Re: Google Cindy Sheehan (was: Re: Peace Mom)


> "Josh Hill" wrote:
>>
>> I just read, though, that there are already at least 16 launch
>> vehicles/launch vehicle stages that have a mass fraction greater than
>> .90, most or all designed before the availability of lightweight
>> composites.

Actually, I thought about this and I wanted to just play with this notion
through an exercise. Let's design a hypothetical SSTO vehicle.

First, we'll start with the Shuttle External Tank. It's made of
aluminum-lithium alloy. Nothing flying is lighter. Dry weight of about
54,000 lb. It holds about 1.6 million lb of propellant at lift off.

Next, let's assume a payload of about 40,000 lb.

Next, we'll need some engines. To get moving off the ground we need a
liftoff thrust to weight ratio of about 1.4. For now I'm going to assume
that we need six SSMEs and then we can check the ratio when we've got all
the parts assembled. SSMEs currently weigh about 7500 lb each, so six of
them are 45,000 lb.

Let's assume that we need some kind of boat tail and thrust structure to
hold the engines. This will also include the feedlines and prevalves as
well as auxiliary power systems. Since I don't work in this area I'm going
to have to guess at this, but I'd assume that it couldn't be lighter than,
say 25,000 lb.

Next, you'll need a carrier for the payload. Again, total guess, but
considering that this is typically more than just a shell, let's say 10,000
lb.

Also remember that not all of the propellants are usable. You need some in
the lines in order to safely shutdown the engines. With six engines it's
going to be more than Shuttle, but lets be kind and call it only 3000 lb.

Okay, add these all together and we find that we have an inert weight of
about 177,000 lb and a gross liftoff of about 1,764,000 lb. If we go back
and look at the thrust-to-weight issue, with 6 SSMEs at 109% power we get a
ratio of 1.39. So it works in that regard. Now, what the usable propellant
fraction? It's about 90%.

Ta-da. Now we can claim to have designed a SSTO vehicle give or take a
percentage point. We'll write a conference paper on these numbers and
declare a paper victory.

Except that we forget a few things. We need a reaction and control system.
We need avionics. We need other stuff as well, stuff admittedly outside my
own area of expertise.

And how good are the assumptions? In particular I'd say that the boat tail
and it's contents are a bit lean. I've been inside the Shuttle boat tail
and it's a busy place crammed tight with lots of heavy stuff (and that's
just for three engines).

Also note what this estimate does not have. It has nothing returning. It
is not a reusable system. There's no landing gear. There's no glide in
orbiter with wings, aerodynamic controls, and thermal protection systems.
There's no provision for landing struts and landing propellant (typically
5-10% of the total propellant mass for VTOVL scenarios).

And, finally, the great big hole in the middle of this is that the
trajectory is probably screwed. Here we're proposing to use six engines
with the same amount of propellant we typically require for three. Thus,
where we currently take about eight and a half minutes to reach main engine
cutoff, here we'll be achieving the same terminal velocity in about half the
time. I don't have any trajectory tools at hand, and it's been years since
I've done any such calcs, but this scenario just doesn't sound good.

So it doesn't take very long to get to a point where one starts to
appreciate the difficulties of SSTO. And it also doesn't take long to get
to a point where one starts to seriously question the reusable SSTO designs
(vertical or horizontal landing) that have been promulgated in the past.


.

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