Expendable Launch Vehicles (ELV)
While we generally believe that a reusable launch vehicle is the way
to go, there are a number of things that make an expendable
launch vehicle more attractive than you might expect.
Mass Production for Low Cost
One of the advantages of using lots of very small rockets instead of a few
very large rockets is that you can use mass production techniques
to reduce the costs. We could be launching a rocket every
90 minutes (or possibly more if more than one launch site). This
would be 16 per day or 5,840 rockets per year.
With these kinds of numbers is makes sense to have an assembly line
and probably robots doing welding etc.
In small quantities you might machine a part for $2,000 but in
large quantities you might amortize the costs of a mold and get
the cost for each part down to $200.
These small rockets could be around the weight of a truck when
empty and simpler to make. If they were produced in the volumes
that trucks are, the costs could be comperable.
It seems very possible to get the costs for a rocket that
can get 4,000 Kg to 5 km/sec to under $100,000 each.
If we can get the cost down to $100,000 each then 3,000 rockets is $300 mil.
This is around half the cost of one shuttle launch, and the total
mass to orbit is far more.
Used ELV is Valuable as Ballast
When growing a tether you need to increase the ballast.
Any mass that makes it to space is valuable as ballast.
An expendable rocket that gets a payload
up to the end of the tether could be taken along with the payload and
used as part of the future ballast.
In this case the used rocket is valuable as ballast.
If the mass of the empty rocket is
equal to the mass of the payload, then using an ELV to build
up a tether might take half as many launches as using
an RLV with the same sized payload. So the ELV could be
cheaper to use while growing a tether even
if it was twice the price per launch.
ELV can Carry More Payload
With an expendable rocket you don't need reentry mass, which lets you
carry more payload. For example you don't need a parachute, landing gear,
tires, doors/motors for extending landing gear, heat shield, etc.
The total of these things can be a significant weight.
No Need to Return to Launch Site
Unless you did something like having 10 launch places spread around the
Earth, or a tether strong enough to carry the whole rocket,
a RLV has the problem of getting back to the launch place.
An ELV does not have this problem.
Ablative Engines are Easier
A rocket engine that is only going to be used once can have an ablative
surface. This is easy and cheap technology. A reusable rocket
probably needs to use "Regenerative Cooling". This is where pipes
run a very cold liquid fuel over the outside of the nozzle to cool
it down before the fuel is put into the combustion chamber. This
extra plumbing costs extra and weighs more.
Reusable Tanks are Heavier
A tank that will be pressurized many times needs to be designed
stronger than one that will only be used once. The extra
tank weight of a RLV hurts it on every flight.
If your tank and engine are only going to be used once,
then it is easier to design to handle the high tank pressure for a pressure fed engine.
Pressure fed engines eliminate the turbo-pumps. This reduces costs.
But without turbo-pumps, the extra pressure lost in regenerative cooling
is a problem. But on an ELV you can use an ablative engine, so
you don't need regenerative cooling.
The tanks have to be stronger to handle the higher pressure in
a pressure fed design, which hurts mass ratios. But it makes
for a very simple design. The extra tank strength means that the stresses of
air-launch are probably not a problem.
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