JP Aerospace Airship to Orbit

I just read the space access 04 review on RLV news
http://www.hobbyspace.com/AAdmin/archive/RLV/2004/SpaceAccess04.html. It
seems that JP Aerospace plans to build a high altitude airship that can fly
directly to orbit using low thrust electric propulsion.

I think that there is no way in hell this is going to work. But JP Aerospace
are certainly no idiots, and they have already produced some impressive
working hardware, so maybe they are on to something.

So what is your opinion? Is there a chance this could work?

best regards,

Rüdiger Klaehn

On Thu, 29 Apr 2004 00:33:23 +0200, in a place far, far away, Ruediger
Klaehn made the phosphor on my monitor glow in
such a way as to indicate that:

I just read the space access 04 review on RLV news
http://www.hobbyspace.com/AAdmin/archive/RLV/2004/SpaceAccess04.html. It
seems that JP Aerospace plans to build a high altitude airship that can fly
directly to orbit using low thrust electric propulsion.

I think that there is no way in hell this is going to work. But JP Aerospace
are certainly no idiots, and they have already produced some impressive
working hardware, so maybe they are on to something.

So what is your opinion? Is there a chance this could work?

It doesn't obviously defy physics, but I'd have to see the simulations
to have confidence.

In article ,
Ruediger Klaehn wrote:

I just read the space access 04 review on RLV news
http://www.hobbyspace.com/AAdmin/archive/RLV/2004/SpaceAccess04.html. It
seems that JP Aerospace plans to build a high altitude airship that can fly
directly to orbit using low thrust electric propulsion.

I think that there is no way in hell this is going to work. But JP Aerospace
are certainly no idiots, and they have already produced some impressive
working hardware, so maybe they are on to something.

So what is your opinion? Is there a chance this could work?

It's certainly a novel concept. I guess the idea is that it doesn't
have to fight gravity at the same time it's accelerating -- it uses
buoyancy instead. My initial reaction was incredulity, like you, but
who knows -- as you say, the JP Aerospace guys are not idiots.

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Joe Strout wrote:

It's certainly a novel concept. I guess the idea is that it doesn't
have to fight gravity at the same time it's accelerating -- it uses
buoyancy instead. My initial reaction was incredulity, like you, but
who knows -- as you say, the JP Aerospace guys are not idiots.

The quote ends with:

Using electric propulsion, the vehicle would gradually gain speed
and take advantage of the slight lift provided by the residual
atmosphere even at such high altitudes. Simulations show such
a vehicle could obtain orbital velocity in about 5 days.

Ok, during most of that time this thing will be hypersonic.
Hypersonic L/D isn't very high (a factor of a few), so the thrust
would have to be a factor of a few less than the mass of the
vehicle for it to accelerate at all. This seems inconsistent
with the '5 day' figure.

What about bouyancy? The bouyancy pressure will be proportional
to density * g * vertical size of the vehicle. The dynamic pressure
will be proportional to density * speed^2. The latter is orders of
magnitude more than the former at hypersonic speeds. So forget
bouyancy, it's negligible compared to lift and drag at high speed.

Paul

On Wed, 28 Apr 2004 22:12:38 -0500, in a place far, far away, "Paul F.
Dietz" made the phosphor on my monitor glow in such a
way as to indicate that:

The quote ends with:

Using electric propulsion, the vehicle would gradually gain speed
and take advantage of the slight lift provided by the residual
atmosphere even at such high altitudes. Simulations show such
a vehicle could obtain orbital velocity in about 5 days.

Ok, during most of that time this thing will be hypersonic.
Hypersonic L/D isn't very high (a factor of a few), so the thrust
would have to be a factor of a few less than the mass of the
vehicle for it to accelerate at all. This seems inconsistent
with the '5 day' figure.

I'm not sure what "hypersonic" means when the mean free path goes to
centimeters or more.

What about bouyancy? The bouyancy pressure will be proportional
to density * g * vertical size of the vehicle. The dynamic pressure
will be proportional to density * speed^2.

Again, I'm not sure that relation applies at low densities.

The latter is orders of
magnitude more than the former at hypersonic speeds. So forget
bouyancy, it's negligible compared to lift and drag at high speed.

Well, as I said, I want to see some numbers, based on PAWKI (physics
as we know it).

Rand Simberg wrote:

Well, as I said, I want to see some numbers, based on PAWKI (physics
as we know it).

You can plug in you own numbers, but order of magnitude calculations
are enough.. And this *is* based on real physics.



Ok, during most of that time this thing will be hypersonic.
Hypersonic L/D isn't very high (a factor of a few), so the thrust
would have to be a factor of a few less than the mass of the
vehicle for it to accelerate at all. This seems inconsistent
with the '5 day' figure.

I'm not sure what "hypersonic" means when the mean free path goes to
centimeters or more.

