Author Topic: Basic Rocket Science Q & A  (Read 281513 times)

Offline Jorge

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Re: Basic Rocket Science Q & A
« Reply #240 on: 05/25/2009 09:39 PM »
The "winner" is a ballistic ascent abort entry and ballistic lunar entry. 

I read that a normal lunar reentry is mostly ballistic anyway, with only the RCS being used for attitude control. Is this true? And if so, how much difference does the attitude control make?

A guided entry uses roll to put the lift vector largely up.  In a ballistic entry with a capsule, roll is set to a constant value and the end result is no effective lift.

Nit: A capsule with an offset CG performs a ballistic entry by setting roll *rate* to a constant, so that the direction of the lift vector averages out.
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Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #241 on: 05/25/2009 09:44 PM »
Nit: A capsule with an offset CG performs a ballistic entry by setting roll *rate* to a constant, so that the direction of the lift vector averages out.

Ah, so does ballistic mean "as if there were no atmosphere and no propulsion" instead of "without propulsion" in this context?
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Offline Jorge

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Re: Basic Rocket Science Q & A
« Reply #242 on: 05/25/2009 09:46 PM »
Nit: A capsule with an offset CG performs a ballistic entry by setting roll *rate* to a constant, so that the direction of the lift vector averages out.

Ah, so does ballistic mean "as if there were no atmosphere and no propulsion" instead of "without propulsion" in this context?

Don't think so; "ballistic" is still reliant on drag for deceleration. I think of it as "drag only, no lift".
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Offline mmeijeri

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Re: Basic Rocket Science Q & A
« Reply #243 on: 05/25/2009 09:49 PM »
Is there a special name for a trajectory when you have no propulsion because something's wrong with your RCS?
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Offline StarStuff

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Re: Basic Rocket Science Q & A
« Reply #244 on: 05/26/2009 02:23 AM »
A ballistic entry has zero lift. The only thing you can control is the angle of entry into the atmosphere. The astronauts would experience at least 17g's. NASA considered that to be not survivable.

The Apollo entry was close to ballistic but with a small change to generate some lift: the CG was offset resulting in a small trim attitude. The lift kept the CM in the upper atmosphere long enough so the astronauts would not experience much over 7g. The lift vector could only be controlled by rotating the CM.

Source: http://www.mae.usu.edu/aerospace/publications/Reno_rev_whitmore.pdf

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060045567_2006144506.pdf

Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #245 on: 05/26/2009 03:31 AM »
The "winner" is a ballistic ascent abort entry and ballistic lunar entry. 

I read that a normal lunar reentry is mostly ballistic anyway, with only the RCS being used for attitude control. Is this true? And if so, how much difference does the attitude control make?

A guided entry uses roll to put the lift vector largely up.  In a ballistic entry with a capsule, roll is set to a constant value and the end result is no effective lift.

Nit: A capsule with an offset CG performs a ballistic entry by setting roll *rate* to a constant, so that the direction of the lift vector averages out.


Not a nit.  A huge difference.  Thanks for the correction.  I edited my post.

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Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #246 on: 05/26/2009 03:33 AM »
A ballistic entry has zero lift. The only thing you can control is the angle of entry into the atmosphere. The astronauts would experience at least 17g's. NASA considered that to be not survivable.

The Apollo entry was close to ballistic but with a small change to generate some lift: the CG was offset resulting in a small trim attitude. The lift kept the CM in the upper atmosphere long enough so the astronauts would not experience much over 7g. The lift vector could only be controlled by rotating the CM.

Source: http://www.mae.usu.edu/aerospace/publications/Reno_rev_whitmore.pdf

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20060045567_2006144506.pdf


The NASA doctors approved 20 G entries for aborts in about 2004.  We looked very seriously at ballistic entries, but I don't think they made the final cut into Orion.

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Offline ginahoy

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Re: Basic Rocket Science Q & A
« Reply #247 on: 05/26/2009 03:55 AM »
I would imagine a 20 g force on a launch abort would be very short in duration -- only in the moments after the initial firing. Sorta like an ejection seat on some fighter jets. Correct?

Offline Jorge

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Re: Basic Rocket Science Q & A
« Reply #248 on: 05/26/2009 04:13 AM »
I would imagine a 20 g force on a launch abort would be very short in duration -- only in the moments after the initial firing. Sorta like an ejection seat on some fighter jets. Correct?

You do get a big g-pulse at LAS firing, but don't forget about the subsequent re-entry, which will be steeper than normal due to the lower horizontal velocity.
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Offline ginahoy

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Re: Basic Rocket Science Q & A
« Reply #249 on: 05/26/2009 04:18 AM »
You do get a big g-pulse at LAS firing, but don't forget about the subsequent re-entry, which will be steeper than normal due to the lower horizontal velocity.

I was only thinking about an escape at launch. I guess the system could be used all the way up to orbit.

Offline Jorge

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Re: Basic Rocket Science Q & A
« Reply #250 on: 05/26/2009 04:25 AM »
You do get a big g-pulse at LAS firing, but don't forget about the subsequent re-entry, which will be steeper than normal due to the lower horizontal velocity.

I was only thinking about an escape at launch.

For that particular case you're correct; there would be one g-pulse at LAS firing but the spacecraft would not get high enough to experience a re-entry.

