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SpaceShipOne and reentry heat



 
 
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  #12  
Old June 24th 04, 05:18 PM
Tony Rusi
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Default SpaceShipOne and reentry heat

SpaceShipOne dropped down from a height of 100km, while a shuttle drops
down from a height of some 400km AND has additionally a horizontal
velocity of some 8km/s. And an Apollo spacecraft dropped down from some
400000km. That's in both cases a *LOT* of kinetic energy more to kill
than SSO had to.


Interesting, for any spacecraft, if more fuel were used on orbit to
bring the (actually tangental) velocity component closer to zero, then
reentry heating shielding requirements are minimal like SS1. Maybe a
heat tile damaged spaceshuttle could re-enter the same way? Maybe this
last crew could have been saved with more fuel and/or less space
shuttle mass and momentum. Maybe they could have taken an engine and
tank off, performed a slowing burn, got into their emergency transfer
beach balls and parachuted from 200k? Iknow probably not enough fuel,
no way to pull an engine and tank off, no transfer beach balls on
board, etc. etc.
  #13  
Old June 24th 04, 05:23 PM
Daniel Walker
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Default SpaceShipOne and reentry heat

Reading these postings about SS1's re-entry and from what we've heard
recently regarding the space shuttle's TPS and its problems, why have
spacecraft tended not to use engine-based deceleration to reduce the
thermal effect of re-entry? If the orbital velocity is a major factor in
re-entry speed, why not take this off and just drop in? Dropping in from
400km would give you a nominal speed of ~2.8km/s if you reached the
Earth's surface (assuming I've done my sums right!).

I imagine the answers will involve the excess fuel needed, manoeuvring,
restartable engines, etc., so at what point (what altitude) does the
weight of thermal protection tiles beat the extra fuel needed for an
orbital craft? I know there's lots of variables involved, but wondered if
anyone had considered this?

Dan



  #14  
Old June 25th 04, 01:00 PM
David Given
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Default SpaceShipOne and reentry heat

On Thu, 24 Jun 2004 09:18:51 -0700, Tony Rusi wrote:
[...]
Interesting, for any spacecraft, if more fuel were used on orbit to
bring the (actually tangental) velocity component closer to zero, then
reentry heating shielding requirements are minimal like SS1. Maybe a
heat tile damaged spaceshuttle could re-enter the same way? Maybe this
last crew could have been saved with more fuel and/or less space shuttle
mass and momentum. Maybe they could have taken an engine and tank off,
performed a slowing burn, got into their emergency transfer beach balls
and parachuted from 200k? Iknow probably not enough fuel, no way to pull
an engine and tank off, no transfer beach balls on board, etc. etc.


There are people here with more technical knowledge than I, but I do know
that the shuttle's main engines aren't restartable. (Do they even have
on-board fuel tanks?) On-orbit, the shuttle manouvers using a completely
different set of engines, the OMS, that has a very small dV capacity. IIRC
it's about the most the OMS can do just to get the shuttle onto a reentry
trajectory.

*However*, it suddenly occurs to me that surviving reentry is actually
pretty simple in the appropriately designed vehicle: passive capsules like
the Soyuz and Apollo return vehicles are old, reliable technology.
(They're aerodynamically stable, and because they're only going to be used
once you can use ablative shielding rather than the TPS tiles.) They're
also small, in mass and volume.

How about fitting the shuttle out with a lifeboat? Stick it somewhere in
the cargo bay. If a shuttle gets sufficiently damaged that it can't
reenter, you use the capsule to get the crew down.

Depending on whether the capsule had its own thruster system, you would
get the choice of putting the shuttle onto a reentry trajectory and then
bailing out, or leaving the shuttle on orbit and just returning in the
capsule. The first option would almost certainly lose the shuttle, but if
you have to use the capsule the shuttle's probably not going to survive
reentry anyway. The second option would leave the shuttle intact in orbit,
where it could (possibly) be repaired, but would require the capsule to
have a fairly decent dV capacity. You'd also have to outfit the shuttle
with an automated station-keeping facility using the OMS; you wouldn't
want it to accidentally fall on someone.

(What's the lightest-weight way of getting a single human down from orbit?
Could you build something like an orbital parachute? If so, would that be
more appropriate than a combined capsule?)

--
+- David Given --McQ-+ "Every planet is weird. I spent six weeks on a
| | moon where the principal form of recreation was
| ) | juggling geese. Baby geese. Goslings. They were
+-
www.cowlark.com --+ juggled." --- Firefly, _Our Mrs. Reynolds_
  #15  
Old June 25th 04, 03:54 PM
Henry Spencer
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Default SpaceShipOne and reentry heat

In article ,
Daniel Walker wrote:
...why have
spacecraft tended not to use engine-based deceleration to reduce the
thermal effect of re-entry?


Because making even a modest difference requires vast amounts of fuel, far
more than can reasonably be made available.

