Atmospheric Re-entry

[mja513]

Can anyone explain why we can't slow down a vehicle (like the shuttle) during re-entry, so the friction and subsequent heat is reduced and removed as a cause for concern? Thanks...

 

[jimfromnsf]

The shuttle uses the friction to slow it .

 

[mja513]

So you're saying that it is a matter of fuel and fuel efficiency? If we had more fuel or if we derived a greater return on the fuel that we do have, we would be able to slow the shuttle down to a point where re-entry would not pose a danger to the craft and crew?

 

[vulture2]

That is technically correct. If fuel was available (perhaps with some future technology) the vehicle could decelerate and land with propulsion alone; this is what Apollo did on the moon, where of course there is no measurable air friction.

 

[holmec]

Actually there is another way, but its impractical.<br /><br />If your craft was light enough and had a mass to air resistance ratio that of a bird to that of a dandylyon seed, then maybe heat would not be a factor. What would be a factor is an astronomical amount of G forces. <br /><br />Another possibility, though not tested, is using solar sails in orbit to get to a higher orbit or to deorbit. Of course the rate of the deceleration would depend on the mass of the craft in relation to the thrust provided by the sails. Theoretically if you had big enough sails and a mass small enough you could fly on the Sun's light hand have no orbital velocity at all but keep a stable distance (or shall we say altitude) from the sun. <div class="Discussion_UserSignature"> <p> </p><p><font color="#0000ff"><em>"SCE to AUX" - John Aaron, curiosity pays off</em></font></p> </div>

 

[j05h]

Instead of needing to refuel completely onorbit, a properly designed system would only require enough propellant to serve two purposes. First, to provide additional thermal protection during initial reentry (transparation through HS or base-first w/ engines throttled down). Second, to give enough delta-V at terminal decent to perform braking and landing. This type of setup (ripped from DC-Y concept) uses a heatshield, reenters base or side first and only needs enough fuel for perhaps 15-30 seconds at landing. In the lower atmosphere, it is only traveling at terminal velocity, subsonic.<br /><br />Josh <div class="Discussion_UserSignature"> <div align="center"><em>We need a first generation of pioneers.</em><br /></div> </div>

 

[rocketwatcher2001]

It's because fuel weighs so much and take up so much volume. If you could figure out how to make an atomic bomb "burn" for 8 minutes instead of .0000000008 seconds, that would be a great power source, and only if you could make it "burn" cleanly. <div class="Discussion_UserSignature"> </div>

 

[henryhallam]

Isn't that called NERVA?

 

[docm]

NERVA wasn't an "atomic bomb" regardless of its run time, it was a reactor for gasifying fuel. <div class="Discussion_UserSignature"> </div>

 

[mja513]

Are we saying that there is no way to slow down a shuttle without the use of friction? We couldn't include solar panels in the cargo bay of a future flight, deploy the panels at the proper time, use the power generated to slow the shuttle to a safe re-entry speed, and jettison the panels prior to re-entry? Is it that the power requirements to slow the shuttle are so great that only massive amounts of fuel would suffice? Obviously I don't know what I'm talking about, and you are the experts. It just seems that there should be some way...

 

[MeteorWayne]

You pretty much have it right.<br /><br />The only way to use electrical power for deceleration is with an ion engine, and that would require weeks or months to slow the shuttle down, not useful when you have 3 days worth of oxygen left <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[askold]

With the shuttle in orbit, you've got 250,000 lbs going 17,500 mph (kinetic energy) at 200 miles (potential energy). To land it, all that energy has to go to zero.<br /><br />Esentially, friction gives you a zero cost way to bleed off all that energy - into heat. Any other way means adding weight to the orbiter - engins, fuel, sails, etc. Which means those things have to be boosted into orbit with the vehicle, which means a higher launch weight, which means even bigger rockets, and so it goes round and round.<br /><br />Dumping all that energy into heat is the most efficient way to go. All you have to worry about is not wrecking your heat protection system from external tank debris .... but that's another story.

 

[h2ouniverse]

The point is the same as for launch. You have to cancel a huge kinetic energy with a duration:<br />* long enough so that you have a reasonable number of g in the cabin<br />* short enough so that you have enough oxygen, and also remain in orbit for the time needed to your deceleration!<br />You end up with thrust values with the order of magnitude of the ones of the launch system...<br /><br />The issue is the same when designing planetary probes: the most difficult case is when you cannot use an atmosphere to aerobrake!<br /><br />Sad, but that's the current situation. Until we discover anti-grav or build the elevator.<br /><br />Regards

 

[holmec]

<blockquote><font class="small">In reply to:</font><hr /><p>Are we saying that there is no way to slow down a shuttle without the use of friction?<p><hr /></p></p></blockquote><br /><br />Technically its not friction but viscosity. Friction involves only solid bodies, viscosity fluid bodies.<br /><br />Using the Earth's atmosphere to slow down a spacecraft for reentry is wise because your making use of the environment where otherwise you would have to carry a lot of weight in propellant just to slow down the craft. <div class="Discussion_UserSignature"> <p> </p><p><font color="#0000ff"><em>"SCE to AUX" - John Aaron, curiosity pays off</em></font></p> </div>