USA Today: Shuttle study finds higher risk of fatal hit by debris

see
http://www.usatoday.com/tech/science...e-debris_x.htm



Posted 6/7/2005 9:18 PM
Shuttle study finds higher risk of fatal hit by debris
By Traci Watson, USA TODAY
A new NASA risk analysis is raising fears that the space shuttle could
stand a higher chance of being destroyed by space debris than previously
thought.

"Jim Oberg" wrote in message
...
see
http://www.usatoday.com/tech/science...e-debris_x.htm



Posted 6/7/2005 9:18 PM
Shuttle study finds higher risk of fatal hit by debris
By Traci Watson, USA TODAY
A new NASA risk analysis is raising fears that the space shuttle
could
stand a higher chance of being destroyed by space debris than previously
thought.

But the preliminary analysis dated April 26 placed the odds
that orbital debris could destroy the next shuttle at a range
from 1 in 54 to 1 in 113. That risk estimate stems from recent
tests showing that the space shuttle's heat shield is more
fragile than NASA had realized.

It's always hard to come up with estimates like this, but if these numbers
are in the ballpark, then this risk is a bit high.

How would you reduce such a risk for a next generation spacecraft?
Certainly better micrometeorite protection would help against smaller
objects, but how do you protect a heat shield on something as big as the
envisioned CEV?

Jeff
--
Remove icky phrase from email address to get a valid address.

In article ,
Jeff Findley wrote:
How would you reduce such a risk for a next generation spacecraft?
Certainly better micrometeorite protection would help against smaller
objects, but how do you protect a heat shield on something as big as the
envisioned CEV?

The fast answer is that you build the heatshield out of something more
durable. In particular, a Mercury/Gemini/Apollo/Soyuz-style ablative
heatshield is tough and can have large safety margins. The heatshield
itself is good for only one use, but the rest of the spacecraft can be
reusable.

If you're really concerned, you can supplement this by putting an
expendable cover on the rear, so the part of the heatshield that really
matters is protected during most of the time spent in orbit.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

"Jeff Findley" wrote in
:

"Jim Oberg" wrote in message
...
see
http://www.usatoday.com/tech/science...uttle-debris_x
.htm

But the preliminary analysis dated April 26 placed the odds
that orbital debris could destroy the next shuttle at a range
from 1 in 54 to 1 in 113. That risk estimate stems from recent
tests showing that the space shuttle's heat shield is more
fragile than NASA had realized.

It's always hard to come up with estimates like this, but if these
numbers are in the ballpark, then this risk is a bit high.

Those numbers are almost certainly high for what the shuttle will actually
be facing post-return-to-flight. They don't account for changes in
shuttle/ISS stack attitude and the extra layer of insulation behind the RCC
to handle "sneak flow" from small holes/cracks. These steps only mitigate
the problem, not eliminate it; orbital debris will remain one of the major
threats to the shuttle.

How would you reduce such a risk for a next generation spacecraft?
Certainly better micrometeorite protection would help against smaller
objects, but how do you protect a heat shield on something as big as
the envisioned CEV?

First you'd recognize that the majority of the debris threat is in LEO and
optimize the design for that. LDEF data shows that the debris threat in LEO
is fairly directional, with the worst impacts along the velocity vector,
whereas the nadir surface is shielded by the Earth filling almost half the
sky.

Operationally, while in LEO, CEV should present the smallest frontal area
to the velocity vector and orient the most vulnerable surfaces to the nadir
and rearward directions. (For the shuttle, this aligns the tail along the
velocity vector and the payload bay to the Earth.)

This extends to ISS docked operations as well; the CEV docking port should
be oriented so as to allow CEV to fly a debris-optimal attitude while
docked. CEV thermal control system design should take this into account.

(The shuttle can't fly a debris-optimal attitude while docked to ISS, owing
to a complex combination of constraints on shuttle/ISS thermal control, ISS
solar power, shuttle/ISS attitude control, constraints on moving the
shuttle docking port, and RCS plume-impingement constraints. The new post-
Columbia docked attitude is a compromise that improves debris protection
without breaking any of the other constraints.)

