Why do space ships require heat shields etc. to slow them down during re-entry?
Why not just make a slower re-entry so the heat doesn't get too unmanageable and has time to radiate/conduct off?

The heat shields aren't to slow them down, the heat shields are so the vehicle actually survives the process of slowing down. The only practical means we have of getting a vehicle out of orbit is getting low enough for atmospheric drag to decelerate the vehicle, and when you're going from a few kilometers a second to a few meters per second that's going to generate a fuck ton of heat.

>>16715176 (OP)
Heatshield is lighter than the extra fuel required to do a powered descent.

>>16715194
What about massively increasing the profile to weight ratio? Make it a huge, lightweight chubby bastard.

>>16715176 (OP)
>slow down re entry
that would require better engineering that nobody wants to do right now for some reason

>>16715199
nobody said anything about powered descent

>>16715211
That's the only alternative to unpowered descent, for which you need a heatshield to not blow up.

>>16715212
no it is not libtard there are various ways of slowing down re entry that do not include powered descent

Why don't they use a giant balloon and a giant parachute?

>>16715194
>>16715199
>>16715212
Yeah but why not just descend more slowly? So slowly that the atmospheric drag and whatnot are lower and the speed gets reduced more gradually, slow enough that the of the craft doesn't overheat even without shielding.
Compare a cannonball vs glider doing descent

>>16715300
>Compare a cannonball vs glider doing descent
Maybe a too small angle to atmosphere bounces the ship out of any entry path and starts an unpredictable "rebouncing" round the globe.

>>16715318
With every bounce you bleed speed.
Besides, if you have any control authority you can stop the bouncing with minimal effort applied.

>>16715206
calculations show that a ballute is less mass-efficient than a heatshield
the smart question to ask is why not decelerate in several steps

>>16715300
Because the fuel to do so would weigh more than the heatshield.

>>16715300
in order to orbit you have to be going like 8 kilometers per second, nothing reentering from orbit could start out slower than that. You could descent slowly using rockets, but it would take just as much energy as it took to get you into space so you would need to launch the rocket you used to get into orbit... into orbit. Slamming into the atmosphere and letting it do all the work is a much easier way to get rid of all that speed.

>>16715300
>Yeah but why not just descend more slowly?
The descent isn't the speed you're really worried about, even in the absence of an atmosphere your top speed falling from 100ish km straight down into the ground would be like 1.5 km/s tops. Factor in the atmospheric resistance and you'd likely never go above a few hundred m/s in a straight freefall.

The problem is you're not descending vertically from rest, you're booking at about 8 km/s tangent to the surface of the Earth to stay ahead of your freefall towards the surface, and that's the delta-v you need to deal with to start actually descending. The only way you could do it without aerobraking would be to carry a metric shitton of fuel with you and burn the lot to deorbit. We can barely get shit into orbit with the fuel-to-dry mass ratio of rockets now, there's no way we could increase it that much and still be able to carry any kind of meaningful cargo or personnel load.

>>16715176 (OP)
An inflatable heat shield can be bigger so you go slower

>>16715360
there's no way in hell that's rotationally stable

>>16715331
What fuel? What fuel does a glider use to do a controlled slow descent?

>>16715347
The atmosphere is not a brick wall, it starts gradually and becomes more dense the deeper you go in.
Why not just gradually slow down at the very top of the atmosphere where there is so little air that the heating is not too much?

>>16715354
Yeah but why not do aerobraking more slowly?
Like a glider gliding on the very top of the atmosphere gradually coming lower and lower as your speed reduces.

>>16715381
I imagine there are probably solutions where you could come in super *super* shallow and slow down very gradually up until the last bit of the flight... but I'll also bet it probably means a reentry takes days or weeks instead of minutes.

>>16715383
It's a complex problem to tackle because as both your velocity and atmospheric density change over the course of reentry, your drag physics are also shifting around, going from scaling linear with velocity to quadratic, from non-turbulent to turbulent, and from one drag profile for a shape behaving one way to another.

It's complicated enough that it's just simpler to slap some ablative material on the underside of a vehicle.

>>16715384
Well of course it would depend on how fast you can get rid of that heat, pretty slow with the lower surface temps, but it seems well worth it considering how dangerous re-entry is.
Especially if your craft is supposed to be reusable.

>>16715386
>too hot
pitch up
>not hot enough
pitch down

Where's the complicated bit?

>>16715389
>too hot
>pitch up
>not hot enough
>pitch down
>too hot
>pitch up
>OH FUCK IT WENT UNSTABLE!
>OH FUCK WE'RE IN A DEATH SPIRAL!
>OH FUCK WE'RE BREAKING APART!
>TELL MY WIFE I L---
Play KSP, anon.

