sci.space.tech Google Group

Re: Sub-Orbital Earth Transits

nos...@127.0.0.1 (David Spain) — Sat, 12 Dec 2009 01:28:17 UT

That depends upon how high in the atmosphere though.
The higher you can fly the faster you can go for a given T correct?
Isn't the maximum T reached when begin your descent?
Don't you start to trade V for T, until your V is low enough
and the air dense enough to allow convection to reduce T?
It would seem to me the limiting factor is the rocket.

Re: Sub-Orbital Earth Transits

alain...@sympatico.ca (Alain Fournier) — Fri, 12 Dec 2009 15:49:43 UT

There is no boundary of the atmosphere. It just gets thinner and thinner.
So it won't leave the atmosphere. It does have to go high enough to
avoid excessive drag. How high would be best depends on lots of things.
Alain Fournier

Re: Sub-Orbital Earth Transits

charliexmur...@yahoo.com (Me) — Fri, 12 Dec 2009 15:49:29 UT

Why does SpaceShipX have to *leave* the atmosphere?
?
So it can coast with no drag and have zero g.
If it stayed in the atmosphere, it wouldn't go very far. Drag would
slow it down. Also it wouldn't be able to fly much above Mach 2 due
to heat build up.

Re: Sub-Orbital Earth Transits

nos...@127.0.0.1 (David Spain) — Fri, 12 Dec 2009 12:35:52 UT

Hmm. You just made me think about another possibility, so obvious I should
have thought of it before! Why does SpaceShipX have to *leave* the atmosphere?
What kind of trajectory could it produce if it stayed low enough not to
build up enough speed to cause trouble. In other words, have far can it
translate on its own rocket power at a shallow angle and far lower speeds?

Re: Sub-Orbital Earth Transits

alain...@sympatico.ca (Alain Fournier) — Fri, 12 Dec 2009 02:08:10 UT

I haven't done any computations on this but I don't think that the
shuttlecock configuration is appropriate if you have significant
horizontal velocity.
What you want to avoid is to be going very fast in a thick atmosphere.
In the case where you just go up then down like spaceship one, the
shuttlecock is a nice idea because you really want to have as much

Re: Sub-Orbital Earth Transits

nos...@127.0.0.1 (David Spain) — Thu, 12 Dec 2009 23:40:36 UT

etc.
Sorry, my original post mentioned a TP skin on the parachute or balloon.
I was thinking this might be needed as a protective measure for either
the parachute or balloon, but only after significant V reduction by the
shuttlecock configuration. I was not intending to say I expected
it to replace the shuttlecock configuration. Only to facilitate further

Re: Sub-Orbital Earth Transits

nos...@127.0.0.1 (David Spain) — Thu, 12 Dec 2009 02:00:42 UT

Not exactly. I was thinking more in terms of a speed brake in order
slow the vehicle enough to reduce aerodynamic forces enough to enable
re-deployment of the booms back to a glider configuration.
So the general idea would be, coat the high T surfaces with TP, to allow
the 'shuttlecock' configuration to be able to withstand the heating of

Re: Sub-Orbital Earth Transits

nos...@127.0.0.1 (David Spain) — Wed, 12 Dec 2009 19:40:57 UT

Yeah, I agree to think otherwise seems unreasonable,
read stupid if you prefer :-)...
I think what happened here, is that my friend and I got off
on a tangent, worrying about the difference between airspeed and
groundspeed. Aerodynamic heating is caused by airspeed, but as you
say, to first approximations, it doesn't really matter, they can

Re: Sub-Orbital Earth Transits

alain...@sympatico.ca (Alain Fournier) — Wed, 12 Dec 2009 02:23:24 UT

If I understand well what you are saying here, you want to deploy
a parachute at hypersonic speeds as a way to lower the the heat
load during atmospheric entry. I think it might be workable, but
it isn't necessarily easy and straightforward. It can't be a nylon
parachute, that would burn in an instant. But you could make it out

Re: Sub-Orbital Earth Transits

jrfr...@ibm-pc.borg (Jorge R. Frank) — Tue, 12 Dec 2009 15:58:05 UT

David, I think you're still not quite getting it.
The Earth's atmosphere, to first approximation, rotates with the Earth.
Therefore thrusting eastward to add to your inertial velocity does *not*
increase your velocity *relative to the atmosphere* any more than
thrusting westward to cancel your inertial velocity; therefore, the TPS

Re: Sub-Orbital Earth Transits

nos...@127.0.0.1 (David Spain) — Mon, 12 Dec 2009 16:04:02 UT

I want to thank Alain Fournier, Charlie Murphy, Jonathon Thornburg,
and Jim Davis for their helpful insights.
I would agree that it seems mitigation via ullage rocketry is impractical
given current technology.
As was also pointed out, you have to overcome the horizontal velocity
component as well, which means expending fuel to reduce it. It is

Re: Sub-Orbital Earth Transits

alain...@sympatico.ca (Alain Fournier) — Mon, 12 Dec 2009 00:53:27 UT

I made a mistake when I wrote that. Because of the slower speed
you need to more than double the atmospheric pressure to double
the deceleration. The atmospheric drag is roughly proportional to
the cube of the speed. So to compensate the speed decrease from
5.6 km/s to 5.5 km/s, you need to increase the air density by

Re: Sub-Orbital Earth Transits

jimdav...@earthlink.net (Jim Davis) — Sat, 12 Dec 2009 02:32:51 UT

David,
1. .5g is still much too high a thrust level to be maintained for
hours waiting on the earth to rotate beneath you. Propellant
consumption is still prohibitive.
2. The mitigation is grossly insufficient. Thrust for .5g still means
you're accelerating downward a 4.9 m/s2. At that acceleration you'll

Re: Sub-Orbital Earth Transits

jth...@astro.indiana-zebra.edu (Jonathan Thornburg [remove -animal to reply]) — Sat, 12 Dec 2009 02:32:16 UT

I asked
and then calculated it to be a (very) impractically large amount.
David Spain <nos...@127.0.0.1> asked
Free-fall (a.k.a. "zero g" starts as soon as the ascent rocket cuts out
(assuming this is above the atmosphere, which is the case for any practical
rocket & trajectory), and it lasts until you hit the atmosphere again on

Re: Sub-Orbital Earth Transits

charliexmur...@yahoo.com (Me) — Fri, 12 Dec 2009 06:43:02 UT

nt
ere?
e
d to
, which
m of
Which still weighs more than TPS. The whole basis premise of your
idea is not viable.
The reason that the shuttle, Soyuz and all entry vehicles use the
atmosphere to reduce speed is because it is more efficient than using
propellant.
This is the line in your proposal where if goes wrong: