Reentry without a heatshield possible on Earth?

[BrickedKeyboard]

I have the "deadly reentry" mod installed. This seems to more or less realistically burn your rockets up if you try to enter at too steep of an angle without a heatshield.

However, I was coming back from the Mun with a couple of goo cannisters, and I had not yet unlocked a method to shield them from reentry heat. So I made my periapsis 33 km (discovered after some trial and error) and found that I could skim the upper atmosphere, bleed off some speed, and not burn up. I still had a rocket motor and a couple of fuel tanks attached as well at this point.

I made two passes, until the friction had moved my apoapsis into the lower atmosphere, and found I was now going slow enough that I wouldn't burn up.

Now, I thought about this maneuver on paper, and I realized that skimming the upper atmosphere without a heatshield is realistic. Technically, low flying satellites are doing this all the time.

However, when you do this maneuver, in the best case scenario, you end up in a suborbital trajectory headed towards the planet at several thousand m/s. (because on your last pass through the atmosphere you lost as much energy as you could, and now you are going to be headed in)

Is this survivable on earth under any circumstances?

I realize that kerth has a smaller atmosphere, and you have less kinetic energy in low orbit. When I right click on the ship parts during this kind of reentry, I can see that they are heating up quite a bit - the rocket motor nozzle peaks at 1100 C, the fuel tanks at about 400 C. However, deadly reentry does not consider this hot enough to destroy these objects, and I can see that, spacecraft aluminum doesn't melt at 400C. However, there's also a force on the thin aluminum skins of a fuel tank, and DR doesn't seem to model that.

So, what's the deal?

[sal_vager]

From orbit? I don't think so, you have too much speed to lose.

But people have fallen from near space without needing heat shields, look at Felix Baumgartner's recent skydive http://www.space.com/14928-space-jump-supersonic-skydive-practice.html

Even in KSP, if you cancel your horizontal velocity before you enter the atmosphere the heating effects are less severe, I've no idea if it could work for real but maybe it could, someone smarter than me would have to run the numbers.

If you could cancel your horizontal velocity and be draggy enough so you didn't build up too much speed as you fell, then you wouldn't have the same heat build up as a craft re-entering, the heat is cause by compression of the atmosphere in front of you, not by friction.

[Kasuha]

On Earth, SpaceShipOne is supposed to reenter without heat shield.

Similar approach is possible in KSP. Or you can thrust downwards to keep your altitude while the atmosphere slows you down. Something like powered landing, but instead of landing you just avoid entering thick layers of atmosphere too fast.

[Tiberion]

Yeah, heat damage is caused by friction with the air, so if you're going slow enough you can keep the heat from reaching critical. You could do this with extended aerobraking in the very upper reaches, or you could do this by retroburning to keep your speed down as you enter.

The reason neither of these are popular in real uses is they require extra resources to be onboard, either in extended food, water and oxygen for the crew to live longer making multiple aerobraking orbits, or with enough fuel to burn the retro-engines long enough to cancel off all that orbital velocity (basically the opposite of how much it required to launch, since you'll have to fight gravity on the way down at least until you're past the danger zone where being in freefall would speed you up enough to cause too much friction again.

[Kilmeister]

While not very practical, a powered descent would fit your criteria.

[Kasuha]

Yeah, heat damage is caused by friction with the air

Nope. It's caused by air compression.

If you fall into water from great height, you will die not because your friction with water is too high but because the water is simply too heavy to move aside fast enough.

[goldenpeach]

Yeah, heat damage is caused by friction with the air, so if you're going slow enough you can keep the heat from reaching critical. You could do this with extended aerobraking in the very upper reaches, or you could do this by retroburning to keep your speed down as you enter.

[...]

Heat damage is done by air compression,even if more air compression usually result in more friction.

[...]

But people have fallen from near space without needing heat shields, look at Felix Baumgartner's recent skydive http://www.space.com/14928-space-jump-supersonic-skydive-practice.html

[...]

He jumped from 39.45KM, at that altitude it's the stratosphere,not space.

It's still hight tough.

To answer your question, it is possible to make a rocket that doesn't need a heat shield.

