Ascender: Airship to Orbit?

This was an interesting article:

http://www.msnbc.msn.com/id/5025388/


Gee, this sounds like it would be a much more enjoyable Space Tourism
experience. Instead of the brief 30-min trip on SpaceShipOne, have a
3-9 day trip to orbit on a larger blimp type of vehicle. The large
Zeppelins used to afford a luxurious trans-atlantic trip in their day,
compared to other modes of travel.

I've heard various people in here comment on the need to make Space
Tourism a more luxurious experience, in order to justify the higher
pricetag and attract the big spenders. So the airship-to-orbit could
provide that.

Their idea of using ion-propulsion to reach orbit also sounded
noteworthy. I've read they're a few times more efficient than chemical
rockets, but if the ion thrust is much lower, then what is
justification of taking the rest of the buoyant mass outside of the
atmosphere all the way to orbit?

But could ion-thrust really get a vehicle to orbit, as they claim?

In article ,
sanman wrote:
But could ion-thrust really get a vehicle to orbit, as they claim?

Assuming this is the system that JP Aerospace described at Space Access
(that URL isn't working for me at the moment)...

No outsider has yet been able to make the numbers add up for the system as
described. It seems to assume impossibly good aerodynamics. With the
caveat that right now there is only very sketchy information about the
design, the general conclusion so far is that either there is some quite
subtle trick involved, or they've goofed.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

(Henry Spencer) wrote in message ...
In article ,
sanman wrote:
But could ion-thrust really get a vehicle to orbit, as they claim?

Ion thrust to my knowledge only works *after* that you are in orbit.

It works for long time, uses little propellant, but has weak thrust,
much less than .1 g, so you can't lift anything with it, from Earth's
surface.

Regards

Carsten Nielsen
Denmark

I'm having a problem witht the ion engines. Even though there is
hardly any air resistance at that altitude, the engines only put out a
few pounds of thrust - at best.

I'm thinking it would be cool to have this sort of thing but allow it
to have a cable going to another, higher, actual space station.

(Henry Spencer) wrote in message ...
In article ,
sanman wrote:
But could ion-thrust really get a vehicle to orbit, as they claim?

Assuming this is the system that JP Aerospace described at Space Access
(that URL isn't working for me at the moment)...

No outsider has yet been able to make the numbers add up for the system as
described. It seems to assume impossibly good aerodynamics. With the
caveat that right now there is only very sketchy information about the
design, the general conclusion so far is that either there is some quite
subtle trick involved, or they've goofed.

I've read they're a few times more efficient than chemical rockets, but if
the ion thrust is much lower, then what is
justification of taking the rest of the buoyant mass outside of the
atmosphere all the way to orbit?
To give them something to reenter with!
I think the principle that JPA is relying on is that a large surface area to
weight ratio will provide a cooler reentry. Bear in mind that their "Orbital
Ascender" is not a jug-shaped capsule that heats-up like a branding iron when
it reenters, it is a *flyable* V-shaped balloon, so it can maneuver in the
higher, thinner regions long enough to slow down cooly.

But could ion-thrust really get a vehicle to orbit, as they claim?
Remember, this vehicle is *floating* on air, thin air but still air. It
therefore takes very little power to accelerate the vehicle, just enough to
overcome the small drag of the thin air. As the balloon accelerates, it is
nudged upwards a little, the air gets thinnner, the drag decreases, the balloon
speeds up and repeats the process.
Thruster power is being continually increased so it is likely the concept
can work.
It's GOT to work, it's the only way for truly CATS.
^
//^\\
~~~ near space elevator ~~~~
~~~members.aol.com/beanstalkr/~~~

In article ,
John Bartley I solved my XP problems w/ Service Pack Linux wrote:
No outsider has yet been able to make the numbers add up for the system as
described. It seems to assume impossibly good aerodynamics...

The analysis does include their three-element system (# 1 ship,
ground-to-station / station in the stratosphere / #2 ship, station to
LEO), right?

Right. The problem is that the orbiter appears to need an impossibly good
L/D ratio to support itself (its buoyancy is helpful only briefly) while
spending several days accelerating to orbital velocity on low-thrust
propulsion. Real-life hypersonic L/D is generally pretty lousy, and while
this thing *is* rather unconventional by hypersonic-vehicle standards,
nobody can see any reason why it would have a radically improved L/D.

Does it also consider the possibility of using hydrogen in the #2
ship; hydrogen, lighter than He, is not very flammable at the low
pressures 20 miles up.

Hydrogen does not, alas, increase the lift very *much* -- it gives 93% of
the lift that vacuum would, while helium gives 86%. The difference is not
big enough to be worth a lot of trouble; the main reason early balloons
and airships used hydrogen was that it's a whole lot cheaper and easier to
get. (The Hindenberg was designed for helium, but used hydrogen because
the US -- which controls most of the world's helium supply -- refused to
sell bulk helium to Nazi Germany.)

...they've got a track record with their PongSats, so perhaps
they don't need any Unobtanium to make it work.

Yeah, if it was just *anybody* proposing this, they'd simply have been
written off as crazies. JP clearly has its act together on dealing with
lesser technical challenges, so insanity is not a convincing explanation
in this case. Hence people are puzzled.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |

Allen Meece wrote:

But could ion-thrust really get a vehicle to orbit, as they claim?
Remember, this vehicle is *floating* on air, thin air but still air. It
therefore takes very little power to accelerate the vehicle, just enough to
overcome the small drag of the thin air. As the balloon accelerates, it is
nudged upwards a little, the air gets thinnner, the drag decreases, the balloon
speeds up and repeats the process.

It's easy to see that bouyancy forces will be small compared to drag
forces at all but low speed (probably below Mach 1).

My take on this idea is that they aren't going to use ion engines as
such, but some higher thrust electric engine. For example, a scheme
that applies an electric field along the passing airstream and accelerates
injected ions, which accelerate the air by ion/neutral collisions.
I also suspect their thrust/weight ratio is intended to be very high,
and they take a long time to reach orbit only because of limits on the speed
of the vehicle due to the extremely flimsiness of the design.

Paul

Allen Meece wrote:


It's GOT to work, it's the only way for truly CATS.
^
//^\\
~~~ near space elevator ~~~~
~~~members.aol.com/beanstalkr/~~~


Be very careful about using phrases like "GOT to work" and "the only
way..."

--

You know what to remove, to reply....

This idea does not make sense.

JP Aerospace claims they will use standard ion drive
(NASA Deep Space 1). The thrust-to-weight ratio of
ion engines is 1/1000 or less. This means that the
lift-to-drag ratio must be more than 1000.
Unfortunately, balloon flying at hypersonic velocity
would have the lift-to-drag ratio of 3 or less.

Pat Flannery suggested that the drag can be reduced
by enclosing the balloon in a plasma sheath.
This does not make sense for two reasons:
- Viscosity of plasma is greater than viscosity of
air of the same density. This means that drag is
greater in plasma than in air.
- Balloons are made of plastics which are burned
by the hot plasma. By the way, the ambient air
would burn the balloon as well.

Solar power density is barely sufficient to keep a
solar powered airplane aloft at noon.

Buoyancy of the balloon is irrelevant at hypersonic
velocity. If you fly the balloon at hypersonic
velocity at the altitude where there is significant
buoyancy (e.g., 40 or 50 km above sea level), its
drag will instantly stop the balloon and burn it.

PS. I have not seen any technical data on the
JP Aerospace web site; only pie-in-the-sky images
and calls for donations.

In article ,
Allen Meece wrote:
Lift here is not aerodynamic lift but is rather, positive buoyancy. So it
makes the L/D ratio irrelevant. The ship is weightless at 200K feet and needs
little aerodynamic lift. As the velocity increases, so does centripetal force,
helping to "lift" the craft.

Unfortunately, at Space Access they said (if memory serves) that it's at
nearly double that altitude at only about Mach 5, still too slow for
centrifugal lift to be very significant.

(A centripetal force by definition pushes *toward* the center of rotation;
the proper word here is indeed centrifugal.)

More generally, the apparent shortfall in L/D is much too large to be made
up that way. Centrifugal force rises with the square of velocity, so you
have to be at 70% of orbital velocity to get half your lift that way, i.e.
to relax the required L/D by a mere factor of 2.

Buoyancy lift may be an important part of the picture, but centrifugal
lift almost certainly isn't.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |