Space Forum / Shuttle / August 2005

"Dead weight" in orbit is a good thing if it gives you greater
reliability and lower system costs.  Propellants are the cheapest part
of getting to orbit and back; losing 1% or so of your vehicles and
payloads to staging-related accidents is ruinously expensive.

A launcher using two parallel stages that has all engines running at
liftoff is nearly as good as SSTO, with the enormous benefit that no one
doubts that it can be done (because it has already been done).

Is there anywhere a proposal to build a disposable SSTO?

While it's true in principle that SSTO does not imply reusable, there
seems little point in building a disposable SSTO. If someone tells me
they're building an SSTO, I will assume that it will be (intended to be)
reusable, and will express considerable surprise if told that that is
not a design goal.

Sylvia.

That's what I mean when I say "cost per lb to LEO".  If you take the shuttle
as an example, only what's in the payload bay (and cargo on the mid-deck,
and the astronauts themselves) is payload that would counted in the "cost
per lb to LEO" calculation.  The mass of the orbiter would be ignored, as
would the dry mass of any upper stage or the dry mass of an SSTO.

My point is, who cares how much dry mass is taken to LEO and back as part of
a reusable launch vehicle?  That "dead weight" quantity does absolutely
nothing to impact the cost per lb (of cargo) to LEO.

An SSTO would clearly place more "dead weight" in LEO (compared to a TSTO
with the same payload), but could still cost less to launch, making the cost
per lb (of cargo) to LEO lower.  Certainly it would burn more fuel than a
TSTO, but as I've said before, fuel costs are a very small percentage of the
total cost to place payload in LEO.

How could an SSTO be cheaper to operate?  There are no staging events to
worry about, so all your engines are started on the ground, so you don't
have to invest time and money to insure that your upper stage engine
actually starts.  Also, while the SSTO may have more dry mass than a TSTO,
you've only got one set of hardware to maintain (e.g. engines, TPS, landing
gear, control surfaces, electronics, and etc.).  So while much of the
hardware itself would be bigger on an SSTO, at least there is only one copy
to maintain, not two.  This reduces maintenance costs considerably, since
you don't have to pay two teams of maintenance crews.

Jeff

re: drymass  and dead weight and payload.

One needs to remember that in a fully multi stage rocket, you have dead
weight at launch time: those engines that will be used only later.

One needs to do math to figure out whether launching greater mass to
have final stage at lower mass  (eg: smaller engines at 100% capacity
for final stage, but which are dead weight at launch) is better than
launching at lower mass (no unused engines at launch)  but having
greater mass for final stage of launch (eg: heavier SSMEs not used at
full capacity for final stage).