However, it would required a lot of fuel(if you decide to use engines) so it's simpler(and more effective) to use a heat shield and let the rocket fall trough the atmosphere.

You might be interested by this article from XKCD:

http://what-if.xkcd.com/58/

[Mr Shifty]

On Earth, SpaceShipOne is supposed to reenter without heat shield.

Similar approach is possible in KSP. Or you can thrust downwards to keep your altitude while the atmosphere slows you down. Something like powered landing, but instead of landing you just avoid entering thick layers of atmosphere too fast.

SpaceShipTwo (One is retired) will be doing suborbital hops only, so won't have to bleed off orbital speeds.

Think of it this way. A vessel in orbit is going about 7500 m/s. The amount of energy that a vessel has is its potential energy (due to its height) plus its kinetic energy (due to velocity.) Use a Mercury capsule (one of the lightest crewed spacecraft ever at 1.4 metric tons) as the example. The potential energy is approximately mass_vessel*g*height = 1400kg*9.8m/s²*3e5m=4.1GJ. The kinetic energy is 0.5*mass_vessel*velocity²=39GJ for a total of about 43GJ of energy that has to be converted (to heat) in order to land the vessel. If you're not going to use rockets to slow down (and it seems wasteful to spend fuel when the atmosphere is capable of doing it), you have to have a heat shield to either carry the heat energy away (through ablation) or absorb the heat while keeping the vessel cool (through insulation.)

Edited November 7, 2013 by Mr Shifty
Fixed my math

[Bunsen]

Reentry heating depends quite a lot on how much surface area you have per unit mass. Large, dense objects require a lot of dynamic pressure to decelerate from orbital speed, and that means lots of compression and high surface temperatures. Small, lightweight objects can do it with lower pressures and temperatures. There have been a couple research projects on developing large inflatable heat shields, which don't need to survive quite such severe heating as their compact counterparts. Some tiny objects (I think the little circuit boards on KickSat are close) can survive reentry without any special shielding.

[PakledHostage]

On Earth, SpaceShipOne is supposed to reenter without heat shield.

What's that they say? Never let the facts get in the way of a good story?

To add to what MrShifty said, re-entry heating is really just all about conservation of energy. A spacecraft in LEO has an immense amount of potential and kinetic energy that must be dissipated during re-entry. The majority of that energy is kinetic energy. An object orbiting the Earth at 400 km has roughly 4 MJ/kg of potential energy and a whopping 30 MJ/kg of kinetic energy relative to an object on the surface. This is why SpaceShipOne can re-enter without heatshields. It has very little, if any, kinetic energy at the top of its ballistic arc. It needs to dissipate less than 2 MJ/kg, whereas the space shuttle needs to dissipate upwards of 34 MJ/kg. An object returning from the Moon has to dissipate close to twice as much energy as an object in LEO, and the Stardust probe's sample return capsule entered the Earth's atmosphere with over 80 MJ/kg!

An aggressive aerobraking pass will cause significant heating and will require a heat shield. The most aggressive form of aerobraking is aerocapture. In the case of aggressive aerobraking, heat dissipation is a further problem because there's no convection in space to cool the shield between aerobraking passes. Radiation is the only cooling mechanism and it isn't as effective as convection. Ablative heat shields designed for multi-pass aerobraking/aerocapture must also tolerate repeated exposure to elevated temperatures without failing or burning through.

The multi-pass aerobraking followed by aerocapture process described in the OP works in KSP because the current re-entry heat model doesn't take into account ablation of heat shields, and the unshielded spacecraft components are more tolerant of high temperatures than real life spacecraft.

Edited November 7, 2013 by PakledHostage
Fixed a number

[Tommygun]

Well if you had materiel strong enough, I guess you could in theory, build a ladder to geosynchronous orbit, park your ship next to it and take the ladder down.

[Lohan2008]

Thanks for the OP, that was an interesting discussion about re-entry theories > I really am a geek.

[K^2]

As some people have mentioned or alluded to, re-entry without heat shields is entirely possible, depending on maximum altitude attained. Re-entry from orbital flight, however, is not. A drop from 100km is only going to have 1.3km/s worth of energy in it. You don't need heat shields to survive that. Re-entry from LEO, on the other hand, is going to be on the order of 7km/s. That's a whole different game.