Quote
I guess the system could be used all the way up to orbit.

Systems with a LAS generally would use it until shortly after staging, then jettison it. After that, the stack acceleration is generally low enough (and the atmosphere thin enough) that the spacecraft can get away from the LV without it, so it is jettisoned to save mass. That is how Apollo/Saturn worked and how Ares/Direct would work.
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Offline Jim

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Re: Basic Rocket Science Q & A
« Reply #251 on: 05/26/2009 11:45 AM »
I would imagine a 20 g force on a launch abort would be very short in duration -- only in the moments after the initial firing. Sorta like an ejection seat on some fighter jets. Correct?

Longer 5 seconds or so and it is 15g's.

Offline hop

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Re: Basic Rocket Science Q & A
« Reply #252 on: 05/27/2009 05:23 AM »
You do get a big g-pulse at LAS firing, but don't forget about the subsequent re-entry, which will be steeper than normal due to the lower horizontal velocity.
Especially if things go wrong like Soyuz 18a. They actually aborted after LAS jettison, but a similar effect.

A Zond ballistic return (i.e. if the guidance failed in a skip re-entry, which it did on a couple of tests) was 20+ as well AFAIK.

Offline veryrelaxed

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Re: Basic Rocket Science Q & A
« Reply #253 on: 05/27/2009 05:55 AM »
20+G for how long.  The final objective upon re-entry is to dissipate the TREMENDEOUS both potential (altitude) AND kinetic energy (angular speed) components into other forms of energy (heat mostly + electro magnetic+air friction<-heat again)  You can do it at lower (+/-)G levels lasting longer, or at higher (+/-)G lasting short time.  And that will determine your vehicle's descent profile.  Biologically speaking a high absolute velocity change in as short period as possible is preferable to extended Gs over entire descent.  (there is much more for mititgation as to the body orientation to the deceleartion vector, and more...)
« Last Edit: 05/27/2009 05:59 AM by veryrelaxed »

Offline C5C6

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Re: Basic Rocket Science Q & A
« Reply #254 on: 05/27/2009 09:14 PM »
I'm embarassed to ask this but I never knew, are ELV stages recovered for burial or recycle or any other use (excepting reuse of course)???

Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #255 on: 05/27/2009 10:13 PM »
I'm embarassed to ask this but I never knew, are ELV stages recovered for burial or recycle or any other use (excepting reuse of course)???

The only ones I know of are some solids on the Ariane that land on land and sometimes they take one down on chutes that land on water.  Not to reuse, but to look at for problems in the design or manufacturing process.

Danny Deger
« Last Edit: 05/27/2009 10:14 PM by Danny Dot »
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Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #256 on: 05/27/2009 10:15 PM »
20+G for how long. 

snip


For about 5 to 10 seconds.

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Offline C5C6

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Re: Basic Rocket Science Q & A
« Reply #257 on: 05/27/2009 10:57 PM »
I'm embarassed to ask this but I never knew, are ELV stages recovered for burial or recycle or any other use (excepting reuse of course)???

The only ones I know of are some solids on the Ariane that land on land and sometimes they take one down on chutes that land on water.  Not to reuse, but to look at for problems in the design or manufacturing process.

Danny Deger
so the ones that don't have chutes and crash on land or water are not recovered not even for cleanup??? I really find it hard to believe they keep throwing rocket stages to some places and are not removed....you'd have a pile of rocket stages in the atlantic only because of KSC launches.....

another embarassing question: most of second or third stages burn up on reentry don't they?? first stages do not acquire enough speed for a destructive reentry do they???

Offline Danny Dot

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Re: Basic Rocket Science Q & A
« Reply #258 on: 05/27/2009 11:22 PM »
I'm embarassed to ask this but I never knew, are ELV stages recovered for burial or recycle or any other use (excepting reuse of course)???

The only ones I know of are some solids on the Ariane that land on land and sometimes they take one down on chutes that land on water.  Not to reuse, but to look at for problems in the design or manufacturing process.

Danny Deger
so the ones that don't have chutes and crash on land or water are not recovered not even for cleanup??? I really find it hard to believe they keep throwing rocket stages to some places and are not removed....you'd have a pile of rocket stages in the atlantic only because of KSC launches.....

another embarassing question: most of second or third stages burn up on reentry don't they?? first stages do not acquire enough speed for a destructive reentry do they???

Almost.  As far as I know the Ariane 4 solids land on land in South America and they are collected and put in a junk pile.  The ones that land in water just stay there.

And you are correct that upper stages for the most part burn up on entry, but some parts make it to the ground.  In the past, upper stages that make orbit often had their tanks over pressure and they blew up making a bit mess on orbit.  Today's upperstages relieve the pressure in the tanks so they stay in one easy to track and avoid piece.

I am not an expert on this, so I might be wrong in a couple of places.

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Offline Antares

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Re: Basic Rocket Science Q & A
« Reply #259 on: 05/28/2009 03:49 AM »
If it made economic sense, they would recover them.  It doesn't.  ELVs also provide a continuous opportunity to improve designs.  It's the state of the art, every time, not 30 year old technology (ahem).
If I like something on NSF, it's probably because I know it to be accurate.  Every once in a while, it's just something I agree with.  Facts generally receive the former.