...so at what point (what altitude) does the
weight of thermal protection tiles beat the extra fuel needed for an
orbital craft?


The fast answer is that there is no crossover point: aerodynamic braking
is *always* a lot cheaper in mass than doing the same braking with rocket
fuel. That's not quite 100% true when you start examining specialized
situations, but for normal reentry from orbit you can take it as given.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |
  #16  
Old June 25th 04, 03:56 PM
Henry Spencer
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Default SpaceShipOne and reentry heat

In article ,
Tony Rusi wrote:
Interesting, for any spacecraft, if more fuel were used on orbit to
bring the (actually tangental) velocity component closer to zero, then
reentry heating shielding requirements are minimal like SS1. Maybe a
heat tile damaged spaceshuttle could re-enter the same way?


Nope. The amount of fuel available is insufficient to make any real
difference. It's not just a little short, but a couple of orders of
magnitude short.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |
  #17  
Old June 26th 04, 02:16 AM
Mike Miller
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Default SpaceShipOne and reentry heat

Daniel Walker wrote in message ...
I imagine the answers will involve the excess fuel needed, manoeuvring,
restartable engines, etc., so at what point (what altitude) does the
weight of thermal protection tiles beat the extra fuel needed for an
orbital craft? I know there's lots of variables involved, but wondered if
anyone had considered this?


Yes, it's been considered.

Getting to orbit typically requires about 90-95% of the vehicle to be
fuel. In other words, for every kilogram put in orbit, 9 to 19
kilograms (or more) of fuel are required to put that kilogram into
orbit.

Equal and opposite reactions: getting out of orbit would require a
similar amount of fuel. The 100 ton shuttle needs 2000 tons of fuel
(give or take) to get into orbit; it would need 2000 tons to get out
of orbit. Of course, to put 2000 tons of fuel into orbit, you need
40,000 tons to put it in orbit...

In comparison, a well-designed heat shield is about 5% of the
re-entering vehicle's weight. A 100-ton vehicle needs about 5 tons,
maybe 10, of heat shielding to re-enter when using ablative or
reusable heat shields.

So: 10 tons of heat shield or 2000 tons of fuel to brake a 100-ton
spacecraft out of orbit.

Even if you just use a little fuel, you'll have a hard time beating
the efficiency of heat shields.

Mike Miller
  #18  
Old June 26th 04, 02:17 AM
Mike Miller
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Default SpaceShipOne and reentry heat

"Rodney Kelp" wrote in message ...
They never reached orbital speed (24,000 mph).


That's closer to escape velocity. Orbital velocity for a low earth
orbit is 17500mph. Escape velocity is 25000mph
  #19  
Old June 26th 04, 07:07 PM
Scott Lowther
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Default SpaceShipOne and reentry heat

Henry Spencer wrote:

In article ,
Daniel Walker wrote:
...why have
spacecraft tended not to use engine-based deceleration to reduce the
thermal effect of re-entry?


Because making even a modest difference requires vast amounts of fuel, far
more than can reasonably be made available.


Good rule of thumb: you'll need about the same detla-v to deorbit as to
get into orbit in the first place, if you wish to dispense with TPS.
This means either refueling on orbit, or extraordinary mass fractions.

--
Scott Lowther, Engineer
Remove the obvious (capitalized) anti-spam
gibberish from the reply-to e-mail address
  #20  
Old June 26th 04, 07:57 PM
Hephaestus
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Default SpaceShipOne and reentry heat

Daniel Walker wrote in message ...
Reading these postings about SS1's re-entry and from what we've heard
recently regarding the space shuttle's TPS and its problems, why have
spacecraft tended not to use engine-based deceleration to reduce the
thermal effect of re-entry? If the orbital velocity is a major factor in
re-entry speed, why not take this off and just drop in? Dropping in from
400km would give you a nominal speed of ~2.8km/s if you reached the
Earth's surface (assuming I've done my sums right!).

I imagine the answers will involve the excess fuel needed, manoeuvring,
restartable engines, etc., so at what point (what altitude) does the
weight of thermal protection tiles beat the extra fuel needed for an
orbital craft? I know there's lots of variables involved, but wondered if
anyone had considered this?


When you see a rocket launch, the vast majority of it (about 95%) is just
there to get the other 5% into orbit. That includes the fuel, engines, and
so on required to get the payload to the required velocity. Conveniently,
zeroing out the velocity like you propose takes exactly the same amount of
work, so if you have a full size rocket in orbit, it can return the payload
to earth via your mechanism.

Unfortunately, since the payload fraction is 1/20th, you've just increased
the amount of weight that must be launched by a factor of twenty! For example,
for a one ton payload, instead of a 20 ton rocket to launch it into orbit, you
now need a 400 ton rocket to launch the payload plus 20 ton return rocket into
orbit. It's easier to build a TPS, no matter how exotic, than to eat a 20-fold
increase in rocket size.
 




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