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samkent
2007-Sep-17, 04:28 PM
What is the minimum delta v needed to deorbit from 120 miles in Earth orbit?
I am assuming that typically they fire the retros opposite to the direction of travel.
How about firing the retros opposite to the center of the Earth? Wouldn't a 90fps change towards the center of the Earth put the craft so deep into the atmosphere in 45 minutes that the craft would deorbit?
Isn't this less energy than the typical deorbit burn uses? Or..
Does the typical burn slow the craft so much that it decreases the heating of reentry?

danscope
2007-Sep-17, 04:50 PM
Hi,
You see,.... the re-entry of a space capsule or shuttle is accomplished at a precise angle . Too steep, and you will surely burn up. No question.
Too shallow, and you will skip off the atmosphere like a pebble off the pond .
It gets tricky.

Best regards, Dan

Larry Jacks
2007-Sep-17, 05:09 PM
You can calculate the delta-v required for a single burn deorbit using the information in this link (http://www.cdeagle.com/pdf/cdeorbit.pdf). Don't let the math scare you.

From the graph on page 3 of the link, a single deorbit burn from 127.6 nm would require a delta-v of 223.2 feet per second. Your 90 fps burn would lower the perigee quite a bit but not enough to deorbit immediately. Your orbit would probably degrade pretty quickly, though.

NEOWatcher
2007-Sep-17, 05:18 PM
...Too shallow, and you will skip off the atmosphere like a pebble off the pond ....
I try to avoid that statement myself.

Skipping off the atmosphere can be very irritating, and screw up a lot of schedules and orbit calculations. But; you will hit the atmosphere on the next trip around, and on some subsequent orbit, the skipping will have slowed you down enough to re-enter.

And per the OP, the most important thing is to lose speed, not to descend. Therefore, the most important vector of thrust is in contrast to the orbit, not the altitude. Descending is a natural result of slowing...

schlaugh
2007-Sep-17, 06:17 PM
That skipping bit is more relevant to craft returning from the moon, as was the case in Apollo. The lunar return speed was more than or close to Earth's gravitational escape speed of 25,000 MPH. So if they "skipped" they would not touch the atmosphere again on the the return (if at all) before the groceries ran out.

Skipping is probably not the right terminology; it's more like plowing through a giant arc of thinner air.

NEOWatcher
2007-Sep-17, 06:42 PM
...The lunar return speed was more than or close to Earth's gravitational escape speed of 25,000 MPH...
Even though it was considerably larger than orbit, it was less than escape velocity , because it was just eccentric enough to put it in the influence of moon's gravity.

...So if they "skipped" they would not touch the atmosphere again on the the return (if at all) before the groceries ran out...
Yes; the main issue.
I would assume the orbit would be somewhere in the 7 to 10 day realm.

Skipping is probably not the right terminology; it's more like plowing through a giant arc of thinner air.
I can see some element of skip (certain vectors of resistance)...but I agree that the prevelant effect is plowing.

astromark
2007-Sep-17, 07:18 PM
In an effort to be precise and accurate confusion and error over come the facts. The reverse thrust burn is primarily to slow the craft so as to allow the re entry via the forces of gravity. If the velocity were not reduced then a very real danger of a plowing or skip occurring. At the higher velocities we are perceptible to excessive temperature transfer at insertion into atmosphere. So reducing that velocity is vital.
Think of the re entry window. At lower velocities that window is bigger than at the higher rate of speed. Yes its rocket science but do-able.

joema
2007-Sep-18, 06:34 AM
...The reverse thrust burn is primarily to slow the craft so as to allow the re entry via the forces of gravity. If the velocity were not reduced then a very real danger of a plowing or skip occurring...
The purpose of a retrograde burn (against the flight path direction) is to alter the flight path angle until it intersects the atmosphere. It is not to slow down the vehicle for a "safe" reentry speed.

This can be seen from a translunar reentry. In that case no slowing reentry burn is done, yet the vehicle is going about 37,000 feet per second. The reason is the flight path angle is already correct for reentry.

For reentry from low earth orbit, the flight path angle must be changed to intersect the atmosphere.

The OP asked why is this done via a retrograde burn, vs thrusting toward the earth. This was discussed in this article on orbital mechanics. The answer is a retrograde burn is about 4x more efficient -- it requires about 1/4 the velocity change (hence propellant consumption) than thrusting toward the earth (121kb .pdf): http://www.jamesoberg.com/orbitology_spt.pdf

To reiterate, the vehicle could reenter safely from LEO using either thrusting method. The speed at entry interface and heating would not be dramatically different. However a retrograde burn requires less propellant, so that's why it's used.

astromark
2007-Sep-18, 07:01 AM
In an effort to be precise and accurate confusion and error over come the facts. The reverse thrust burn is primarily to slow the craft so as to allow the re entry via the forces of gravity. If the velocity were not reduced then a very real danger of a plowing or skip occurring. At the higher velocities we are perceptible to excessive temperature transfer at insertion into atmosphere. So reducing that velocity is vital.
Think of the re entry window. At lower velocities that window is bigger than at the higher rate of speed. Yes its rocket science but do-able.

Firstly lets understand what I have said. OK done that ...I understand me perfectly...:)

What you have said is also correct. We are not contradicting each other...I agree with you. The re entry window at insertion from lunar orbit can be correct... I am talking of from orbit to re entry as the OP asks... The burn does reduce the forward momentum allowing gravity to pull the vehicle into the re entry trajectory you are speaking of... yes its about conserving energy and getting the result for the least bucks... but thats a different subject? yes. mark.

Count Zero
2007-Sep-18, 09:13 AM
Essentially, the retrograde burn lowers the perigee of the orbit to an altitude where atmospheric drag becomes significant. The drag further lowers the speed, which drops the perigee until it intersects the Earth. As the spacecraft continues to slow, the down-range point where the trajectory intersects the surface keeps shortening. This continues until the craft reaches terminal velocity in a more-or-less vertical fall (either with or without a parachute), lands (like the Shuttle) or impacts the ground.

samkent
2007-Sep-18, 02:06 PM
I'm missing something.

Wouldn't 90fps put the craft 1 mile per minute closer to the Earth?
After 45 minutes wouldn't it be at an altitude of about 75 miles?
The linked chart shows at 127 miles you need a delta v of 223 fps to deorbit.
90fps is less than 233fps.
Or is it just my greymatter computer missing the mechanics of it?

samkent
2007-Sep-18, 02:15 PM
I just had an epiphany …
The ‘towards the Earth’ is only for an instant of time.
After that the change (delta v) begins to shift towards the direction of travel.
And in 90 degrees it is 100 percent in the direction of travel.

duh

dhd40
2007-Sep-18, 03:29 PM
I wanted to post a reply ... but then ....

... deorbit from 120 miles in Earth orbit ...

... a 90fps change ...

... a single deorbit burn from 127.6 nm (my insert ! ???????) would require a delta-v of 223.2 feet per second ...

... 90 fps burn ...

... gravitational escape speed of 25,000 MPH ...

... yet the vehicle is going about 37,000 feet per second ...

Wouldn't 90fps put the craft 1 mile per minute closer to the Earth?
After 45 minutes wouldn't it be at an altitude of about 75 miles?
The linked chart shows at 127 miles you need a delta v of 223 fps to deorbit.
90fps is less than 233fps

Sorry, I couldn´t resist. I know, it´s difficult for you to switch to metric (SI) units as it is for me to switch back to non-SI-units

cjl
2007-Sep-18, 03:46 PM
Honestly, it's difficult for me to read that too...
Despite being here in the US, in everything orbital, I have all my knowledge primarily in metric.

It would be wonderful if everyone just used the standardized units...

mugaliens
2007-Sep-18, 05:14 PM
I try to avoid that statement myself.

Skipping off the atmosphere can be very irritating, and screw up a lot of schedules and orbit calculations. But; you will hit the atmosphere on the next trip around, and on some subsequent orbit, the skipping will have slowed you down enough to re-enter.

And per the OP, the most important thing is to lose speed, not to descend. Therefore, the most important vector of thrust is in contrast to the orbit, not the altitude. Descending is a natural result of slowing...

And skipping can turn ugly, depending on how much energy was lost during the skip. It could have been enough to dangerously steepen the next entry.