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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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Space Walker
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Post    Posted on: Wed Nov 09, 2005 3:46 pm
...pretty sad when I can't even remember simple Mechanics... but I took that class over 20 years ago, so I have an excuse...


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Post    Posted on: Thu Nov 10, 2005 1:37 am
SawSS1Jun21 wrote:
...pretty sad when I can't even remember simple Mechanics... but I took that class over 20 years ago, so I have an excuse...


Hey, you only forgot the scalar constant. I've done dumber things in my Physics classes.

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Post    Posted on: Thu Nov 10, 2005 7:14 am
Interesting...

I wrote a suborbital simulator using VB.Net with each loop representing 10 milliseconds. It covered the ascent as well as the descent.

I used the below algorithm (pinched from the NASA website) for atmospheric density, but my coeficient of drag was a bit of a WAG.
I think I took the drag of a sphere of the same radius as my rocket, and doubled that at supersonic speeds. I also threw in air brakes to increase drag on descent.

My results are similar, although peak G on descent from 100 km was 5 G.
With less drag, peak G is a little higher, but occurs closer to the ground.

Clearly, vertical descent is the worst case, for G's and heating.


Select Case Altitude

Case Is > 25000
Temperature = 141.89 + (0.00299 * Altitude)
Pressure = 2.488 * ((Temperature / 216.6) ^ -11.388)

Case 11000 To 25000
Temperature = 216.64
Pressure = 22.65 * Math.Exp(1.73 - (0.000157 * Altitude))

Case Is < 11000
Temperature = 288.14 - (0.00649 * Altitude)
Pressure = 101.29 * (( Temperature / 288.08 ) ^ 5.256)

End Select

AirDensity = Pressure / (0.2869 * Temperature)


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Post    Posted on: Thu Nov 10, 2005 2:23 pm
I looked at that density model too but decided it was too complicated, didn't cover the full range of altitudes I wanted and was not needed for the simple first order approximation I was doing. The density model I am using is just EXP(-Alt/8.42) where Alt is in km. I assume temperature is constant, density at sea level is 1 and drag is just density times velocity squared. There are values for atmospheric density, space craft mass and drag coefficient in real physical units that would give the same results as mine, but I haven't worked it out. However it is safe to say that it would be a pretty "fluffy" space craft with low mass and large drag coefficient, something like a large SSTO vehicle with nearly empty tanks, because I am getting slightly lower G loads for the 100 km free fall than SS1 actually experiences.


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Post    Posted on: Thu Nov 10, 2005 4:33 pm
Would resonance be a problem with those empty tanks on return?


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Post    Posted on: Thu Nov 10, 2005 10:07 pm
Unless I'm very much off, a few pieces of rubber spacer can solve nearly every resonance problem in existence -- the rest are damped with springs. And resonance shouldn't be a problem anyway, unless something about your ship causes it to wiggle and/or vibrate in a periodic manner.

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Post    Posted on: Thu Nov 10, 2005 10:22 pm
Well if we can't find a way to bring big empty tanks safely back to the ground, then the whole reusable space ship idea is in the toilet, isn't it?


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Post    Posted on: Fri Nov 11, 2005 12:21 am
You gotta admit, he kinda has a point.... :shock:

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Post    Posted on: Fri Nov 11, 2005 8:23 am
Regarding big empty tanks bringing safely to the ground there is an alternative which keeps reusability working: Keeping the big empty tanks in space and refueling them by delivery of propellant by small tankers.

This would be based on t/Space's tanker-idea and is a little bit assisted by an article about mega-modules.



Dipl.-Volkswirt (bdvb) Augustin (Political Economist)


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Post    Posted on: Wed Nov 16, 2005 5:15 pm
But the mega-module as tanker concept works the best. In this way, with two HLLV launches, you can transfer a good load of fuel at one time with minimal boil-off as opposed to multiple launches of craft yet to be demonstrated.


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Post    Posted on: Wed Nov 16, 2005 7:39 pm
By the way, a Ballute (Balloon – Parachute) should work to produce reentry drag, and could be either deflated or cut free to turn the orbital reentry deceleration into a double humped curve with about &frac12; the peak g loads.

The old Allen and Eggers paper notes that (with a logarithmic air density and fixed geometry) the peak deceleration does not depend on drag area, scales as V^2, and is moderated by low angle entry into the atmosphere. You may already have all this in your equations.


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Post    Posted on: Wed Nov 16, 2005 8:00 pm
I did some quick experiments with the Excel file. I multiplied the drag terms by 100 to simulate a ballute. I could not get reentry G forces below 3 and I could not get a double hump profile. All that happened was that the peak G forces occurred at a higher altitude.

Now about the slightly off topic subject of reusable space ships. I don't consider a ship that leaves its tanks in orbit or cuts it's ballute loose on the way down to be fully reusable, unless those parts are retrieved and reassembled on the ground for another launch. If the ship can't launch again with all the same parts that were launched before, it isn't fully reusable.


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Post CONTROLLED Ballutes   Posted on: Thu Dec 08, 2005 12:44 am
The point is that a Ballute need not be a fixed geometry, since any fixed geometry produces almost identical deceleration. If it is partially or slowly deflated (or even cut loose) before the desired g force is exceeded, then you can get double or multiple humps, or a flat top deceleration curve.


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Post    Posted on: Thu Dec 08, 2005 2:04 am
:idea: I hadn't thought of that. What a great idea.


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Post    Posted on: Thu Dec 08, 2005 3:19 am
Wow. Awesome.

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