Re: Minimum development cost space transport

>> According to wikipedia, the voyager had a mass ratio
>> of more than 10! Quite impressive for such a complex
>> and fragile airplane.
>
>That is impressive. I had not realised it was so high.
>
A mass ratio of 10 is much more difficult for an airplane than for a
rocket, so this is very impressive.

> While harder, there are still methods possible for building small scale
> very thin wall tanks. Small scale low pressure engines do not fare so
> well with regard to minimum gauge constraints.
>
> Small scale SSTO vehicles must still achieve the same mass ratios and
> performance as large scale SSTO vehicles. I doubt one would use pressure
> fed at larger scale, so I do not see why one would use it at smaller
> scale.
>
I guess you are right. While a pressure-fed air launched SSTO is
certainly possible, you do not have a big margin left. So maybe it is
indeed better to use a pump. But then it might be useful to keep the
tank pressurized from the launch to the landing to give it some more
stability. That way you could probably get away without milled
structural stiffeners.

> Some of the lifting tank approaches might also use aerodynamic lift
> during launch for a minimum mass penalty. Consider the aerodynamic lift
> from three tanks stacked side by side.
>
I think that aerodynamic lift is just not worth it, especially when you
launch from high altitude anyway. Just get the rocket vertical using
the AirLaunch air drop method and then get out of the atmosphere as
fast as possible.

> The thing about horizontally landing a fluffy near SSTO vehicle is that
> you can mostly use the body for lift such that the wings can if need be,
> be little more than small control surfaces.
>
x-33 tried that.

They started with a pure lifting body with very small flaps. In the end
the flaps grew into quite large wings, and they had huge problems with
the center of gravity.

I think that they were even thinking about putting *ballast* into the
nose to get the CG in the right area. It does not get any more absurd
than that.

So I think making a vehicle that is stable during the subsonic and
supersonic parts of launch, during reentry and during subsonic gliding
flight without ballast or large control surfaces is quite complex.

> A VTVL space transport might
> use control surfaces during reentry anyway - as per DC-X, so this may
> cost little. I also quite like the idea of a water landing as this might
> negate the need for landing gear.
>
What would you need control surfaces for? The DC was supposed to do a
nose-first reentry and use flaps to satisfy the military crossrange
requirements. I would not bother with crossrange at all. Just do a
water landing and place the recovery ship under the orbit before
reentry.

> Yes, rocket engines actually work the best though at these heights one
> might just use an old jet engine.
>
> The way the trade offs work out, one tends to either go for a
> conventional aircraft at lowish altitude, or a purpose built rocket
> powered aircraft staging at very high altitude and say mach 4+.
>
I think a dedicated plane to get the vehicle to a low pressure
environment like the Rutan very large airplane does make a lot of
sense. But only if you can afford it.

> Significant oxygen addition to an air breathing engine must be matched
> by an increase in cooling, this requires a from scratch engine design
> which is prohibitive. Adding LN2 for cooling is easier but gets heavy.
> Air launch is only viable if easy, cheap and routine.
>
Why not use liquid air? It is even less expensive than liquid oxygen,
and it has exactly the ratio of o2 and n2 the engine is designed for. I
would not worry about weight too much. You do not want to win an
endurance record or something.

A mechanism that runs liquid air through a heat exchanger with the
exhaust and feeds the internal combustion engine should not be that
hard.

.

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