Re: Book, Single Stage to Orbit

Henry Spencer wrote:
 >On Fri, 12 Mar 2004, Ian Woollard wrote:
 >>However, compared to a similar construction TSTO I found my payload
 >>dropped precipitously, by 75%.
 >>That's *really* bad.
 >
 >No; why should it be?  They're different classes of vehicles.

They're both strictly TSTO designs, and should be pretty comparable.

 >The original Kistler scheme, with the LAP, is *not* a TSTO -- it's an
 >assisted SSTO.

I don't really see that. As soon as you have that staging event, it's 
clearly not SSTO. Or if you look at it as a near SSTO, SSTOs are 
relatively heavy, and the assist has to lift that lot up to a decent 
fraction of 100km. I think this vertical lift scheme is actually the 
worst of all possible worlds.

 >And as Max Hunter said, the bad thing about assists is that as soon as 
 >you start planning for them as part of the baseline, people start
 >mumbling about optimized staging, and the idea of building an
 >almost-SSTO -- in hopes of getting to a real SSTO in the second
 >generation -- quietly dies.

Yes. That's a good point.

But I think it's a better scheme to use strap-on boosters if you want to 
go the SSTO route. That's what Rotary planned.

 >>So I have to either build my launch vehicle 4x bigger or launch 4x 
 >>more often. IMO neither seems like a brilliant idea.
 >
 >On the contrary, both deserve serious consideration, and either might 
 >well be the way to go.

I thought about it fairly carefully. It seems to me to be unlikely 
though in this case. Making the vehicle bigger means 4x more dry mass; 
with big penalties during testing and construction and launch.

The other option of launching 4x proportionately reduces the life of the 
vehicle. The vehicle cost is one of the biggest costs of launch, 
eclipsed only by R&D; and this scheme quadruples it per unit payload 
(ok. rather less, as there are economies of scale.)

Bearing in mind that there are other potential reusable schemes such as 
flyback boosters or soft landing the first stage downrange or even SSTO 
which don't seem to suffer nearly this much overall penalty, it doesn't 
seem to me to be such a wise strategy. Even expendable boosters probably 
work better.

 >It's abundantly clear that if you are trying to minimize gross liftoff
 >mass for a given payload

Sure, but I was actually trying to minimise cost/kg. It just doesn't 
seem to me at the moment that vertical staging helps with that.

And please don't misunderstand me; the equation is different for SSTO; 
there the trajectory is better, and there's no staging, so less vehicles 
to design and build, and no reassembly etc. etc. And I'm not even sure 
whether a 1.5STO vertical staging is as bad.

 >But gross liftoff mass is a really stupid figure of merit, unless
 >you've got outside constraints like "must fit in a submarine crosswise"
 >or "must be carried aloft by a 747".

Ok, you're right there.

 >Almost all that mass is fuel, which costs almost nothing.

But so far as I can see, if you reanalyse the situation in terms of dry 
mass; the same pattern arises- there's a factor of 4 adrift here.

 >And spending more to develop a more complex vehicle that will fly less
 >often is *not* a smart economic move.

My point is that the TSTO vertical staging scheme never breaks even 
compared to the other schemes; it doesn't seem to matter how many times 
you fly it.

 >                                                          Henry Spencer
 >                                                    [email protected]