Inflatable capsules and space transports

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.

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.

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.

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.

Pete Lynn wrote:
[snip]
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.

There is a german project for an expendable, fully inflatable SSTO. But
unfortunately I did not bookmark the link, and I could not find this on
google. I will let you know if I find it again.

I found it:
http://www.flightglobal.com/Articles/2005/12/20/Navigation/200/203670/Inflatable+launcher+more+than+hot+air.html

It is a little low on details, but it looks like it uses an annular
aerospike nozzle.

It also has a very low payload fraction. Fueled weight is 47t, payload
into an unspecified low earth orbit is just 150kg.

I wonder why they have separate propellant tanks instead of integrating
them with the aeroshell.

"Rüdiger Klaehn" wrote in message
oups.com...
I found it:

http://www.flightglobal.com/Articles...n/200/203670/I
nflatable+launcher+more+than+hot+air.html

It is a little low on details, but it looks like it uses an
annular aerospike nozzle.

It also has a very low payload fraction. Fueled weight
is 47t, payload into an unspecified low earth orbit is
just 150kg.

I wonder why they have separate propellant tanks
instead of integrating them with the aeroshell.

Me too. This is a very interesting but strange design.

It says that such a vehicle if used for Mars sample return, would be one
third the weight of a conventional vehicle. I am not sure that it uses
an annular aerospike, it would seem silly to embrace such added
complexity at this stage, though the position of the helium tank does
suggest it.

Strange things:

- It uses spherical tanks, inflatable tanks like composite tanks do not
need to be spherical.

- It looks pressure fed, requiring heavy tanks. While inflatable tanks
still have an advantage at high pressure they have a far greater
advantage as low pressure balloon tanks. Inflatable tanks do not suffer
the same minimum gauge constraints and buckling during ground handling
is not a problem.

- There seems to have been no effort to make the tanks structurally self
supporting, they are supported by a separate carbon fiber frame which
seems totally unnecessary and negates many of the advantages of using
such tanks.

Having said all that it still looks very interesting indeed and to have
a much lower drymass than conventional vehicles. A definite positive
step in the right direction.

Having the propellant tanks inside the inflatable aeroshell does allow
one to collapse the tanks so as to use the last drops of propellant. It
also helps prevent icing on the LOX tank as the air inside the aeroshell
becomes dehumidified. Nor does this necessarily require much more mass
as the aeroshell pressure adds to the tank pressure. One serious concern
I have with such a configuration is that a small propellant leak could
lead to a very nasty explosion inside the aeroshell. They obviously
think this is not prohibitive, though I think I would still favour an
external tank approach, primarily for this reason. I also like the idea
of having both the propellant tanks sitting on the engine, saves on
plumbing and tank structural support.

Another trick that seems possible using said inflatable materials is the
construction of a very light weight positive displacement turbo pump
equivalent. Kind of like a cross between a Flowmetrics pressure pump and
a turbopump, it should have most of the advantages of both. This might
be ideally suited to such small low cost launch vehicles.

Pete.

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.

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

Mr. Spock - shields?
Shields are inflated to 70% Captian.

I think the mass ratio they mentioned is an extremely conservative
estimate. They have probably a 500kg allowance in their calculations to
allow for weight gain. So a payload of up to 500kg is probably
possible.

Another trick that seems possible using said inflatable materials is the
construction of a very light weight positive displacement turbo pump
equivalent. Kind of like a cross between a Flowmetrics pressure pump and
a turbopump, it should have most of the advantages of both. This might
be ideally suited to such small low cost launch vehicles.

Could you elaborate on that? Or is that idea confidential?

I don't really see what is wrong with turbopumps. If you have decent
margins, turbopumps can be very reliable and also easy to manufacture.

The V2 turbopumps were assembled under very bad conditions in forced
labor camps, and yet they managed a decent reliability.

And modern airplane turbine blades are operating in an extremely
hostile environment, yet they manage to work for many thousand hours
without major maintenance.

"Rüdiger Klaehn" wrote in message
oups.com...
I think the mass ratio they mentioned is an extremely
conservative estimate. They have probably a 500kg
allowance in their calculations to allow for weight
gain. So a payload of up to 500kg is probably
possible.

Yes 150 kg payload sounds small, 47 ton GLOW infers drymass plus payload
being something like 2500-3000 kg.

Another trick that seems possible using said
inflatable materials is the construction of a very light
weight positive displacement turbo pump
equivalent. Kind of like a cross between a
Flowmetrics pressure pump and a turbopump, it
should have most of the advantages of both. This
might be ideally suited to such small low cost launch
vehicles.

Could you elaborate on that? Or is that idea
confidential?

Roll sock seals/pistons, (similar to diaphragms) are nothing new. A roll
sock seal is a tube of flexible material that is rolled back inside
itself, like turning a sock half inside out. One end is attached to the
top of the cylinder, the other to the top of the piston with a small gap
between the two surfaces. As the piston goes up and down the sock rolls
up and down with it, sealing like piston rings but without the friction
or leakage.

Roll sock seals are more efficient than rings, can perhaps handle higher
temperatures do not leak, and do not require lubrication or accurately
machined surfaces. Their major draw back is the longevity of the roll
sock, not such an issue for a rarely used rocket engine, and the roll
socks could be cheap to replace anyway.

The respective power and pump 'pistons' could act directly on one
another, no bearings. The roll sock material and the 'cylinder' in which
it operates can be thin wall pressure vessels - very light weight.

I don't really see what is wrong with turbopumps. If
you have decent margins, turbopumps can be very
reliable and also easy to manufacture.

The V2 turbopumps were assembled under very bad
conditions in forced labor camps, and yet they
managed a decent reliability.

And modern airplane turbine blades are operating in
an extremely hostile environment, yet they manage to
work for many thousand hours without major
maintenance.

I would mostly agree with this however the main problem with turbopumps
is probably their greater development cost. As development cost is by
far the dominant cost, development has to be minimised in the short
term. A positive displacement pump is probably also easier to fool
around with and adapt to different engines, they are far more
controllable.

Like gas turbines turbopumps become increasingly inefficient at small
scale, not that such inefficiency is as critical on a rocket engine. For
a small space transport, if engine development cost has to be covered,
then this probably favours using multiple even smaller engines.

Pete.