>>16715395
Bro just make it a simple aerodynamically stable shape like pic related
it's not rocket science

>>16715398
You're still thinking too hard

>>16715395
>>16715398
the sane option is to do two or more aerobraking passes, where the first one puts you in a very elliptical orbit around the target body
having an atmosphere to brake against is a luxury anyway, I don't know why modern day faggots like to complain so much, the guys who did Apollo would have had this licked before their liquid lunch, using only slide rules and bits of string

>>16715409
>I don't know why modern day faggots like to complain so much
It's their only well-developed skill

>>16715206
This actually works. A large enough Starship could re-enter on bare steel
>>16715327
Skipping causes a number of other problems

>>16715381
below a certain velocity you won't gain altitude, as you slow your trajectory inevitably curves down more steeply. You can kind of do what you were describing with aerodynamic lift though, "skipping" has been used on many spacecraft to reduce heat load.

>>16715436
the only one I'm aware of is longer trip time

>>16715409
>where the first one puts you in a very elliptical orbit around the target body
that requires having a much higher velocity at perigee, which means more velocity, which means more friction, which means more heat.

>>16715450
much higher than what

>>16715176 (OP)
>Why not just make a slower re-entry
orbits are fully determined by position and velocity
by slowing reentry, you change your velocity, putting you into a steeper dive, causing you to go through less atmosphere, such that you don't slow down enough before you get to 0 m altitude and hit the sea at 200 m/s.

>>16715176 (OP)
earth is just a little bit too big for this. Since earth's mass is this strong, it requires a lot of energy or speed to get into orbit.

To break the spacecraft you have two options:
> engine break
> aerodynamic break
turns out with our current technology we can't do anything useful in orbit and have enough fuel to do engine breaking, so the only way is aero breaking with heat shields.

Starship Flight 6 proved this is possible for the first time.

>>16715194
>The heat shields aren't to slow them down, the heat shields are so the vehicle actually survives the process of slowing down.
idiot, you're so obtuse you're avoiding OP's question, he's implying that there would be no need for heat shields if the descent through the atmosphere was slower and he's right

>>16715398
>simple
the pilot was not allowed to touch the controls during hypersonic flight because the controls would be reversed, there was nothing simple about it

>>16715438
>aerodynamic lift
exactly, there should be plenty of lift to control flight altitude when flying at multiple km/s

>>16715490
>steeper dive
just use your wings to generate lift and make it less steep

>>16715300
You’d by trying to glide down. Generating lift becomes ever harder with increasing mach, see Kuchemann equations. Best achievable lift to drag at orbital speeds is less than 1/1.
Play KSP with FAR and RSS and you’ll see what I mean.

auto pilot isnt good enough to make adjustments

>>16715643
Is this really what it comes down to? A shallow enough descend angle simply not being possible due to inefficient aerodynamics at high mach?
Kind of frustrating.

>>16715320
>With every bounce you bleed speed.
That's why it's unpredictable. You can come to steep and vapor your spacecraft to plasma

>Besides, if you have any control authority you can stop the bouncing with minimal effort applied.
But you have to and you do not even know where you ship is when over the pacific. No chance to know 3D direction and momentum. Spaceflight is very danger and you choose the most secure path. Methink random bouncing around the globe isn't that one.

>>16715633
LOL. The Shuttlification of Starship is complete.

>>16715633
>just use your wings to generate lift and make it less steep
You need to brake anyway and all momentum is transformed to heat. So your spacecraft will see extended heating time. Heatshield are useless then, the friction heat will creep through instead taken away with the vaporized material.

>>16715630
For each unit of lift you use to maintain altitude, you also experience a certain amount of drag. The amount of drag experienced per unit lift varies with the wing design, called the lift-to-drag ratio of the wing; more efficient wings incur less drag to generate the same amount of lift. But even the most efficient wing designs still produce more than enough drag for the lift required to stay airborne to require heat shielding.

>>16715860
Yeah but if the heating is low enough it could be radiated away maintaining a constant temperature.

>>16715176 (OP)
fuck, this anon has noticed the conspiracy.

>>16715633
but muh dry mass!

>>16715911
>Yeah but if the heating is low enough it could be radiated away maintaining a constant temperature.
Just a layman but friction heat is way more than radiated one. Further control over the cooling phases is very limited because the re-entry is a passive process ( enough fuel would avoids any meddling with friction but theer is no).

>>16715318
>muh atmospheric bouncing
nasa niggers are fucking retarded