This does suggest one way to return from orbit without heat shields, however. Just do a retrograde burn to kill all your orbital velocity and drop almost straight down into the atmo. Problem is that the fuel required to do that is going to be heavier than a heat shield.

[Tech Support]

From orbit? I don't think so, you have too much speed to lose.

But people have fallen from near space without needing heat shields, look at Felix Baumgartner's recent skydive http://www.space.com/14928-space-jump-supersonic-skydive-practice.html

If you could cancel your horizontal velocity and be draggy enough so you didn't build up too much speed as you fell, then you wouldn't have the same heat build up as a craft re-entering, the heat is cause by compression of the atmosphere in front of you, not by friction.

this video should provide some illumination to this debate

In the end he does suggest that if we could could a very angled trajectory we could do without heatshields

Edited November 8, 2013 by Tech Support

[K^2]

In the end he does suggest that if we could could a very angled trajectory we could do without heatshields

Nah. I did the math on that once. It's a good idea on paper, but by the time you have enough lift, you're in too dense an atmosphere moving too fast to survive without a heat shield.

You might be able to do something with a powered descent, but all of the "simple" things I've tried result in you burning as much or more fuel than you'd need to simply kill your orbital velocity and drop down.

[drakesdoom]

So right now everyone is talking about 1 shot ships and using a heat shield because it is lighter.

What if we wanted a ship that could land multiple times from orbit in multiple locations on multiple bodies in locations where replacing a heat shield is not an option but refueling is.

[Thiel]

Simple, make a tougher heatshield and pay the weight penalty

[Tommygun]

So right now everyone is talking about 1 shot ships and using a heat shield because it is lighter.

What if we wanted a ship that could land multiple times from orbit in multiple locations on multiple bodies in locations where replacing a heat shield is not an option but refueling is.

Well transporting a heat shield to the landed ship is still easier than transporting the heavier and explosive fuel to it.

There aren't that many places that produce rockets fuels around the world. If you want space ports, then you need to pre-place the necessary parts and infrastructure to do it.

Just look at that big Airbus A380. It can't just go to any airport, it has to go to airports that have been prepared for it.

[K^2]

What if we wanted a ship that could land multiple times from orbit in multiple locations on multiple bodies in locations where replacing a heat shield is not an option but refueling is.

It is an option. I don't think anyone is saying it is not. You are not going to do worse landing than you are taking off. In fact, you can look at LM as an extreme case. Lithobraking wasn't an option, so they had to burn fuel to land. Same deal with aerobraking on re-entry.

The problem, of course, is that it takes an enormous amount of fuel to do powered landing on Earth. As I've mentioned above, I haven't come up with anything that improves dramatically on simply killing (almost) all of your orbital speed. And that's 6-7km/s worth of fuel. When your typical rocket is capable of something on the order of 10km/s total, adding 6-7 more on top of that is insanely challenging. Right now, it's simply cheaper to have craft that are either not re-usable at all, or require servicing of the heat shields. And we can't do better with chemical fuels.

We are going to have to find a completely different kind of fuel/propellant for our ships before we start looking at a heat shield as too much hassle. MSMH could do it, but we don't know if that mythical substance even exists. Much less how to produce it in necessary quantities for cheap. At higher energies are the core electrons, which nobody knows how to work with, or nuclear. Fission or fusion are going to be very troublesome for small ships, but maybe we don't have to go small. There is also some promise in nuclear isomers. Since there is no decay involved with these, the only radiation you have to deal with is gamma, making for a much cleaner reactor. Nuclear isomer batteries are also rechargeable, at least in theory, but that might not be worth the trouble. At any rate, there are some options in power generation.

The biggest limitation with alternative power sources is getting decent TWR. If you want to do powered landing, you don't need quite as high TWR as you do for liftoff, but anything significantly less than 1 is going to be problematic. The best we've got is NERVA, and that only gives you about a fifth of its own weight in thrust. You might be able to do powered landing on one of these if that's all you are carrying. But you aren't lifting off on that. And once you add everything you need for liftoff, the ship's too heavy for powered landing on NERVA. (Naturally, I assume that multi-stage defeats the very purpose of building something that can be simply refueled.)

