[notact]

Can someone explain why reentry must be so hellish? The energy gained during the rocket burn into orbit must be bled off during reentry, and that energy is enormous. However, why must reentry occur so quickly? It seems if the descent into the atmosphere was slower, the heat shield would be able to radiate the heat energy away more effectively, thus lowering skin temperatures, and significantly reducing the engineering challenge.

[magicalhippo]

I tried what you said in the most realistic simulator we have, Kerbal Space Program, assuming like you that a gentler approach would be better. And I learned that no, that most certainly is not better.

What you need to protect is on the inside of the heat shield. Heat conduction is based on temperature difference and time[1] and the conduction of the material[2]. Since the heat shield tiles have a very low thermal conductivity, it takes a long time for significant heat to pass through.

Yes a more aggressive approach will lead to a greater temperature, but it'll also provide significantly greater drag, thus the the extreme temperatures only exist for a relatively short amount of time, and thus it doesn't have time to pass through the tiles and heat up the inside.

A very shallow approach has significantly less drag, and you spend significantly longer slowing down. The temperatures might be a fair bit less, but the much longer time spent decelerating means it has a chance to make it through the heat shield tiles.

It's not entirely unlike iron meteorites which can still be cold when landing, as they only spend a brief time in the atmosphere[3] and thus don't have time to heat up.

[1]: https://en.wikipedia.org/wiki/Heat_equation#Interpretation

[2]: https://en.wikipedia.org/wiki/Thermal_conductivity_and_resis...

[3]: https://earthscience.stackexchange.com/questions/127/what-te...

[ActorNightly]

What happens if you do a geostationary deorbit? I.e straight down.

[dotnet00]

You need to balance peak heating and heating duration. Shallower reentry means lower peak heating, but higher heating duration. Steeper entry means higher peak heating, but lower heating duration.

The heat shield material can handle a certain amount of heat and a certain maximum temperature before it starts to ablate away, so you're forced to thread the regime where both variables are within its tolerances.

[saratogacx]

Scott Manley has a good video looking at this question and goes into a bit of a dive into the physics and engineering issues involved

https://www.youtube.com/watch?v=5kl2mm96Jkk

[CarVac]

To get a gentler reentry, you need a greater lift-to-drag ratio.

To have a better hypersonic lift-to-drag ratio you need significantly more wing area, which is dead weight (and drag and a control problem) on the way up.

[avhon1]

Exactly! Re-entry is the transition from orbital dynamics to aerodynamics. If you want the transition from orbit to flight to occur at a lower speed, then you need to be able to produce lift equal to your weight at that speed, at the altitude where you will hit that speed.

[GMoromisato]

Not sure if you got an answer already, but the reason is gravity. Gravity pulls the ship down, so you have to move very quickly horizontally so that the curvature of the Earth starts falling away from you. For LEO that's something like 25,000 kph.

If you move slower, you are no longer in orbit and your trajectory will intersect the ground.

If you tried to slow down more gradually, your orbit would keep dropping until you suddenly hit the ground.

Think of it this way: orbital speed is the speed required for a ship to stay in orbit without thrust. If you had infinite thrust, you could land on the ground at any speed you wanted. But without thrust, you have to go from orbital speed to 0 in less than one orbit.

[jtriangle]

If you want to do slow star-trek style landings, you need star-trek level tech. Namely, propulsion tech that doesn't exist.

That doesn't mean that it's impossible, just means that it'd require things that don't exist yet.

Worth mentioning that, additionally, reentry heating isn't a huge problem, and you're not going to create new propulsion tech to counter it, you're just going to make better heat tiles. What you need new propulsion tech for is doing expanse type stuff, where you can accelerate for months at 1G so you essentially have artificial gravity and can get places extremely fast. If you're into sci-fi, the show/books "The Expanse" goes into what that looks like in practice fairly well.

[PaulHoule]

A positive way of framing it is that atmospheric recently is free. If the Earth didn’t have an atmosphere it would take just as big a velocity change to land as it does to get into orbit and getting to be orbit would be as hard as an interplanetary flight. It's worse than it sounds because the rocket equation has a logarithm in it...

https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

double the Δv means you square the mass ratio. The space shuttle had a mass ratio of about 16, a mass ratio of 256 would be absolutely insane.

