Brute force re-entry

I think the point that he was trying to make was that given the steep
angle of re-entry that NASA always uses, you might as well be "running
directly into the planet at full speed" instead of "skipping repeatedly
off the top of the atmosphere to shed speed"
True. Flying into reentry is more elegant than plunging downward and also
cheaper and easier compared to heavy duty thermal tile systems. It's what we'll
need to do for CATS, cheap access to space.
Critics wrongfully claim it'll take lots of braking fuel to get the speed
down but that's boar wash. The ship is nearly empty so short thrusting will be
enough to slow it down enough to start skipping into the outer atmosphere.
^
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"Jorge R. Frank" wrote in message
...
Alcore wrote in
:

On Sun, 15 Aug 2004, Bill Bonde ( ``Soli Deo Gloria'' ) wrote:

Lizerd wrote:

Brute force re-entry

I wouldn't have expected using the atmosphere to slow you down would
be considered using a 'brute force' method of slowing down. I would
think it would be considered elegant, and spending fuel to slow down
or just running directly into the planet at full speed would be
consider the brute force approaches.

I think the point that he was trying to make was that given the steep
angle of re-entry that NASA always uses,

Incorrect. The space shuttle re-enters at a flight path angle typically
between -1 and -1.5 degrees. That's hardly "steep".

True, 1 to 1.5 is shallow....
But I was picturing the shuttle reentering at closer to 15 to 20.
I do know it is piched up at 33 to 35 degrees.
From Hollywood, (I know that they only produce fact!), it looked
more like about 45 degrees.
(Now that is steep....)

I think the basic idea here is that there is a *lot* of energy being
shed by steep re-entry... and if there's enough energy to heat the air
blasting past the spacecraft into a plasma, is doesn't *seem* like so
much of a stretch to try and use some of that energy to alter the
spacecraft trajectory upwardly... in order to deliberately remain in
the thinnest air possible or even deflect completely outside the
atmosphere briefly. Which in turn, should reduce the heat loading.
(Or at least stretch out the heat loading over a long enough period of
time to allow some scheme to manage it more efficiently.)

The shuttle already does this, to the extent possible. It reduces the peak
heating, at the expense of increasing the total heat load.


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In article , Alcore
writes:


I think the basic idea here is that there is a *lot* of energy being shed
by steep re-entry... and if there's enough energy to heat the air blasting
past the spacecraft into a plasma, is doesn't *seem* like so much of a
stretch to try and use some of that energy to alter the spacecraft
trajectory upwardly... in order to deliberately remain in the thinnest air
possible or even deflect completely outside the atmosphere briefly.
Which in turn, should reduce the heat loading. (Or at least stretch out
the heat loading over a long enough period of time to allow some scheme to
manage it more efficiently.)


The Shuttle's angle of attack in the initial reentry thru max heating and I
believe all the way thru max-Q (dynamic pressure) is 30 degrees! With the
lousy lift to drag ratio associated with the high speeds in this phase of
reentry, it is hard to see how you could generate any more upward dynamically
than is alreadly being generated. However, If I understand the profile
correctly,
the deorbit burn places the craft in a trajectory with a perigee well within
the
upper reaches of the atmosphere, perhaps much more well within than is
strictly necessary. Rather than trying vainly to generate more up with the
wings,
perhaps the enlightened path to a more gentle reentry is to generate less down
with the deorbit burn.
At any rate, the reentry phase of the Shuttle is not really broke, and
doesn't
need to be fixed. It worked a gazillion times, and the one time it did not
work
was because a piece of a _new_ insulation material fell off of the ET and hit
the
wing, knocking part of the leading edge tiles off durring the _ascent_. The
new
insulation is broke and needs fixing or simply to go back to the old. The
reentry
works fine if you leave the tiles on. They are not just for show, they are
there
for a reason.
What the Shuttle needs, besides a new airline, is some small, incremental
improvements. Liquid flyback side boosters, rather than the dangerous, messy,
and operationally (rather than conceptually) complex solid ones. And make
the new Shuttles out of titanium rather than aluminum. Maybe a newer, lighter,

more sophisticated avionics package, stuff like that.
We need a stay-in orbit OTV, with a solar powered ion or plasma drive for
carrying unmanned payloads from LEO to higher.
A nice thing for smaller payloads launched with multistage rockets might
be
a small, ultralight high performance last stage which delivers a payload to
LEO.
The payload separates and the OTV transfers the payload to its operational
orbit.
Then it returns to transfer the light, empty top stage to a rendezvous with an
empty
Shuttle which has delivered a payload to orbit. The Canadarm slaps the upper
stage into the empty cargo bay, slam the bay doors shut and it returns to Earth

with the Shuttle to be reused. The weight and performance of upper stages has
a disproportional impact on the overall performance of a multistage delivery
system. You can afford to spend more on materials and fine tolerances if you
can reuse the vehicle. And in this case, you get it back without having to
design, build, or haul a reentry system. Just design the uppermost stage to
be as light as possible when empty, and to fit and attach in the Shuttle cargo
bay. The Shuttle's rated reentry cargo wieght capacity is over 14,000 kg,
almost half of it's rather large lifting capability prograde to LEO.
(Source: Illustrated Encyclopedia of Space Technology)
By way of comparison the Agena D 3rd stage of the Titan 23B is:
Weight: 7160 kg (15800 lb) - fueled
Empty Weight: 2300 lb (1045 kg) (Source http://en.wikipedia.org/wiki/Titan_23B)
Of course the Agena would not fit, but you get the idea.
Also, custom made ET substitutes could be quite a payload in their own
right. Skylab was a stretched Agena 3rd stage with fittings, what could be
engineered with an ET's volume and most of the Shuttle's ordinary cargo
capacity
as a mass budget for internal fittings, plumbing, etc?! Nothing this size has
ever been boosted to orbit in one piece or assembled. The Saturn V never did
it. The
biggest thing the Russians orbited was a mere Mir, which was much smaller than
Skylab, and an ET dwarfs both.
And use it as a truck, not a camper or a portible lab. Can we stop with
the
mission to planet Earth and the ants in orbit already? I hereby declare the
experimental phase of cislunar explorational phase over. Let the operational
phase of space development begin already! I would give an arm and a leg for a
quarter of the orbital lift that has been squandered on idiotic and
rerereredundant
experiments. In fact as a US taxpayer I effectively have, in vain! In the
space
age, "What goes up must come down" is a rule made to be broken. If those
hundreds of Shuttle missions had ascended with judiciciously selected
infrastructure, and returned with only personnel, we could have been well on
our
way to delivering lunar oxygen to LEO and looking to expand a lunar mining and
industrial base.
If launch expense is the cost driver, why are we squandering it? There is
a
short and obvious answer, the purpose of a bureaucracy is to spend money
without making waves. Yet one would think that the thickest heads among
them surely know why their agency exists, the goals and aspirations of those
without which their agency would not and should not exist. If they don't
personally believe in them the honest thing to do is seek more gainful and
meaningful employment in the IRS or the Department of Redundancy
Department. But I suppose this rant is redundant, as well, preaching to the
choir.soulful sigh

(Allen Meece) wrote in message ...
Critics wrongfully claim it'll take lots of braking fuel to get the speed
down but that's boar wash. The ship is nearly empty so short thrusting will be
enough to slow it down enough to start skipping into the outer atmosphere.

How much delta-V were you thinking of getting from braking fuel? 100m/s? 1000m/s?

Mike Miller, Materials Engineer

"Allen Meece" wrote in message
...
I think the point that he was trying to make was that given the steep
angle of re-entry that NASA always uses, you might as well be "running
directly into the planet at full speed" instead of "skipping repeatedly
off the top of the atmosphere to shed speed"
True. Flying into reentry is more elegant than plunging downward and
also
cheaper and easier compared to heavy duty thermal tile systems. It's what
we'll
need to do for CATS, cheap access to space.

In the world of aerospace "elegant" and "cheap" generally have a low
correlation coefficient.

A counter-argument is offered by Jeff Bell on spacedaily.com:
http://www.spacedaily.com/news/rocketscience-03zy.html

I.e. that a straightforward semi-ballistic capsule is the way to go to get
costs down, and safety up, for the next generation of manned spaceflight.
This philosophy is consonant with the "big dumb launcher" school of thought
for launch systems.


Critics wrongfully claim it'll take lots of braking fuel to get the
speed
down but that's boar wash. The ship is nearly empty so short thrusting
will be
enough to slow it down enough to start skipping into the outer atmosphere.
^
//^\\
~~~ near space elevator ~~~~
~~~members.aol.com/beanstalkr/~~~

"Allen Meece" wrote in message
...
I think the point that he was trying to make was that given the steep
angle of re-entry that NASA always uses, you might as well be "running
directly into the planet at full speed" instead of "skipping repeatedly
off the top of the atmosphere to shed speed"
True. Flying into reentry is more elegant than plunging downward and
also
cheaper and easier compared to heavy duty thermal tile systems. It's what
we'll
need to do for CATS, cheap access to space.

It may appear to be more elegant, but it's much harder to do. This is
because you either need a great deal of L/D, or you need to burn fuel to get
back out of the atmosphere every time you "skip". Either way, this would
only be a net "win" if you could shead heat from your TPS quickly while out
of the atmosphere. This limits your TPS materials greatly (shuttle tiles
don't shead heat quickly). If your TPS can't shead the heat while out of
the atmosphere, the "skipping" trajectory will be worse in terms of total
heat load than a "traditional" re-entry.

