/sci/ - Science & Math » Thread #11971914

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Anonymous Tue 04 Aug 08:17:11 2020 No.11971914 View Reply Original Report

Quoted By: >>11971972 >>11972014 >>11972033 >>11972823 >>11974726

Why does atmospheric entry have to happen so fast that you need huge heat shields to deal with the temperature? Why can't the entry vehicle just descend really slowly so it doesn't heat up?

Anonymous

Anonymous Tue 04 Aug 2020 08:44:14 No.11971972 Report

Quoted By: >>11971998

>>11971914
you can, but then you have to spend about the same amount of fuel as it took to get into orbit coming back down instead of just letting the atmosphere slow you down for free

Anonymous

Anonymous Tue 04 Aug 2020 08:52:47 No.11971998 Report

Quoted By: >>11972016 >>11972038 >>11972081 >>11974581

>>11971972
I'm not saying to use fuel, just glide down slowly like any airplane, they don't require fuel to reduce speed, nor do they need a heat shield because they don't decelerate so fast.

Anonymous

Anonymous Tue 04 Aug 2020 08:56:47 No.11972014 Report

Quoted By: >>11972115

>>11971914
because it's a feedback loop: the lower you are, the more the atmosphere slow your craft, slowing means you're lowering your orbit so you lose altitude, which means more braking, etc.
There's no stable position if you're going at 4km/s (orbital velocity is ~8km/s): if you're anywhere close to Earth, you're going to fall and you need to stop before you meet the ground

Anonymous

Anonymous Tue 04 Aug 2020 09:00:38 No.11972016 Report

Quoted By: >>11972115

>>11971998
You can't go slowly.
They're coming in at orbital velocities, at best you can change your angle of attack to reduce the rate you slow down.

Anonymous

Anonymous Tue 04 Aug 2020 09:10:30 No.11972033 Report

Quoted By: >>11972168

>>11971914
There is no better technique than playing Kerbal Space Program to understand why.

Anonymous

Anonymous Tue 04 Aug 2020 09:15:24 No.11972038 Report

Quoted By:

>>11971998
space shuttle would have done this if it were possible

Anonymous

Anonymous Tue 04 Aug 2020 09:42:44 No.11972081 Report

Quoted By: >>11972118

>>11971998
orbiting is fast, by the time you are in atmosphere thick enough to do any significant slowing or "gliding" you are moving ~30 000 km/h or more

Anonymous

Anonymous Tue 04 Aug 2020 10:04:03 No.11972115 Report

Quoted By: >>11972841

>>11972014
>>11972016
That's the part i don't get, couldn't you use wings to provide lift so you don't immediately plummet down and burn up even though your velocity isn't orbital anymore?

Anonymous

Anonymous Tue 04 Aug 2020 10:05:04 No.11972118 Report

Quoted By: >>11972150 >>11972663 >>11974595

>>11972081
I mean if the air resistance is strong enough to burn you to a crisp through friction shouldn't it be enough to give you plenty of lift?

Anonymous

Anonymous Tue 04 Aug 2020 10:06:22 No.11972121 Report

Quoted By:

rocks dont burn, right ?

Anonymous

Anonymous Tue 04 Aug 2020 10:25:33 No.11972150 Report

Quoted By: >>11975439

>>11972118
you dont need lift you just need to slow down

Anonymous

Anonymous Tue 04 Aug 2020 10:36:06 No.11972168 Report

Quoted By:

>>11972033
Seriously this game taught me more about orbital dynamics and kinetic energy than anything else.
I still rely on the planner though and can't plan my hand

Anonymous

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Anonymous Tue 04 Aug 2020 15:00:51 No.11972663 Report

Quoted By:

>>11972118
it does
you'll still have to deal with the heat regardless

Anonymous

Anonymous Tue 04 Aug 2020 16:09:53 No.11972823 Report

Quoted By: >>11975439

>>11971914
Do you understand how fast an orbital object is moving relative to the Earth?

You could hypothetically slow a rocket down before reentry but even then there will still be some reentry heating. You would need to launch your orbital capsule with nearly as much fuel as what it took to get into orbit in the first place though, and in the end you will need a much larger launch vehicle to get all that fuel up in space in the first place. It would be too expensive for too little gain, particularly when reentry heating is manageable with current tech anyways. A heat shield costs far less than having to launch an entire deorbit rocket into space strapped to the capsule.

Anonymous

Anonymous Tue 04 Aug 2020 16:15:16 No.11972841 Report

Quoted By: >>11974706 >>11976471

>>11972115
Here:
https://www.youtube.com/watch?v=HgTNzDCc0gk

Anonymous

Anonymous Wed 05 Aug 2020 00:02:29 No.11974581 Report

Quoted By: >>11975439

>>11971998
>they don't require fuel to reduce speed
How are they going to reduce speed then?

Anonymous

Anonymous Wed 05 Aug 2020 00:03:54 No.11974595 Report

Quoted By: >>11974706 >>11975439

>>11972118
But you don't want lift. You're trying to re-enter the atmosphere. You're going down.

Anonymous

Anonymous Wed 05 Aug 2020 00:15:48 No.11974706 Report

Quoted By:

>>11974595
That's not entirely true, you actually do want some lift so you don't reenter to fast and decelerate so quickly the G forces kill you.

See >>11972841

A good example of this actually is the Falcon 9, which more or less does what OP suggests, albeit not from orbital velocity. The G forces a Falcon 9 undergoes during it's reentry and landing are several magnitudes greater than what would be safe for a human. If a person was on a Falcon 9 landing they would pass out because the atmospheric deceleration is way more intense than a controlled capsule reentry where the capsule slowly eases back down into the atmosphere.

Anonymous

Anonymous Wed 05 Aug 2020 00:19:49 No.11974726 Report

Quoted By: >>11975439

>>11971914
>object going around the earth fast
>enters atmosphere
>parachutes and front thrusters hit the brakes
>people inside hit the walls at escape velocity and turn to modern art

Anonymous

Anonymous Wed 05 Aug 2020 03:40:34 No.11975439 Report

Quoted By: >>11976471

>>11972150
>>11972823
>>11974595
>>11974726
What I'm suggesting with the lift part is that the craft keeps itself high up in the thin atmosphere with its wings until its speed has slowed down enough so that it can safely descend to the lower and denser atmospheres.
>>11974581
Are you really asking me how airplanes reduce speed if not through the usage of fuel?

Anonymous

Anonymous Wed 05 Aug 2020 11:17:50 No.11976471 Report

Quoted By:

>>11975439
>What I'm suggesting with the lift part is that the craft keeps itself high up in the thin atmosphere with its wings until its speed has slowed down enough so that it can safely descend to the lower and denser atmospheres
Again, see >>11972841
That's literally what manned space vehicles already do. Even in the thin upper atmosphere the friction is so great because of the speed that reentry heating occurs.

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