Speed of sound is defined even in gas with very long MFP. In this
case, hypersonic implies speeds the average thermal speed in the gas.



What about bouyancy? The bouyancy pressure will be proportional
to density * g * vertical size of the vehicle. The dynamic pressure
will be proportional to density * speed^2.


Again, I'm not sure that relation applies at low densities.

Of course it does. The bouyancy pressure applies even if the
MFP is infinite, it just requires that the height of the vehicle
be small compared to the scale height of the atmosphere (which it
will be.) Similarly, the drag relationship is a consequence
of conservation of momentum, and would apply even if MFP is infinite
(although the constant might change).

Paul

On Thu, 29 Apr 2004 00:22:26 -0500, in a place far, far away, "Paul F.
Dietz" made the phosphor on my monitor glow in such a
way as to indicate that:

Again, I'm not sure that relation applies at low densities.

Of course it does. The bouyancy pressure applies even if the
MFP is infinite, it just requires that the height of the vehicle
be small compared to the scale height of the atmosphere (which it
will be.) Similarly, the drag relationship is a consequence
of conservation of momentum, and would apply even if MFP is infinite
(although the constant might change).

As I said, while it may be so to you, it's not obvious to me that it
will or won't work upon inspection. You may well be right, but
there's no cost to me to wait until they show something more
substantive to issue a judgement.

Ruediger Klaehn wrote:

I just read the space access 04 review on RLV news
http://www.hobbyspace.com/AAdmin/archive/RLV/2004/SpaceAccess04.html. It
seems that JP Aerospace plans to build a high altitude airship that can fly
directly to orbit using low thrust electric propulsion.

I think that there is no way in hell this is going to work. But JP Aerospace
are certainly no idiots, and they have already produced some impressive
working hardware, so maybe they are on to something.

So what is your opinion? Is there a chance this could work?


Orbital dirigibles... I have now officially heard _everything_!
Is it supposed to be solar powered, or driven by some other form of
energy- such as beamed microwaves from the ground?

Pat

Joe Strout wrote:

It's certainly a novel concept. I guess the idea is that it doesn't
have to fight gravity at the same time it's accelerating -- it uses
buoyancy instead. My initial reaction was incredulity, like you, but
who knows -- as you say, the JP Aerospace guys are not idiots.


But it starts out (the orbiter part) at 140,000 feet, and is supposed to
use ion drive to climb to orbit; since Dark Sky Station is hovering up
there, there is obviously some air around; and that air is going to
generate a lot of drag on the Orbital Ascender as it tries to accelerate
under ion power...how exactly powerful are these ion motors supposed to
be? You are going to have to generate tens of thousands of pounds of
thrust to get the thing accelerating- unless the idea is to turn the
whole envelope into a giant electrostatic ion motor and flow the air
around it to generate propulsion energy (i.e. It wouldn't so much push
itself forward as suck itself along, like a giant floating "Ionic
Breeze" air cleaner, effectively generating no drag at all.)
To me the problem seems to be related to how much such a system would
weigh compared to it's buoyancy; at 140,000 feet you aren't going to get
much lift in comparison to the size of your gas bags.
Still, it's a very wild and fun idea, isn't it?

Pat

Paul F. Dietz wrote:


Ok, during most of that time this thing will be hypersonic.
Hypersonic L/D isn't very high (a factor of a few), so the thrust
would have to be a factor of a few less than the mass of the
vehicle for it to accelerate at all. This seems inconsistent
with the '5 day' figure.

I think the thing uses a plasma sheath over its entire surface to
accelerate the air around it as it accelerates; that's why the paintings
show it as blue- it's glowing from the fact that its outer surface is
ionized; I would think that this would make it effectively dragless, it
sucks up the air around its entire surface and accelerates it
backwards...probably off the upper tips of the "V" shape; as the air
ahead of it is drawn towards it, it itself is constantly drawn forward
by the inertia of the air that is being accelerated towards its leading
edge- in short, it's constantly pulling the air ahead of it towards it,
while at the same time being constantly pulled forward by the air ahead
of it.
They've got a real problem in relation to their Earth surface to Dark
Sky Station Ascender though; at 140,000 feet the gas (hydrogen?) is
going to be around twenty times the volume that it is at the Earth's
surface, and assuming they vent gas all the way up to 140,000 feet to
prevent the envelope from exploding, they are either going to have to
either carry spare gas to refill it during descent; or use an internal
air-filled ballute that takes up most of it's internal volume if they
expect it to return in the same shape it lifted off in (I guess you
could just pump outside air into it and skip the ballute; but I would
suggest using helium -not hydrogen- if that's the case, unless one wants
a great big blue flash and "kaboom" sound to be the end of a lot of
descents). This is going to affect the cargo carrying capacity very
adversely.

Pat