Why does reentry have to be so fast

Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Can a reentry be done slowly? Ask Question. Asked 3 years, 4 months ago. Active 4 months ago. Viewed 8k times. As an alternative, what would it take to "fly" a slow reentry? This would be more of a "balance lift and drag to fly at a slowly decreasing speed while maintaining altitude" Is there any research available on what such a profile might look like, and what lift and drag performance would be needed to pull it off?

Improve this question. Bob Jacobsen Bob Jacobsen Which I think fights against the "stay high for a long time" goal. Good paper here: emits. I was told that best managed total energy. If you're avoiding the engineering questions regarding materials, risk, etc. It's absolutely brutal to fight gravity for a lengthy period of time. Show 5 more comments. Active Oldest Votes. Or at least not easily.

However the "why" is a bit tricky. At low speed, you can survive going through the lower atmosphere so you can generate the lift aerodynamically However, in-between is where things get difficult. Improve this answer. ANone ANone 3, 6 6 silver badges 20 20 bronze badges. Add a comment. But it would be extreamly expensive. Uwe Uwe He basically says the same thing as this answer, talking about the tyranny of the rocket equation and the high velocities involved.

Check out the article if you want a more in-depth and humorous explanation. I'm pondering the difference in heat between the heat of compression vs. Cooler, but "cool"? Not sure. That's roughly 7 times larger than the Saturn V Mg.

Calmarius Calmarius 1 1 silver badge 5 5 bronze badges. We're home to five men's and six women's athletics teams and a variety of intramural sports opportunities. Keep up with Union University events on campus and student, faculty and alumni engagement around the world. Site Map Employee Directory. Re-entry is a particularly dangerous time for the shuttle, a time during which the shuttle experiences tremendous stress and high temperatures.

There are actually two different phenomena at work to heat the shuttle, compressive heating and friction. Perhaps you have noticed when you use a bicycle pump that the fitting at the end of the pump gets very hot, very quickly. That heat comes primarily from the action of your muscles pushing on a plunger and compressing the air in the pump. When air or any gas, for that matter is compressed it heats up; conversely when it expands it cools.

Now consider the re-entry of the space shuttle or the fall of a meteor through our atmosphere. Initially, the shuttle moves around Earth in the emptiness of space at a tremendous speed. The astronauts slow down by firing some thrusters and gravity begins to pull the shuttle to a lower orbit. 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.

It inflates before it enters the atmosphere see patent for details , and rather similarly to the JP Aerospace idea it decelerates slowly in the upper atmosphere, so generating much less heat because of its low ballistic coefficient.

They hope it can be used for Venus, and also Titan, and possibly Mars. It would only descend as far as the Venutian upper atmosphere, at the cloud tops, where temperatures and pressures are the same as on Earth.

The cloud tops also have natural protection from cosmic radiation, and nearly all the ingredients for life. Indeed there are suggestions that it could be a good place for humans to settle outside of Earth. Some astrobiologists think there may be life in the upper Venus atmosphere, already, which could have migrated there long ago when Venus was more habitable. The Russians are interested to search for this life, and may include an unmanned aerial vehicle, possibly VAMP in their Venera D mission to Venus in the mid s.

So it could also be used for Earth re-entry. It might be useful for surveillance, photographing the Earth from above, and also for scientific studies of the upper atmosphere.

Can this be improved on? This absorbs most of the heat, all the way through the early stages of re-entry, until the spacecraft is traveling slowly enough to drop the aeroshell and deploy parachutes.

The spacecraft hits the atmosphere at many kilometers per second, so there is a lot of heat to dissipate. The main methods they use to keep the temperatures within reasonable bounds are:. They reached rather higher temperatures for the Apollo return from the Moon, as their re-entry was at a higher velocity.

It is useful, as it is also good at conducting heat and electricity. Titanium and zirconium diboride have similar properties. You might wonder why the astronauts returned from the Moon at such a high speed. After all, it orbits the Earth at a speed of only 1. If you come back in a transfer from the Moon you hit the atmosphere at Starting from a higher orbit just makes things worse for you. The one thing you can do to help with re-entry speed is to orbit the Earth in the same direction that it spins.

So if your satellite is orbiting in the same direction as the Earth in an equatorial orbit, West to East, it has 0. This makes re-entry just a little easier.

An orbit in this direction also makes the launch easier. You need around 0. This is why it was such a major gaffe for the Gravity film when it showed all the orbital debris orbiting Earth from East to West, as Neil deGrasse Tyson tweeted.

But you can achieve a much gentler re-entry using a ballute - a cross between a balloon and a parachute. It works like an aeroshell but decelerates much higher in the atmosphere.

It combines some of the approaches of the previous ideas. The space engineers in the early s explored many other such ideas detailed here: Rescue.

Some seem rather hair-raising, including the Paracone —the astronaut just sits in a seat, with their back towards the Earth, and aims towards the center of a large continent, as its margin of error is kilometers.

When it re-enters, then a large inflatable aeroshell deploys with a crushable cone. There is no parachute—it relies on the aeroshell crushing during landing to protect the astronaut. The astronaut has an inflatable aeroshell stowed away in the seat. During re-entry this deploys. Unlike the Paracone, you do have a parachute as well, for the landing. This is another idea originally developed for Gemini in the early s. For a while, before they settled on the familiar parachutes, the engineers thought that after the fiery stage of re-entry, the capsules would glide down to Earth beneath a parasail or paraglider.

Those tests were quite promising, though they ran into many issues; for instance, getting the glider to unfold. Eventually this line of research ended in when they changed to the parachutes as used by Apollo. The Russians also used parachutes for the Soyuz flights. For details of the paraglider research, see: Coming Home.

Anyway, at around the same time in , the engineers came up with the idea of using the same paraglider approach to go all the way from orbit right down to the surface, without an aeroshell. It could be folded up into a small cylindrical package that would be kept docked to a space station, much as our modern Soyuz TMA is. In an emergency, the crew enter this cylinder, and separate.

The paraglider then inflates and deploys. It would re-enter at an angle of 1 degree, with the paraglider angle of attack of 70 degrees. It would approach the speed of sound at 43 km altitude, and from there it would be able to glide km horizontally before eventually landing.

The Spaceship-One uses a different idea for re-entry. This is only for a sub-orbital hop at present. The first demonstration of the feather system was in The Virgin Galactica crash in was a result of the pilot accidentally unlocking the feathering system too soon.