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Author Topic: Why Can't A Rocket Just Drop Down Nice And Slow ?  (Read 2973 times)

Offline neilep

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Dearest Re-entryologists,

As a space sheepy I of course luff buzzing round the Uni freaking out isolated communities by appearing out of nowhere and wearing my ' blurry camouflage ' so that any pictures/film/video taken is always fuzzy !

Having watched lots of films I always follow suit and land back on Earth after negotiating a rapid re-entry angle thingy. It was always get me hot though....So....take a look at my proposed new re-entry flight plan thingy



My Proposed New re-Entry Flight Plan Thingy Just Moments Ago.

......looks great yes ?

I'm just gonna hover over where I want to land and drop down nice and slow and use the thrusters built into my Pradas. This will solve all that re-entry malarky won't it ?

If it's a no-go though....why so ?....Why could not any space-mobile just go into a Geostationary orbit and drop down nice and slow  ?


You're gonna ruin my plan aren't ya ?



Hugs and shmishes


mwah mwah mwah



Neil
Landing Gear Down
Lands On Neighbours Roof at 3am !
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx









 

Offline wolfekeeper

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Re: Why Can't A Rocket Just Drop Down Nice And Slow ?
« Reply #1 on: 24/02/2012 14:02:15 »
You can do this to some degree.

Reentry is to do with 'ballistic coefficient'  - how heavy it is per unit area, and lifting coefficient (for these vehicles a lift/drag ratio between 0 and about 4).

Basically, if you have a light, big vehicle with lots of lift (which is hard to do at reentry speeds) then you can take much longer to reenter, but you reenter at higher altitude. It turns out that the air temperatures are higher, but the skin temperatures you face when you do that are lower.

But there's a price to be paid.

Although the peak temperature is a lot lower, the total heat soak is a lot higher because reentry takes longer; so the vehicle usually gets hotter- it has to deal with more heat energy!

If I remember correctly, Apollo reentries were fast and heavy, the Shuttle is also pretty fast, although takes a bit longer, the UK's Skylon spaceplane design is pretty slow.

But the idea that you could spend a LONG while reentering; you can't, after the reentry interface you're travelling at high hypersonic speeds, and the achievable lift/drag ratio there is about 1. If you could get say 15 you could spend hours reentering, and everything would be much better, but with a ratio near one, once you're past the reentry interface, you're coming down in half an hour or maybe 45 minutes, and you're down.

Thrusters aren't a lot of use either, reentry shields give you much, much MUCH better performance for any given weight.
« Last Edit: 24/02/2012 14:17:05 by wolfekeeper »
 

Offline Gordian Knot

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Re: Why Can't A Rocket Just Drop Down Nice And Slow ?
« Reply #2 on: 24/02/2012 14:20:17 »
You DO have heat retardant wool, right?????
 

Offline neilep

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Re: Why Can't A Rocket Just Drop Down Nice And Slow ?
« Reply #3 on: 24/02/2012 18:07:42 »
You can do this to some degree.

Reentry is to do with 'ballistic coefficient'  - how heavy it is per unit area, and lifting coefficient (for these vehicles a lift/drag ratio between 0 and about 4).

Basically, if you have a light, big vehicle with lots of lift (which is hard to do at reentry speeds) then you can take much longer to reenter, but you reenter at higher altitude. It turns out that the air temperatures are higher, but the skin temperatures you face when you do that are lower.

But there's a price to be paid.

Although the peak temperature is a lot lower, the total heat soak is a lot higher because reentry takes longer; so the vehicle usually gets hotter- it has to deal with more heat energy!

If I remember correctly, Apollo reentries were fast and heavy, the Shuttle is also pretty fast, although takes a bit longer, the UK's Skylon spaceplane design is pretty slow.

But the idea that you could spend a LONG while reentering; you can't, after the reentry interface you're travelling at high hypersonic speeds, and the achievable lift/drag ratio there is about 1. If you could get say 15 you could spend hours reentering, and everything would be much better, but with a ratio near one, once you're past the reentry interface, you're coming down in half an hour or maybe 45 minutes, and you're down.

Thrusters aren't a lot of use either, reentry shields give you much, much MUCH better performance for any given weight.

Gosh !..THANKS wolfekeeper. very grateful for your kind response here. So, which ever way I try..I'm never going to make my approach at 30 mph !...I'm always going to be able to fry eggs on the hood !
 

Offline neilep

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Re: Why Can't A Rocket Just Drop Down Nice And Slow ?
« Reply #4 on: 24/02/2012 18:09:32 »
You DO have heat retardant wool, right?????

Of course Gordy !   ;)...I paid for the fire retardent optional extra !..If I do get it damaged then I get a free refit by a sheep tailor !   ;D
 

Offline greeniemax

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Re: Why Can't A Rocket Just Drop Down Nice And Slow ?
« Reply #5 on: 25/02/2012 08:21:08 »
There are several problems with this because if you are going to hover than you'll need fuel to keep the "mobile" afloat, because gravity in near space doesn't change much that means your rocket will have to be much larger than normal.

If you are using thrusters to slow down on reentry than you'd have to carry the fuel to do that and you would need almost the same amount of fuel you had to use to go up because you are slowing down against the same gravity.

The speed of entry depends on speed at which the shuttle travels outside the atmosphere around the world (dropping to keep itself in 0 gravity), in case its slowed down it'll drop before toward earth as its not travelling away from its gravity anymore.
 

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Re: Why Can't A Rocket Just Drop Down Nice And Slow ?
« Reply #5 on: 25/02/2012 08:21:08 »

 

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