Maglev assisted launch SSTO

Henry Spencer wrote:
In article ,
Roger Stokes wrote:

...Mach 5 is a bit fast inside an atmosphere. Even at the top
of the highest mountain on Earth, the air will still be thick enough that
the vehicle will need a thermal protection system ON THE WAY UP...

If the vehicle is SSTO, it will already posess a thermal protection system
for reentry at mach 25, which should be only lightly loaded at mach 5-10
for a few 10's of seconds on the way up, even at greater air density.


Alas, that doesn't follow. Reusable reentry vehicles do most of their
decelerating at extremely high altitude, in very thin air, where thermal
loads are modest. Slamming through thick air at hypersonic speed, even
briefly, is a far more severe problem.

Roughly speaking, heat flux scales with air density times the cube of
velocity. (The kinetic energy per kilogram of the incoming air scales
with the square of velocity, and how much air you hit per second scales
with density times velocity.) At 1/3 of the speed, you get about 1/27th
of the heat flux; that's good. But the air density at 8km (highest
mountains) is about 1,000,000x that at 100km (typical reentry altitude);
that's really bad. The multiplication is left to the student. :-)

Cube root(1,000,000) = 100. So the 8 km sled would need to be going
1/100 of typical reentry velocity for same amount of heat?

Mach 5 is about 1.7 kilometers/second? And the reentry velocity you're
thinking of is about 5.1 km/sec? 1/100 of this speed is .051 km/sec.
This is only about 1/6 of mach 1. Hasn't the Concorde flown at mach 1
at lower altitudes and not burned up? I know I'm missing something here.

Hop
http://clowder.net/hop/index.html

Henry Spencer wrote:


Note that I said "roughly speaking". :-) There are a bunch of complicating
factors, mostly notably the fact that only a small fraction of the air's
kinetic energy actually shows up as heat at the surface. Just how small
that fraction *is*, becomes very important. That's heavily influenced by
things like vehicle shape and air flow characteristics (which vary a lot
with both speed and density).

Changing the velocity by a factor of 100 is too much for this simple
approximation; it breaks down.

I googled mag lev trains and learned that existing trains achieve a
speed of .120 km/sec. The equator speed is about .464 km/sec so the sled
might achieve .584 km/sec at lift off. At the mountain top you seem to
have a weight savings of about .2 percent. This doesn't seem like a
great improvement.

The air pressure seems to be about 2/5 of sea level. Would the reduced
drag at ignition be a significant advantage?

Looking at a globe there seems to be some real estate east of the Andes
at the equator. I don't think folks living east of the launch site would
be happy.

This tentative examination seems to be deflating a fantasy I've enjoyed
entertaining in the past.

Hop
http://clowder.net/hop/index.html

Henry Spencer wrote:

In article ,
Roger Stokes wrote:


...Mach 5 is a bit fast inside an atmosphere. Even at the top
of the highest mountain on Earth, the air will still be thick enough that
the vehicle will need a thermal protection system ON THE WAY UP...


If the vehicle is SSTO, it will already posess a thermal protection system
for reentry at mach 25, which should be only lightly loaded at mach 5-10
for a few 10's of seconds on the way up, even at greater air density.



Alas, that doesn't follow. Reusable reentry vehicles do most of their
decelerating at extremely high altitude, in very thin air, where thermal
loads are modest. Slamming through thick air at hypersonic speed, even
briefly, is a far more severe problem.

Roughly speaking, heat flux scales with air density times the cube of
velocity. (The kinetic energy per kilogram of the incoming air scales
with the square of velocity, and how much air you hit per second scales
with density times velocity.) At 1/3 of the speed, you get about 1/27th
of the heat flux; that's good. But the air density at 8km (highest
mountains) is about 1,000,000x that at 100km (typical reentry altitude);
that's really bad. The multiplication is left to the student. :-)


I think that this is technically true but misleading. I don't think the
100km typical re-entry
altitude is where the heat flux is highest. Do you need any serious
thermal protection
if you are at orbital speed at 100km (other than because you would need
it a few minutes
later). At what altitude does a typical re-entry experience its maximal
heat flux?
The important point remains true, the air density at 60km is about a
thousand times
that at 8km and a heat flux of 1000 * 1/27 is a no no (orbital speed at
60km is
surely a severe heat flux).

On a related topic, does any one know if Columbia was far from its
maximal heat
flux when it started to break up? At what altitude during descent does
the protective
tiles stop being necessary?

Alain Fournier

"Hop David" wrote in message
...

I googled mag lev trains and learned that existing trains achieve a
speed of .120 km/sec.

Presumably this is because they are designed to compete with traditional
steel-rail trains - a ticket can't cost more than a few dollars (or euros).
A maglev launcher is competing against a shuttle launch at $500 million a
pop - so there is incentive to innovate.

Hop David wrote


I googled mag lev trains and learned that existing trains achieve a
speed of .120 km/sec. The equator speed is about .464 km/sec so the sled
might achieve .584 km/sec at lift off.

A maglev isn't really the best way to go about it.

For non-human cargo which can take a bit of g then a big airgun, using
hydrogen instead of air as working fluid, will get you about 2 km/s and be a
lot cheaper than a maglev. No carts, just a light, reuseable sabot. I once
costed one at UK £4 Bn (approx $6 Bn), but that was a while ago.

