Author Topic: Re-entry Question  (Read 1476 times)

Offline kenneylee

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Re-entry Question
« on: 04/24/2009 07:01 PM »
My apologies in advance if this is the incorrect forum, and for my lack of knowledge in this field, but I have a question that may seem stupid to someone with knowledge of this material, but it is something I have been curious about, and could not find an answer in a search of these forums.

I have just finished watching Apollo 13 for probably the 10th time, and my only knowledge of space flight is 99% from movies.

I know on Apollo 13 they had very, very limited options, but in re-entry in general, why is it that they are always traveling at an ungodly speed and trying to find this minute angle so they don't skip off the atmosphere or burn up in the atmosphere?

I'm sure this will probably sound ignorant and if possible they wouldn't spend so much energy and money building heat shields, but why can't they fly in perpendicular to the Earth, slow to a near stop and just let the Earths gravity pull them directly into the atmosphere. Its obviously something. Guess I want to know what prevents this.

Thanks in advance for your responses, and if I am in the wrong forum, I will appreciate your direction to the correct forum.

Online rdale

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Re: Re-entry Question
« Reply #1 on: 04/24/2009 07:06 PM »
You need to carry a LOT of propellant to come to a standstill if you're coming back from the moon.

Offline kenneylee

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Re: Re-entry Question
« Reply #2 on: 04/24/2009 07:19 PM »
Thank you, I appreciate your response. So, it isn't really something that is physically impossible, just more impractical.

Offline Laryb

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Re: Re-entry Question
« Reply #3 on: 04/24/2009 07:46 PM »
How does the mass of the fuel needed to slow from moon return speed to orbital speed compare to the mass of the heat shield, parachutes, extra structure, etc, needed for re-entry? 

In other words, is carrying the launch/entry vehicle to the moon and back more mass efficient than an earth orbit rendezvous and using space only vehicle for going to the moon and back?

I'm just asking about mass. I know there are other considerations.

Offline Jorge

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Re: Re-entry Question
« Reply #4 on: 04/24/2009 07:56 PM »
How does the mass of the fuel needed to slow from moon return speed to orbital speed compare to the mass of the heat shield, parachutes, extra structure, etc, needed for re-entry? 

In other words, is carrying the launch/entry vehicle to the moon and back more mass efficient than an earth orbit rendezvous and using space only vehicle for going to the moon and back?

I'm just asking about mass. I know there are other considerations.

The mass of the propellant would be orders of magnitude larger than the heat shield, parachutes, and structures. Carrying the return vehicle is far, far more mass efficient than attempting earth orbit rendezvous on the return trip.
JRF

Offline Spacenick

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Re: Re-entry Question
« Reply #5 on: 04/24/2009 08:10 PM »
Unless one uses aerocapture on the way back, a Zond wouldn't have needed much fuel to get into LEO after the first skip, however you'd still need a heat shield.

Offline Kaputnik

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Re: Re-entry Question
« Reply #6 on: 04/25/2009 10:34 PM »
Conservation of energy. When you 'fall' back to the Earth from the moon you arrive with the sort of energy you needed to leave- so you'd need a Saturn-V rocket to make tha braking manoeuvre!
A heatshield is a bit more practical!
Waiting for joy and raptor

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