So in short, yes, powered landing can replace heat shields. Yes, it would be very useful to have that capability. But no, we don't have anything even close to the tech you'd need to make it happen.

[Savage117]

So right now everyone is talking about 1 shot ships and using a heat shield because it is lighter.

What if we wanted a ship that could land multiple times from orbit in multiple locations on multiple bodies in locations where replacing a heat shield is not an option but refueling is.

This made me think, what other ways could one land a ship that still has to deal with aerobraking without using an ablative heatshield? For instance, lets say our ship is interstellar and has established orbit around a Earth-like planet, we only have one lander and that lander needs to be able to land, takeoff, and land again in another location from orbit, say because it needs to refuel at the mothership. If its a large lander, which it will be due to the needed fuel to make a powered landing on Earth and takeoff, an ablative shield will also have to be large and heavy, and storing several of those shields on the mothership would be impractical for many reasons, most of them due to safety concerns. So how would you do it? I'm no expert so these ideas I'm gonna throw out are just brain storming. We have materials that could easily survive the heat of re-entry without damage, the problem is they wouldn't dissipate the heat very well, basically cooking our poor astronauts before they got to the ground. So how bout active cooling? Or maybe passive cooling methods like Heatsinks, these are usually heavy things especially if they needed to dissipate heat quickly, but they might work, right?

Second theory: since sky diving from orbit would turn you into a crispy critter without a heatshield. Could you not just send a rocket straight up to 300km without establishing orbit, jump out and fall straight back down to Earth without deadly re-entry heat. Or would the rotational speed of the planets atmosphere be enough to cook you on the way down? Just a thought.

[K^2]

Second theory: since sky diving from orbit would turn you into a crispy critter without a heatshield. Could you not just send a rocket straight up to 300km without establishing orbit, jump out and fall straight back down to Earth without deadly re-entry heat. Or would the rotational speed of the planets atmosphere be enough to cook you on the way down? Just a thought.

300km is pushing it. Not because of Earth's rotation, but because a fall from 300km is going to get you going at nearly 2km/s by the time you enter thick enough atmosphere to start slowing down. But what use is that, anyways?

[Savage117]

300km is pushing it. Not because of Earth's rotation, but because a fall from 300km is going to get you going at nearly 2km/s by the time you enter thick enough atmosphere to start slowing down. But what use is that, anyways?

Never said it would be useful, just awesome. I thought smaller objects create less compression, so would a man sized object be able to survive 2km/s worth of aerobraking? Thats 1/4th the speed of a spacecraft returing to Earth. How fast do you need to be going to get deadly re-entry heat anyway? When I thought about this I was thinking of Star Treks space diving that was briefly mentioned in several episodes and shown in the new Star Trek movie. After reading this and playing KSP I know that was a bunch of movie magic, handwavyness and impossible as shown in the movie, but could it actually be possible under the right circumstances, was my thought?

[K^2]

There are two regimes. First in upper layers of the atmosphere, where density is very low. The in-falling object will collide with individual air molecules there. There is no hydrodynamics. Every molecule on your path you are going to hit. Here, the shape is absolutely irrelevant. Heat generation can be quite severe, but falling straight down you also pass this region pretty fast. Then you hit thicker atmosphere where air starts to behave as a fluid. The in-falling object generates a shock wave, and that helps reduce heat transferred to the body. On the other hand, air is denser, so way more energy is dissipated here. If there is a problem, it will happen here. And yeah, shape and size will matter. Slow down too gradually, you burn up. Slow down too fast, G-forces will get you. The later shouldn't be a problem for a falling human. The heat, though, I don't know. This is Mach 6, so it's going to be bad. But it could be brief enough to be survivable.

[BrickedKeyboard]

This is Mach 6, so it's going to be bad.

Indeed. Hope he's wearing an armored spacesuit with armored underpants.

[Simon Ross]

It really depends on what you define as a heat shield. On most traditional capsule type re-entry vehicles it has been an ablative type heat shield. On the Shuttle it was ceramic charring heat shield. In reality, there are a number of metals you can use as a heat shield, all of which will dissipate heat faster then they will absorb it during re-entry and are re-usable. Only problem is, most of them are very heavy.