Re: Minimum development cost space transport

"Rüdiger Klaehn" <rudi@xxxxxxxxxxxxxxxxxxxx> wrote in message
news:1134067395.853965.319620@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

> The only way to save engine mass is to launch more
> or less horizontally and then use the lift of the vehicle
> during the powered acceleration phase to turn it to
> almost vertical. I think pegasus does it that way.

Yes, Pegasus seems to do it at quite high accelerations.

An air launch speed of say 250 m/s seems a useful bit of delta v that it
would be nice not to waste.

> This might save you a bit of engine mass, but makes
> the structure of the vehicle much heavier because it
> has to take loads in different directions. It also
> drastically increases drag losses because lift always
> comes with drag and you have to stay in the dense
> lower atmosphere for longer.

Why would a lifting tank design be any heavier due to such horizontal
loading? External aerodynamic pressures will be much less than internal
tank pressures.

> Besides, a lifting tank design might be quite fluffy
> when empty, but when full it will have aerodynamic
> properties that make the shuttle look good in
> comparison. At least if you use dense propellants.
>
> So if I had to choose between an initial L/D of, say,
> 1.2 and do a ballistic ascent and an initial L/D of 0.8
> and the complexities of a lifting ascent, I would
> choose the ballistic ascent any time.

L/D in both the subsonic and hypersonic regimes should not be that
dissimilar to the Shuttle. I was assuming an aggressive aerodynamic
climb through the upper atmosphere until such point that T/W allowed a
more vertical trajectory. With such a step climb, L/D is actually not so
critical, though I really should get around to modelling it properly.

> How do you propose connecting the tanks? Have
> two cylindrical tanks and then connect them with
> some struts, or have something like a common
> bulkhead over the length of the cylinder?

Connect them at a few points as per normal. Reentry shielding will want
to bridge any gap, assuming there is one.

> I completely agree with this part. A pressurized
> stainless steel tank would never have any hotspots
> because all temperature differences would
> immediately be equalized by the pressurant gas. And
> by releasing some of the pressurant gas you can get
> some nice evaporative cooling. Add some water if
> that should not be enough. But I suspect that that
> might not even be needed.

With low wing loadings and nickel alloy tanks one can perhaps do it
passively, though water injection would help. An additional problem is
that after atmospheric cooling, one almost has to start re-pressurising
the balloon tanks for structural strength prior to landing.

> > - If the TPS is up to it, do a horizontal water
> > landing. This should avoid the need for landing gear,
> > a mass and design headache.
> >
> But that would place large mechanical loads on the
> design, wouldn't it?

Yes, though ideally reentry shielding should be a structural part of the
tank wall, providing extra thickness and strength. A double wall nickel
alloy heat shield on the lower surface, (with insulation in between)
might be up to such water landings.

> And it might be quite a challenge to keep it under
> control while it skips over the waves.

Hopefully not too bad, boats manage and this thing will have a low
landing speed and a multi-hull shape. Obviously one would favour flat
fresh water landing areas.

> A vertical water landing with a parachute and/or
> braking engines would be more gentle.

Comparing a capsule landing to a float plane landing, I very much doubt
it.

> You suggested water landing for your design. What
> makes vertical water landing and recovery so much
> more difficult than horizontal water landing and
> recovery? The only way to avoid some kind of
> recovery operations is to land and _launch_ out of the
> water.

I was hoping to land like a float plane at a convenient place, perhaps a
small lake next to a launch area.

> With your "inflatable boat" design it would be silly not
> to use the crossrange. But I still don't think that
> significant crossrange is required. Even if you want to
> do one launch per day: a ship can easily do 500km in
> one day.

Such ships are quite expensive - there are also weather constraints.

> But a motorized parachute powered by an internal
> combustion engine with some additional air feed
> would be much more low tech. This would be
> something even a dedicated amateur could build.

Altitude is but one advantage of air launch, it is perhaps better to
concentrate on the other benefits than waste development effort pushing
the high altitude limits. Stage to compensate.

Pete.


.

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