SSTO propulsion overview

(Henry Spencer) wrote:

[...] If you could somehow stabilize the tank, so it reentered
broadside-on (for maximum frontal area) with a slow spin on its
axis (so heating would be spread evenly instead of being
concentrated on one side), in theory it should survive reentry.
Landing it intact is left as an exercise for the student. :-)

Would it slow to subsonic speeds before hitting the ground?
If so, maybe it could deploy a hydrogen gasbag to stay aloft.
A 40 m diameter balloon ought to be enough lift for the empty
tank, if I've done my sums correctly.


;K

In article ,
Allen Meece wrote:
I read that the ET is broken up by explosive charges so that it burns better
during reentry.

No, it's not. Even the destruct charges that used to be there (fired by
command only) are no longer present.

Makes you wonder why nasa didn't go the other way and try to make it burn
less and be recoverable.

Because it was designed from the start to be expendable. Recovering it
would be difficult, and refurbishing it for re-use would be tricky and
expensive. (Even refurbishing the SRBs probably does not actually save
NASA any money.)
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

In article ,
Allen Meece wrote:
That was how the old DynaSoar was going to do it. Quick little dips down
into thick air to scrub some speed but then skipping back up into thinner air
to cool off a bit before gliding back down.

Unfortunately, while this may improve the thermal situation somewhat --
it's not as clear-cut as the Dyna-Soar promoters thought -- it doesn't
actually help the aerodynamics. To achieve the same duration of flight,
you need the same L/D. Averaged over one cycle, you need the same amount
of lift, and hence at the same L/D, you will incur the same amount of
drag.

I think that's how civilian entrepreneurs will design their reentry ships -
more like an airplane than a bowling pin.

There's no such pattern visible in how civilian entrepreneurs have
designed their vehicles to date. A long, slow, high-L/D reentry has
major disadvantages as well as advantages.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

John Carmack wrote:

JC I had heard it as "if it could be stabilized for a
JC sideways reentry and spun along the long axis to even
JC out the heating, it could reenter by itself", which
JC is a bit different than just letting it fall. Yes,
JC it is due to having a very low ballistic coefficient.
JC Some plans have been proposed for reentry with extreme
JC area to mass ratios using ballutes and such, which can
JC theoretically be done with very modestly heat resistant
JC materials.

Heat load during reentry is on the order of 100,000 W/m^2.
Spinning the craft will reduce the heat load by a factor
of 2. Reentry temperatures are over 10,000 Kelvins (plasma).

The external tank of the space shuttle is made of a cryogenic
aluminum-lithium alloy. Such alloys have melting point
temperature of about 600 degrees Celsius.

NASA tested ballutes made from Kapton film reinforced with
PBO fiber - they fail when ambient temperature exceeds 600
degrees Celsius. (http://www2.jpl.nasa.gov/adv_tech/ballutes/tech.htm)

Kris Cowart made charts depicting heat loads and temperatures
during reentry:
http://www.ssdl.gatech.edu/main/ssdl...4806charts.pdf

We are not in the ballpark - spinning, empty, aluminum-lithium
tanks cannot survive the reentry unless they have walls at least
one foot thick.

================================================== =====================

Allen Meece wrote:

AM That was how the old DynaSoar was going to do it.
AM Quick little dips down into thick air to scrub some
AM speed but then skipping back up into thinner air
AM to cool off a bit before gliding back down.

Easier said than done. Thin wings of the spacecraft will
melt down in a few seconds, long before the spacecraft
returns to the thin air.

The thing to remember is that reentry heat is due to compression of the air in
front of the vessel. A vessel shaped like a bowling pin will more compress
impinging air than the leading edge of a wing.

Allen Meece wrote:

AM That was how the old DynaSoar was going to do it.
AM Quick little dips down into thick air to scrub some
AM speed but then skipping back up into thinner air
AM to cool off a bit before gliding back down.

Easier said than done. Thin wings of the spacecraft will
melt down in a few seconds, long before the spacecraft
returns to the thin air.
^
//^\\
~~~ near space elevator ~~~~
~~~members.aol.com/beanstalkr/~~~

(Allen Meece) wrote in message ...
The thing to remember is that reentry heat is due to compression of the air in
front of the vessel. A vessel shaped like a bowling pin will more compress
impinging air than the leading edge of a wing.


Yes, but Those Who Know point out that the compressed air is a lot
*closer* to the wing than to the bowling pin, *transferring* more
heat.

/dps

In article ,
Abrigon Gusiq wrote:
Does a craft coming in, have to come down hard and fast? or can it come
down slow and soft? Reducing friction?

Yes and no.

There are different kinds of reentries, mostly depending on how much lift
you have. With none, or only a little (Apollo had some -- at hypersonic
speeds, even a blunt object generates some lift if it flies at an angle),
you get a fairly short sharp reentry. With more, like the shuttle, you
get a longer and slower reentry.

But there are no aerodynamic miracles to be had. At hypersonic speeds,
the amount of lift available (for a given amount of drag) is limited. The
old science-fiction idea of decelerating very slowly and gradually, so
heating never gets severe, *does not work* -- there is no way to *stay up*
that long. There's a limit to how long you can stretch out the reentry.

Moreover, the long slow reentry isn't necessarily an easier engineering
problem. The heating rate is lower, but the total amount of heat which
has to be dissipated somehow is actually quite a bit higher. The simple
and robust solutions which work spendidly for the short, sharp reentry
don't work for the long, slow one.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

In article ,
Abrigon Gusiq wrote:
Nearly half a century ago, we already understood that an airplane shape
was not the best choice for reentry.

A better shape is round and like an asteroid?

A rounded shape with an almost flat front surface is the winner in terms
of handling heat. The exact shape you end up with depends on how you
handle the rest of the vehicle. A sort of lens shape has excellent
reentry properties, but has a relatively cramped interior unless you make
it very large (which would lead to problems with launching it). So you
usually end up bulging the rear surface out to make more room inside,
leading to an Apollo or Soyuz shape.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

Henry Spencer wrote:
A sort of lens shape has excellent reentry properties, but has a
relatively cramped interior unless you make it very large (which
would lead to problems with launching it).

Wouldn't it turn sideways during reentry? Why not?
--
Keith F. Lynch - http://keithlynch.net/
Please see http://keithlynch.net/email.html before emailing me.

In article ,
Keith F. Lynch wrote:
A sort of lens shape has excellent reentry properties, but has a
relatively cramped interior unless you make it very large (which
would lead to problems with launching it).

Wouldn't it turn sideways during reentry? Why not?

No more than an Apollo shape would -- since the cone is in the wake, all
the air sees of an Apollo is the base, which looks just like the base of a
lens shape. With the center of mass in the right place, it's stable.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |