"SpaceShipTwo could be single stage to SUBorbit"

On 5/22/2010 12:16 PM, Fevric J. Glandules wrote:
Pat Flannery wrote:

On 5/21/2010 2:50 PM, Fevric J. Glandules wrote:
...and I still don't understand the problem. The gas cells can be
slightly over-pressurised, and made of material that doesn't stretch.
Inside the gas cells are stretchy balloons, into and out of which you
pump air as needed.

What am I missing?

That the total ability of the airship to lift its own weight and its
payload is a fixed amount, and the fuel has weight, so as the fuel is
burnt its weight is lost and the airship becomes lighter and tries to
rise to a higher altitude.

Oh, I understand that bit. But I don't understand why controlling
bouyancy is such a problem. All you need, or perhaps that is
"all" you need, is to regulate the volume displaced by the
lifting gas cells. This could be done using *pressurised*
ballonets.

What am I missing?

The weight of ballonets capable of doing that, which would cut badly
into the lifting capacity of the airship.
I once thought about using another concept; instead of pressurizing the
ballonet, use a compressor to take some of the helium and store it in
high pressure tanks till it's needed to generate lift again.
But again, weight is the problem with that concept, although maybe you
could make some sort of carbon fiber composite tanks that wouldn't weigh
too much.
An idea that has repeatedly come up in the history of both airships and
balloons, is heating the lifting gas to decrease its density and
increase its lift and controlling your altitude by varying the gas
temperature; its a simple and elegant concept, but how exactly to
efficiently heat the gas is the problem. Obviously, helium would be a
far safer choice to do this with than hydrogen.
A idea that has been used with success is to combine a hot air balloon
with a gas filled one and use the hot air one to control the buoyancy.
This idea goes clean back to 1785, although the first test of it proved
fatal: http://www.spartacus.schoolnet.co.uk/AVrozier.htm
http://xplanes.tumblr.com/post/22117...ancois-pilatre


Thanks, that's some interesting stuff.

If I've got this right, in a blimp the system is essentially stable
and self-compensatory. As the blimp gains altitude, the lifting gas
expands, due to reduced pressure; but the weight of displaced air
remains the same, as it follows the same equation, and hence the
blimp continues to have neutral bouyancy [1]. The whole system
operates at essentially ambient pressure, with a bit of over-pressuring
to keep the envelope taught [2].

That's the idea, by giving the helium room to expand inside the envelope
as the blimp rises by the use of the ballonets, its buoyancy remains stable

My proposal involves running the system at higher pressures; have
pressurised ballonets [3] that reduce the volume of lifting gas
as required. That would involve a non-stretchy gas cell.

There's something like that in use already called a superpressure
balloon; in its case the balloon is filled completely with helium on the
ground, and the gas can't expand as it ascends due to the strength of
the envelope. These are used for high altitude meteorology in regards to
winds, as they will ascend to a particular flight level and stay the
http://www.space-travel.com/reports/...ctica_999.html
I don't know if anyone has used this concept in relation to a manned
balloon though.

Or use another method: have stretchy gas cells contained within a
net. By mechanical means, compress the gas cells to reduce the
volume of air that they displace. This might be expensive in
terms of energy, but it would only be for fine-tuning: the main
lifting cells would operate on the blimp / ballonet principle.

What am I missing? What is so difficult that it requires
condensing exhaust gases, etc., etc.?

It's a matter of weight again... though now we have a lot more
sophisticated materials than they did in the age of the great
dirigibles, so maybe something along the lines you are suggesting could
be done.
Considering that it was both successful and trouble free*, its odd the
Navy didn't follow up the metal-skinned ZMC-2 with more work on that
concept: http://nasgi.org/zmc2.htm
The odd thing about the water recovery gear is that they were actually
coming out ahead on the deal in regards to ballast water; not only was
there water in the exhaust from the burning of the gasoline, but the
water in the air the engine was ingesting would condense also, so you
ended up with more weight in water than the weight of the gasoline you
were burning.
You can see the water condensers on this photo of the USS Macon:
http://www.history.navy.mil/photos/i...00/g441983.jpg
They are the lines of rectangular things above the engines.

[1] Ignoring temperature changes, and assuming the ideal gas law,
PV=nRT, says he, to demonstrate that he has actually studied physics
to at least 'O' level, which you'd probably call "high school level".
[2] Saggy blimps can't be good for business, after all.
[3] inside gas cells inside a rigid airship.

They have actually gotten hot-air blimps to work, although obviously
they can't fly any too fast or the bow will cave in due to the headwind:
http://www.personalblimp.com/
....not for use in high winds. ;-)

* Which by Navy standards meant it didn't crash, unlike all the other
rigid airships we had other than the one we got from the Germans.
The one the British built for us came apart in mid-air before it could
even be delivered.

Pat