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Aerobraking and reentry

Posted by: campbelp2002 - Fri Oct 28, 2005 6:08 pm
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Aerobraking and reentry 
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Post    Posted on: Thu Dec 08, 2005 7:19 pm
Well, the reducing ballute didn't work out after all. First I set up the best reentry I could with a 100x ballute. That is, I multiplied drag in the simulation by 100 and adjusted the entry angle for lowest peak G load, which turned out to be about 2.8. Then I tried cutting the ballute loose when the Gs got to 2. The G force built back up to a new peak of 5. The problem seems to be that you don't loose speed fast enough after reducing the drag but you do fall into denser air. I also tried cutting it in 50% and 10% steps, but the result was still a high G peak after the ballute was gone. Surprisingly the 10% steps were worse than either the 50% step or just cutting it loose. The problem is that your speed, which is not graphed here, is still way too high after cutting the last 10%. The net result is that the ballute has just forced you onto a bad entry angle. What I get from this is that variable drag is no good. You need lift too, like the shuttle's wings can provide. So far I have not tried to simulate that.
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Post    Posted on: Thu Dec 08, 2005 11:39 pm
Hrm. Well, one idea down. Peter: could you send me or post the coding for that simulation of yours?

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Post    Posted on: Fri Dec 09, 2005 1:24 am
There is a link to it in my October 31 post, earlier in this thread.
Here it is again.
http://home.austin.rr.com/campbelp/astro/reentry.xls

This is the basic code. You will need to multiply the drag expression by 100 or, whatever amount you want for your ballute, and copy lots of rows. And plug in appropriate starting conditions.


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Post    Posted on: Fri Dec 09, 2005 5:53 am
Okay, didn't know if you'd changed it since then or not.

By the way, what type of simulation is this? I've seen ones like it before, but I don't know what they're called.

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Post    Posted on: Fri Dec 09, 2005 7:49 am
I think there's a mistake in the drag function

K9=exp(-I9/8.42)*C9*abs(C9)
should be
K9=exp(-I9/8.42)*C9*H9
and
L9=exp(-I9/8.42)*D9*abs(D9)
should be
L9=exp(-I9/8.42)*D9*H9

This has to be doing something odd. Curiously the differences won't show up under one or two straightforward testing conditions and the total amount of drag won't ever change by more than about 50%.


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Post    Posted on: Fri Dec 09, 2005 3:15 pm
Hi spacecowboy

I don't know a name for it. It is just the same orbital simulation from that other thread which I though I had invented but have since learned that Euler invented in the 1700s. All I have done is add a drag term. It is basically a numerical solution to a differential equation, but WAY simpler.

Hi nihiladrem

I have to think about that for a while.


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Post    Posted on: Fri Dec 09, 2005 6:11 pm
Hi again nihiladrem,

OK, I have thought about it and you are absolutely right. :oops: I have made that change and uploaded the file to the same place. Now I no longer get the nice gentle 3G LEO reentry, it peaks above 7Gs! I haven't played with it much yet, but I will.


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Post    Posted on: Fri Dec 09, 2005 6:28 pm
...


Last edited by whonos on Thu Jun 07, 2007 7:16 pm, edited 1 time in total.



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Post    Posted on: Fri Dec 09, 2005 7:12 pm
That kinda makes sense: increasing the drag won't decrease the peak deceleration, it just makes it happen sooner. The only way to decrease peak deceleration is to actually decelerate slower, not sooner. So we need something that helps to slow us down over all points of the re-entry, *especially* before we slam into the atmosphere (unless the atmospheric pressure gradient is perfectly linear, which I rather doubt). Retro-rockets, anyone? Or would they add an unacceptable complication?

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Post    Posted on: Fri Dec 09, 2005 7:52 pm
Rockets would work but require way too much propellant, more than the final mass that reenters. The Shuttle gets it's gentle 3G reentry with lift from it's wings. It can stay in the thinner upper atmosphere longer because of lift. I tried the deflating ballute again but it still does not work. You end up not slowing down enough while still high in the thinner atmosphere and after the ballute is totally gone you are too low in the thicker air for the speed you still have and deceleration just builds back up to a peak over 7Gs.

Actually my original intent with this simulation was to see how hard aerobraking into LEO would be when returning from the Moon. I am glad to see that it is still quite gently, with the peak under 3Gs and the duration over 2Gs only about a minute. So the lunar tourists (me, ooooh me, me pick me!) will comfortably aerobrake into LEO, transfer to a winged shuttle (or maybe a space station and then a shuttle, like in 2001 a Space Odyssey) and have a nice easy 3G reentry.


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Post    Posted on: Fri Dec 09, 2005 8:10 pm
That sounds good.


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Post Numerical integration   Posted on: Sat Dec 10, 2005 5:26 am
campbelp2002 wrote:
It is basically a numerical solution to a differential equation, but WAY simpler.

Several kinds of numerical integration typically more accurate than using Euler steps are not significantly more difficult in concept or implementation. Often even when lots of accuracy is not required these methods prove necessary. Generally difficulty in numerical integration isn't closely tied with the order of integration, but usually to do with some property of the system involved or the manner in which it is described. These equations of motion involving drag and gravity are very forgiving in this regard, all the time derivatives are explicit and nearby trajectories are all physically meaningful and lead to similar solutions.

Though they have a place, I generally see a spreadsheet as a kind of lowest available denominator. Higher order methods may simply involve changing from having very many rows to several times more columns. Neither is appealing to me and if I were writing a simulation using one I wouldn't know at this moment how to effectively hide that complexity.


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Post    Posted on: Sat Dec 10, 2005 5:48 pm
Excel was not my first choice either, but it is available to almost everyone. I also did this is Java, but that is not as portable as they claim. I have also done it is BASIC and C, but those are least portable of all now days.


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Post    Posted on: Sat Dec 10, 2005 5:51 pm
Actually, I've seen and heard of Excel used as the initial simulation for a lot of projects. Remember: the more accurate the simulation, the more processing time it takes, so Excel spreadsheets are about at the limit of our personal computers. I mean, sure, you can run some very complex simulations, but you'll more than likely end up with smoke pouring out of your computer's ventilation grates. Especially considering the fact that this is more a toy (all these numbers have been proven out to the n-th decimal by someone with access to much bigger computers and more accurate algorithms than us) than a tool, Excel should be just fine.

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Post    Posted on: Sun Dec 11, 2005 1:08 am
Peter, I know you were just making a spreadsheet available to make it understandable to the widest group of people. C is pretty portable up until when you need graphics or sound support :( , and is probably even less transparent than a spreadsheet. The graphing facilities of spreadsheets are handy.

Spacecowboy, I'm not sure I understand what you mean.

Today's personal computers are just as capable as the mainframes of a decade ago and can be used in the same way. Unless you go wild with plugins, trying to do much of the calculations using a spreadsheet is just a waste of computing resources, however much of those you have.


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