--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

I read the UN Repot on Space Debris, and it is quite alarming. We need
to clean up our act before it gets even worse. There are studies to
prevent spent boosters from exploding, etc, but that will not remove
the junk that is already there...

Matthew Ota

In article .com,
Matthew Ota wrote:
I read the UN Repot on Space Debris, and it is quite alarming. We need
to clean up our act before it gets even worse. There are studies to
prevent spent boosters from exploding, etc...

More than studies: most new launchers take precautions against it, and
even many of the old ones have been retrofitted with changes for that.
(For example, the Delta second stage used to be notorious for exploding
eventually -- debris from those stages is a sizable part of the total
debris population -- but that was fixed quite a while ago.)

but that will not remove the junk that is already there...

Unfortunately true. Doing anything about the existing junk is hard.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Henry Spencer wrote:
[...]
but that will not remove the junk that is already there...

Unfortunately true. Doing anything about the existing junk is hard.

It would vary, I think, with what you'r trying to clean up...

For a collection of small pieces in reasonable proximity, could you
launch a foam dispenser at a closely matching orbit (station keeping)
slightly ahead of the debris, dispense the styrene/urethane/whatever,
let drag slow it down just enough that the debris impacts at low
relative velocity, and then let drag lower the orbit?

Of course, drag effects are only useful for LEO orbits, but isn't that
where the problem is most acute?

Large skin pieces will have their own drag, and probably are self
cleaning in this region.

Large heavy pieces may require an autonomous docker that can do a retro
burn.

I think there would be a real problem, though, with small isolated
pieces -- hard to track, and you don't get many of them per cleaning
trip.

For those you can detect on orbit as they approach, maybe you could
throw aerogel foam in their path? Urethane wouldn't slow them much if
they were fast enough to be a problem for the current-technology
shields, although I guess aerogel would get poofed pretty fast, too.

/dps

In article .com,
snidely wrote:
For a collection of small pieces in reasonable proximity, could you
launch a foam dispenser at a closely matching orbit (station keeping)
slightly ahead of the debris, dispense the styrene/urethane/whatever,
let drag slow it down just enough that the debris impacts at low
relative velocity, and then let drag lower the orbit?

Feasible in principle (although some details need work), but it depends on
having a collection of small debris in essentially the same orbit.

Unfortunately, most any event that spreads small pieces gives them enough
of a shove that they end up in slightly different orbits. Not only do
they spread out along the original orbit because they have slightly
different orbital periods, but worse, the rate at which an orbit's plane
precesses around Earth's axis depends on its period. So they soon end up
so spread out that there's no way you can have them all hit at low
relative velocity.

I think there would be a real problem, though, with small isolated
pieces -- hard to track, and you don't get many of them per cleaning
trip.

Exactly. And as per above, that describes essentially all of the small
debris. You could consider sending a tug to collect big stuff, but no
way is that feasible for the small stuff.

For those you can detect on orbit as they approach, maybe you could
throw aerogel foam in their path? Urethane wouldn't slow them much if
they were fast enough to be a problem for the current-technology
shields, although I guess aerogel would get poofed pretty fast, too.

Unfortunately, there's basically nothing you can put in their path that
will help. Analogies from lower speeds fail; at relative velocities of
multiple km/s, most any impact is an explosion, regardless of the exact
nature of the materials, because there is more than enough kinetic energy
there to vaporize *any* material. This actually tends to make things
worse, because shock waves from the initial contact blow the two colliding
objects to bits, and the two debris clouds then pass through each other.

Aerogels are a partial exception -- they're thin enough to decelerate an
incoming object slowly and spread the energy out over both time and
material -- but you would need enormous volumes of aerogel to decelerate
macroscopic objects that way.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

"Jim Oberg" wrote:

A new NASA risk analysis is raising fears that the space shuttle could
stand a higher chance of being destroyed by space debris than previously
thought.

One wonders if this isn't NASA covering it's butt by announcing a
potential LOCV cause rather than hiding and handwaving as has been the
case beforehand.

D.
--
Touch-twice life. Eat. Drink. Laugh.

-Resolved: To be more temperate in my postings.
Oct 5th, 2004 JDL