Pete Lynn wrote:
Inflatable habitats and reentry shields have reached some level of
acceptance. With this in mind there no longer seems any good reason why
the two should not be combined to construct significantly lower drymass
capsules and even space transports.

Interesting subject, inflatable reentry shields. I hadn't looked into
it until you brought it up.

http://www.jamesoberg.com/112003irv_his.html
http://snipurl.com/n4nq
http://www.space.com/missionlaunches...ne_000816.html
http://www.strangehorizons.com/2006/...g-hell-a.shtml
http://www.engr.uconn.edu/~adstc/PUB...aareview01.pdf

The only problem is I haven't seen anything that shows one working
well.

The Inflatable Habitats are also interesting...

http://www.space.com/news/businessmonday_040524.html
http://snipurl.com/n4op
http://snipurl.com/n4pl
http://www.thespacereview.com/article/292/1
http://www.thespacereview.com/article/293/1

However, I may have a more efficient design which accomplishes more.
Something to discuss later, sometime.

Inflatable propellant tanks would seem less challenging development-wise
than either inflatable habitat modules or heat shields. And so fully
inflatable space transports seem potentially quite possible.

I am guessing you are referring to some craft inflated in space to
transport between another destination in space. I am finding it hard
to picture a launch vehicle with inflatable tanks. My interest in tanks
stems from the Hydrogen Economy on Earth. Hydrogen is a persnickety
element frustrating to generations of engineers for over 30 years
trying to make compact vehicle fuel tanks work in cars. All the
problems of making H2 tanks work in cars are compounded making them
work on Mars (or to Mars).


In adopting such an approach drymass can be greatly reduced, perhaps by
as much as a half depending on details, tank mass can become near
negligible, as can structural mass. Hence payload might also be greatly
increased.

In space-to-space transport, size is no limitation, but volume does
have a cost in terms of skin sheathing. A kilogram of liquid Hydrogen
is 14 liters, but a kilogram of Hydrogen gas is 11.98 cubic meters, or
11,978 liters. Obviously it takes more material to contain Hydrogen gas
than to contain Hydrogen liquid. That extra size of sheathing
containment material costs mass. Hydrogen is the smallest and leakiest
material there is. It can find ways through most materials, sometimes
destroying the material on its way through. You will also notice that
it doesn't pack much power in its volume either -- those 11,978 liters
of H2 gas has about the same fuel energy as a gallon of gasoline packed
in 3.8 liters, but only has that power if four times it's mass of
oxygen is present. Hydrogen does not burn without oxygen. (Technically,
the stochimetric ratio is ideally 1:8 H2 to O, but realistically H2
burning in rocket engins is burned "rich" with 2x the H2 required
ideally.)

I am not knocking H2. I said I came into this from an interest in H2. I
just like working with the realities that H2 presents and solving those
problems in realistic fashion.

When you take a kilogram of H2(l) and heat it to vapor, 20.28 K
(-252.87 °C, -423.17 °F), those 14 liters become 11,978 liters of
H2(g). If the tank is rigid the pressure will be in the scores of
thousands of psi. I don't really know because nobody ever did that and
survived to tell about it. At 800 bar H2 only condenses down to 27.7
liters, which is at 11,603 psi.

You can't just double H2 pressure and reduce the volume in half. H2 has
fierce positive charge repulsion.

In short, there is a big quantum leap between carring significant
volumes of H2 as liquid and letting it go to vapor. Quite probably the
Hindenberg zeppelin did not carry the load of H2 that the Shuttle
carries for launch. I haven't been moved to do the math. If you want to
replave the Shuttle External Tank with an inflatable, you won't be
carrying LH2 but gasous H2, and then you have a Hindenberg strapped to
the SRBs and Shuttle at launch. I have a feeling that the mass savings
for so much skin and so much air resistence will not turn out to have
much value after all.

Inflatable LH2 technology has not been demonstrated. Whether the
container is metal, rigid composite (like the Space Shuttle) or
inflatable, the LH2 must be kept cryogenic, which requires
refrigeration equipment and power supply operating continuously.

If the temperature ever gets to boiling the pressures of thousands of
pounds per inch square will pop the balloon.


At such low drymass fractions the design margins are greatly eased such
that SSTO becomes favoured with regard to development costs. For
example, the Falcon 5 lower stage so modified could become a reusable
SSTO of modest payload. Suborbital applications promise similar
advantages.

If the time is not yet right to bite the bullet of inflatable space
transports, then it is not far off. It definitely has the potential to
quickly make a lot of more traditional approaches obsolete and I am
somewhat surprised someone is not already covering the possibility.

Pete.