Critics wrongfully claim it'll take lots of braking fuel to get the
speed
down but that's boar wash. The ship is nearly empty so short thrusting
will be
enough to slow it down enough to start skipping into the outer atmosphere.

This is what the shuttle and capsules already do. The re-entry burn is only
big enough to make the orbit intersect the atmosphere. The reason they
don't typically skip is due to the L/D and materials issues.

Jeff

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

Henry Spencer opined


That said, I'm going to assume you were expressing yourself poorly -- that
you meant the spacecraft should stay up in *extremely* thin air, shedding
speed very slowly over a period of hours rather than minutes, until it's
lost almost all of it.

That's a very pretty picture. It's how a lot of people thought it would
be done, in the days before anyone seriously investigated the details.

But just try and make it work numerically! It's a happy fantasy having
nothing to do with the real world. It's not possible. There is no way to
*hold the spacecraft up* that long.

Aerodynamic lift simply isn't enough. It always comes packaged with a
certain amount of drag, and at hypersonic speeds actually quite a bit of
it. When you do the numbers, it just doesn't work. If the drag is low
enough to stretch the deceleration out that long, you don't have enough
lift to hold you up. Period. Full stop.

Does that also mean that the Sanger skip bomber would not work?


-ash
Cthulhu for President!
Why vote for a lesser evil?

They could send up some solid fuel boosters that the shuttle could dock with
and use for braking. How how long would it take to brake from 18k mph to
about 200mph without exceeding 2 G's?

"Henry Spencer" wrote in message
...
In article ,
Lizerd wrote:
Early on in the space program, the space capsule used brute force
re-entry.
IE: it slammed into the upper atmosphere at high speed to slow down for
return.

That is the only method anyone has ever used for reentry, from that day
to this: atmospheric braking. The details have gotten fancier (in most
cases), but the basic scheme of things has not.

The space shuttle is a lifting body.
Why can't it fly back???

It does. The Apollo and Gemini capsules were lifting bodies too, by the
way (and so is Soyuz). They all use aerodynamic lift to stretch their
reentries out as much as they can. But there are severe fundamental
limits to what can be done. Even pushing it as far as the shuttle orbiter
does incurs serious penalties, notably a thermal protection system which
is complicated and rather fragile compared to the simple and robust
heatshields the capsules used.

If the shuttle hit the atmosphere slower, use aero braking and descend at
a shallower angle, the shuttle could return at a slower decent rate, and
not
be subjected to the high temptures.

The longer, slower reentry the shuttle uses makes its thermal problems
*worse*, not better. The prolonged baking is actually rather harder to
handle than a quick blowtorching.

In any case, this isn't a question of the shuttle being deliberately
operated in some stupid, suboptimal way. It *already* uses aerodynamic
lift as much as it can without melting something off.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |



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"Rodney Kelp" wrote in
:

They could send up some solid fuel boosters that the shuttle could
dock with and use for braking. How how long would it take to brake
from 18k mph to about 200mph without exceeding 2 G's?

The question is not how long it takes but the mass of the SRB required.
Slowing from orbital speed to practically zero as you suggest would require
an SRB almost big enough to do the reverse (i.e. get the orbiter from the
ground to orbit). That's an awfully big SRB; it would have to do the work
of both SRBs *and* the propellants in the external tank. There's no rocket
in the world that could lift such an SRB into orbit.

OK, quick calculation: the shuttle's SRBs have an Isp of 269 s. Slowing
from 18000 to 200 mph with such a rocket would require a mass ratio of
about 20.4, according to the Rocket Equation. For a worst-case orbiter mass
of 250k lbm, the required SRB propellant mass is 5.1 million lbm. And that
doesn't even account for the SRB casing, which on the shuttle accounts for
15% of the SRB mass.

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"Lizerd" wrote in
:

"Jorge R. Frank" wrote in message
...
Alcore wrote in
:

I think the point that he was trying to make was that given the
steep angle of re-entry that NASA always uses,

Incorrect. The space shuttle re-enters at a flight path angle
typically between -1 and -1.5 degrees. That's hardly "steep".

True, 1 to 1.5 is shallow....
But I was picturing the shuttle reentering at closer to 15 to 20.
I do know it is piched up at 33 to 35 degrees.

That's the problem: people's mental "pictures" of shuttle re-entry are
entirely contrary to the fact. I suspect a lot of them are confusing angle
of attack (40 degrees) with flight path angle (-1 to -1.5 degrees).

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