A slightly cheaper system using air will give about 900 m/s, plus the 468
for Earth's orbit, and be suitable for humans.

Even building both together would be a lot cheaper than a maglev. Maglev's
are very expensive.


--
Peter Fairbrother

Roger Stokes wrote:

"Gordon D. Pusch" wrote in message
...
"Roger Stokes" writes:

The track would be dug into a groove to direct sonic booms upward so the
NIMBYs wouldn't whine.

Shock waves don't work that way.

Thanks - I guess I was visualizing from the anti-sound walls sometimes seen
along freeways.

How do shock waves propagate from a supersonic object in a trench?

violently

Peter Fairbrother wrote:
Hop David wrote


I googled mag lev trains and learned that existing trains achieve a
speed of .120 km/sec. The equator speed is about .464 km/sec so the sled
might achieve .584 km/sec at lift off.


A maglev isn't really the best way to go about it.

For non-human cargo which can take a bit of g then a big airgun, using
hydrogen instead of air as working fluid, will get you about 2 km/s and be a
lot cheaper than a maglev. No carts, just a light, reuseable sabot. I once
costed one at UK £4 Bn (approx $6 Bn), but that was a while ago.

A slightly cheaper system using air will give about 900 m/s, plus the 468
for Earth's orbit, and be suitable for humans.


900 m/s is about Mach 2.65? I believe the Concorde attains Mach 2.5 so I
am venturing to guess Mach 2.65 is plausible.

so that totals about 1.4 km/sec. A typical orbit speed is about 9 km/sec?

(Mass payload + Mass fuel)/mass payload = e^(Vfinal/Vexhaust)?

4 km/sec is a plausible exhaust speed?

e^((9-1.4)/4)=6.7
e^(9/4) =9.5

This seems like quite an improvement!

I guess this wouldn't be truly SSTO but actually TSTO where the first
stage (aka the big air gun) is ground based and reusable?

Hop
http://clowder.net/hop/index.html

(Henry Spencer) wrote:

In article ,
Hop David wrote:
Looking at a globe there seems to be some real estate east of the Andes
at the equator. I don't think folks living east of the launch site would
be happy.

That area isn't heavily populated, at least not until you get considerably
farther east, but it's certainly an issue.

It is also an area that a large variety of folks are increasingly
interested in removing existing disturbances and maintaining that
which is currently undisturbed in that state.

Dropping spent or failed stages there will be a tough sell.

D.
--
The STS-107 Columbia Loss FAQ can be found
at the following URLs:

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Corrections, comments, and additions should be
e-mailed to , as well as posted to
sci.space.history and sci.space.shuttle for
discussion.

(Gordon D. Pusch) wrote in message ...
"Roger Stokes" writes:

"Gordon D. Pusch" wrote in message
...
"Roger Stokes" writes:

I have read in this newsgroup that chemical fuel SSTO proposals have
a mass ratio of around 8:1, leaving very little left over for payload.
However if the launch could be assisted by an initial velocity of about
2 km/sec the vehicle mass ratio would drop to about 4:1, permitting a
much bigger payload.

2 kps is Mach 5. Mach 5 is a bit fast inside an atmosphere. Even at the
top of the highest mountain on Earth, the air will still be thick enough
that the vehicle will need a thermal protection system ON THE WAY UP, as
well as on the way down.

If the vehicle is SSTO, it will already posess a thermal protection system
for reentry at mach 25, which should be only lightly loaded at mach 5-10
for a few 10's of seconds on the way up, even at greater air density.
(Although this is just intuition, since I don't know how to calculate it)

There is a =BIG= difference between doing Mach 25 through the near-vacuum
at 250,000 feet versus through a substantial fraction of an atmosphere.
Even Mach 5 is =WAY= to fast at the pressures you find on mountaintops !!!


-- Gordon D. Pusch

perl -e '$_ = \n"; s/NO\.//; s/SPAM\.//; print;'



Well, troopers, a mag lev launcher might do for returns from
planets/moons without atmospheres, eh? Now all we need it 'da juice.

In article ,
Hop David wrote:
Looking at a globe there seems to be some real estate east of the Andes
at the equator. I don't think folks living east of the launch site would
be happy.

Indeed, the city of Mecapa is located almost precisely on the equator
(longitude 51 W), just inland from the Atlantic coast, where the Amazon
river flows into the Atlantic ocean. I can't find Mecapa's population in
my atlas, but it's a fairly major city (it's the capital of Amapa state).
I don't think its residents, or their representatives in the Brazilian
Congress and Senate, would appreciate being downrange of such a launcher.

--
-- "Jonathan Thornburg (remove -animal to reply)"
Max-Planck-Institut fuer Gravitationsphysik (Albert-Einstein-Institut),
Golm, Germany, "Old Europe" http://www.aei.mpg.de/~jthorn/home.html
"Space travel is utter bilge" -- common misquote of UK Astronomer Royal
Richard Woolley's remarks of 1956
"All this writing about space travel is utter bilge. To go to the
moon would cost as much as a major war." -- what he actually said