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Sub Orbital v. Orbital Flight

Posted by: chaseries - Wed Nov 05, 2008 2:56 am
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Sub Orbital v. Orbital Flight 
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Post Sub Orbital v. Orbital Flight   Posted on: Wed Nov 05, 2008 2:56 am
I was wondering if anyone could tell me why so much speed and heat are generated during atmospheric re-entry. I understand that orbit requires a certain speed, but why can't a spacecraft slow down its orbit so that it free falls down and use a parachute as human beings do? And if the gravity is too strong while the atmosphere is too sparse and a spacecraft would gain too much speed even with a parachute or something, why can't there be a kind of propulsion system to counteract its free fall acceleration until the atmosphere is sufficiently dense?

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Post    Posted on: Wed Nov 05, 2008 3:21 am
Ok, one point at a time.

1: Vehicles decelerate just enough to drop out of orbit, which doesn't take much of a change in delta v, so they reenter at near orbital velocity.

2: The heat is generated by friction between the air and the body of the vehicle

3: I'm sure we could build orbital vehicles with additional fuel and an engine and slow them drastically before much drag is generated. However, that same fuel and engine are mass that could have been used for payload. Its just cheaper and more efficient to use a heat shield for the bulk of deceleration.

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Post    Posted on: Wed Nov 05, 2008 4:06 am
I kind of assumed that was the reason, but I wasn't sure just how much extra fuel or technology it would require to slow it down significantly from orbital velocity. But then again, is the reason SpaceShipOne equipped with a feather solely to slow it down because, again, the paucity of atmosphere in conjunction with still-present gravity? I realize it's just semantics between "orbital" and "suborbital" space, but couldn't you just zip around in space around earth and have nothing to do with orbit? Wouldn't you then be in "suborbital" space simply by the virtue of not being orbiting a planetary body or otherwise?

(And thanks for the feedback on the last one.)

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Post    Posted on: Wed Nov 05, 2008 4:27 am
You would need the same fuel that is needed for accelerating minus the (not much) fuel needed during ascent because of gravitational losses and air drag. To have that fuel available in orbit you would need of course much much more fuel for the ascent then.

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Post    Posted on: Wed Nov 05, 2008 3:05 pm
Orbital or sub-orbital is not a place. Its a matter of velocity. I could be 1000 km above the earth, but if I travel too slowly and/or in the wrong direction, I'm sub-orbital.

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Post    Posted on: Fri Nov 07, 2008 8:47 pm
The denser the atmosphere the quicker you'll slow down, so the hotter you'll get.

Based on that, the best reentry idea goes to JP Aerospace, for using a blimp to decelerate in the upper atmosphere, reducing heating.


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Post    Posted on: Fri Nov 07, 2008 10:06 pm
Terraformer wrote:
The denser the atmosphere the quicker you'll slow down, so the hotter you'll get.

Based on that, the best reentry idea goes to JP Aerospace, for using a blimp to decelerate in the upper atmosphere, reducing heating.


IIRC, they're actually still ascending when the balloon bursts and throw a parachute on the way back down, but they've also got essentially zero velocity at the top (no intentional lateral movement) so they start the descent phase from a dead stop - easiest of all possible re-entry scenarios. A VERY nice system, but it's still pretty much the same as any sub-orbital vehicle re-entry...


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Post    Posted on: Sat Nov 08, 2008 2:54 pm
I don't get that impression from what they say on their website.

Oh, and it's an orbital reentry. So they have lots of lateral speed.


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