with a crew of to assist. Its design is much lower in density than the space shuttle, once it has used up its fuel to get into orbit. So it slows down in the atmosphere at higher altitudes on the way down. What really matters is the mass per cross sectional area it presents to the atmosphere or more exactly, its ballistic coefficient.
Skylon could slow down even higher in the atmosphere if it presented a large blunt face like an aeroshell, but it has to be streamlined for the other stages of its flight. However, it is also able to compensate for that to some extent by steering during the early part of the flight to slow down more quickly. It flies to orbit from a normal length runway, reinforced to take the weight of fuel on lift off and may fly in the s.
It is heavy when it takes off, but during the landing, having used up most of its fuel, it is low density and so slows down much higher in the atmosphere than the Space Shuttle. As a result, it will reach lower temperatures than the Space Shuttle on re-entry though higher than a supersonic jet at Mach 3. Here are a few figures for skin temperatures for comparison, hottest first.
These are the figures for the hottest parts of the spacecraft or plane:. But the Skylon uses a structure much more like a zeppelin or a small plane. It has aluminium propellant tanks suspended inside it.
Covering that, it has a thin outer aeroshell of a high temperature silicon carbide fibre reinforced glass ceramic material. For details see page 2 of this report. This ceramic outer skin is black, which is why Skylon is shown that color in most of the artist renderings. This is an animation to show the concept for a mission to orbit and back by Reaction Engines, who developed the idea. Re-entry starts about seven minutes into the video.
This approach of reducing the density of the spacecraft to lower its re-entry temperature is taken much further with the plans of JP Aerospace. Their kilometer scale orbital airship is filled mainly with hydrogen.
It only operates above , feet and is balanced for the upper atmosphere. It also has a huge cross section which it presents to the atmosphere.
This spaceship design consists of a near vacuum of hydrogen floating in a near vacuum of normal air. If they succeed in building it, then it will be able to slow down just through friction in the very tenuous upper atmosphere. It would be a leisurely journey, as you would get there slowly over several days. Although it may not look it, its huge V shape is designed to be aerodynamic at hypersonic speeds in the near vacuum upper atmosphere.
They have done modeling, calculations, and wind tunnel tests with scale models to test this. So on the way up, it gradually accelerates to supersonic speeds, then to hypersonic speeds by which time it is already in a near vacuum.
It has solar panels over its vast upper surface to generate power, and uses these to power ion thrusters. These let you accelerate with a very high exhaust velocity, and so, with a small total amount of fuel, so long as you have plenty of power. It would have no shortage of power with such a large area of solar panels. It has no internal girders.
Its outer shell covers an interior of many large bags of hydrogen to give it rigidity and to stop the gas bunching up at its nose. It also has inflatable trusses, with nitrogen filling the gaps in between these components.
The nitrogen is vented if necessary and then replaced from liquid nitrogen tanks. It is balanced to float at , feet altitude in the atmosphere. But since it is aerodynamic, it also behaves like a glider on the way down. It doesn't look much like a glider to our eyes perhaps, but that big voluminous V shape makes a great glider in the very tenuous upper atmosphere during re-entry.
So what keeps it up is partly aerodynamic lift and partly buoyancy. The aerodynamic effects keep it higher in the atmosphere for longer, and so keep it cooler on the way down. On page they say: "By losing velocity before it reaches the lower thicker atmosphere, the reentry temperatures are radically lower This makes reentry as safe as the climb to orbit. Instead, every stage along the way pays for itself.
At present they pay for the tests through pongsats and other ways to lift material to the edge of space. Their tests involve high altitude balloons and V-shaped airships rated for the lower atmosphere. They have also tested a high altitude balloon-based airship design. JP Aerospace holds the altitude record for an airship , propeller driven, remotely controlled from the ground, and flying at a height of 95, feet above sea level. It gets the name because at that height the sky will be dark even in daytime, as for the Moon.
Next, they plan small airships doing test hypersonic glides back to Earth. Finally, they do test flights to orbit with smaller airships, then the first human pilots to orbit, and then huge orbital airships with passengers and cargo. The idea started off as a US Air Force contract for a near space reconnaissance airship.
It was only rated as sturdy enough for launch in a 2 mph wind at the time an airship is particularly vulnerable in the short time it takes to launch it from the ground. They did this with some reluctance - and it blew apart in the strong winds, causing some minor injuries. The inventor himself sustained three broken ribs. That was enough for the US Air Force to cancel the contract. JP Aerospace have now solved the problem and can launch their lower atmosphere V-shaped airships in any wind conditions.
You can read their account of this story here. You might wonder what happens if the airship is hit by a meteorite or orbital debris. From page of the book:. A balloon pops because the inside is at a higher pressure than the air on the outside.
The inner cells of the airship are "zero pressure balloons". There is no difference in pressure to create a bursting force. All a meteorite would do is to make a hole.
The gas would leak out staggeringly slowly The JP Aerospace orbital airships are so lightweight they could never survive at ground level. The slightest wind would tear them apart. If you want to fly all the way down to ground level on Earth in one go, then you need a more massive airship.
It still gets quite hot during the descent. Under my orange launch and entry suit, I wore a water-cooled set of long johns that kept me cool. My body heat was transferred from the water-cooled underwear into the cabin air by a briefcase-sized thermoelectric chiller.
The cooling system removed excess heat from the air, electronics and the astronauts, and kept the cabin at a comfortable temperature, dumping the heat into the vacuum outside using a water-to-steam evaporator. Continue or Give a Gift. Daily Planet. Flight Today.
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