You get this velocity change at the cost of dealing with the heat and all but a tiny fraction of that heat ends up immediately in the atmosphere.

[garaetjjte]

If you are coming at higher speed eg. from the moon, then it's possible to slow down to get reentry equivalent to low earth orbit one. But you can't really slow down much more because you would just plunge into atmosphere at steeper angle. Some vehicles utilize skip reentry trajectories, where it does high altitude pass through atmosphere and then goes in second time: https://en.wikipedia.org/wiki/Non-ballistic_atmospheric_entr...

[bagels]

It would take a tremendous amount of fuel to do what you're imagining, probably to the point of making the craft impossible to build with current technology.

Your orbit would have to be high enough to do a burn to cancel your orbital velocity (lots of fuel), then you have to burn against gravity for a slow vertical descent (lots of fuel). The rocket equation says... you'll need a larger craft and more fuel to carry the extra fuel in to orbit. It gets pretty out of hand.

Instead of using fuel to slow down, spacecraft make a small burn to have the orbit intersect the atmosphere, and then use drag instead of fuel to slow down.

[notact]

I'm not sure why people are misunderstanding my question as "Why not bring more fuel and burn the rockets in reverse". I am simply asking: why not reenter the atmosphere at a shallower angle, spreading the atmospheric braking friction over a longer period of time, which I'd expect would allow more time for the accumulated heat to radiate away before it becomes catastrophic.

[friend_and_foe]

So, speed of reentry is directly a consequence of surface area (or energy expended by fuel to counter, in the case of the booster which does not use friction with the atmosphere to slow down) of one side. You'll produce the same amount of heat (and sound, light, but let's keep the model simple so we can understand better) no matter how fast you come in and no matter how wide your surface area is (assuming the same mass), it's just the thermal properties of the material and the surrounding environment dictate how quickly that heat dissipates, and the surface area determines how distributed the heat is, and the speed it's entering determines how quickly the heat is generated.

So to slow down more evenly and have less heat at the max point per square inch, you need wider surface area (or you need to expend fuel firing engines in the opposite direction of travel, what both parts do at the end to slow to 0, and a problem due to the rocket equation, fuel has mass and so increases the amount of kinetic energy you must dissipate), and that means more mass and more engineering and a bigger vehicle. The goal ultimately is of course optimizing all these variables.

[yalue]

The velocity of a spacecraft in low earth orbit is over 15,000 miles per hour. Smashing into the atmosphere is perhaps the most fuel- and cost-efficient way to slow down to a speed at which landing is possible.

[93po]

It doesn't really answer the question though. Why not descend slower so that the 15k MPH isn't meeting so much air? And bleed it off much slower so there is less heat

[tsimionescu]

It's because slowing down from 26000 km/h to something that wouldn't cause extreme heating (say, 1000km/h) would only be possible by firing thrusters in the opposite direction. Otherwise, any contact with a medium that could slow you down would lead to the same extreme heating.

And of course, the thrusters you'd need would add huge complexity for the shape, and need extra fuel in the stage 2 itself, greatly reducing its cargo capacity.

[stevage]

If you're reentering from say the moon, you don't really have a choice. If you reenter too slowly, you won't end up landing at all, and will skip out the other side. Or you would have to do multiple passes, which would take days to weeks.

[tocs3]

They use the atmosphere to help slow the ship down. It takes most of the tank of fuel to get up there and moving so fast. It would take most a tank to slow down. So, they would need about double the fuel plus some for landing.

P.S. I have not done any of the math (I might be able to figure it out but it might take a week or two to figure it out).

P.S.S : Maybe if they could refuel in space efficiently (asteroid mining?) it might be worth looking at but it will be a while before I would expect anything like that. It would just be the ship.

[notact]

I understand the atmosphere is used to slow the vehicle - it's basically free brakes that you don't have to carry with you. I never suggested using rockets in reverse to slow the vehicle down. What I am asking is, instead of effectively standing on the breaks and generating enormous amounts of friction in a short period of time, why can't the vehicle ease onto the breaks and spread the friction out over time so it can be more safely dissipated (via a more shallow reentry angle).

[bagels]

Its more than double.

[chasd00]

Slowing down from Mach 20-something takes a huge amount of energy in its own right.

[robertsdionne]

You are not riding the atmosphere down, the atmosphere is riding you.