On 3/2/2014 1:28 PM, Niels Jørgen Kruse wrote:
Anthony Frost wrote:

In message
David Spain wrote:

In this scenario, a smaller trans-stage is attached to the top

There are a couple of advantages to this approach:

1) The entire propellant load of the first stages could be used for ascent.

Not an advantage. You still need a similar amount of fuel for the
landing, all you've done is add extra tanks to hold it, increasing the
mass, and added more engines, also increasing the mass.

But the main engines don't have to be restartable and the thrusting
engines are farther from the ground in landing, blowing up less
shrapnel.


Yes I considered that as well. Shrapnel that could get into your firing
engines at a very inopportune time. But I would think that could be
easily avoided by keeping the landing pad clean and free of debris.

Anthony's observation about the additional mass is a good reason why
this is not a good option for a single booster. I really was only
thinking about it in terms of a multiple-booster cluster.

OTOH you have to view that against the added wet+dry mass of the
trans-stage. And the fact that the CG change during ascent possibly
moves along a line that is in a direction less favorable over time,
depending upon that wet+dry mass of the trans-stage versus the dry mass
of the boosters below.

I'll throw in one more what-if. And that would be what-if the
trans-stage used a different engine & fuel. One that offered much higher
ISP and with a denser fuel than the first stage could afford to use on
its own? That might swing the balance in the favor of this approach. But
then consider, who is going to give you that other rocket and fuel for
free? If you were a legacy company with lots of different rocket designs
sitting around that approach might make some sense. But if you are just
starting up, wouldn't it makes more sense to develop your best solutions
for the first stage and go with what you have there? As SpaceX is doing now?

Some things to mull over.

Dave