An Aerospike engine is nearly as efficient at low altitudes as well as
high altitudes

http://www.aerospaceweb.org/design/a...ensation.shtml

The RL-10 engine hardware was used to build an aerospike engine in
1967 for the FDL-5 classified spacecraft.

http://www.astronautix.com/engines/amps1.htm#AMPS-1

The particulars are;

Isp 468
18,000 lbf (80 kN)

With a 9% structural fraction, and seven stages clustered together, -
viewed from above - numbered 1 through 7

(1)(2)
(3)(4)(5)
(6)(7)

With 1 and 6 feeding 3
With 2 and 7 feeding 5
With 3 and 5 feeding 4

1,2,6,7 drain first - forming a first stage
3 and 5 drain next - forming a second stage
1 drains last - forming a third stage

Each peels away, and comes back down-range, folding out switchblade
wings and gliding to be recovered by a loitering airplane - and towed
back to the launch center.

With 18,000 lbf thrust on each stage, and 1.4 gees at lift off we have
a total stage weight for each unit as 12,857 pounds. With a
structural weight of 9%, 1,157 pounds of weight is structure leaving
11,700 pounds of propellant. This along with the Isp lets us figure
the payload this system can put in orbit 10,000 pounds of payload with
recovery of all components.

10000 payload

Total Wt. Prop Wt. Prop f mph

Stage 1: 99,999 46,800 0.4680 6,473
Stage 2: 48,571 23,400 0.4818 6,742
Stage 3: 22,857 11,700 0.5119 7,356

TOTAL: 20,571

The cool part about this is that ALL the seven elements are exactly
the same, and cost about $3 million each to build, with $5 million for
the engine. That's $8 million per piece, and $56 million total per
vehicle.

With 1,000 uses, this would contribute $56,000 to each launch - in a
simplified accounting.

A more complex accounting would take time value of money into
consideration, and launch rates would be important. The ability to
recycle the spacecraft for say twice a week use, and setting up the
logistics so that it could fly twice per week would allow 100 flights
per year - and a 10 year life span. At a 6% discount rate this
becomes $77,900 per flight.

At $2 per pound for propellant, propellant costs are $23,400 per
element, or $163,800 per launch. Adding this to the payments for the
capital expense we have $241,700 per launch.

Reducing launch rates to twice per month, or 24x per year and 10 years
- 240 flights - we have $324,600 capital expense per launch, which
when added to the $163,800 propellant cost per launch obtains
$488,400.

Cost of launch infrastructure contributes also the the cost of each
launch, then there's the recurring costs associated with the labor for
returning the vehicle to flight status and maintenance and so forth.

A cost of about $1 million per launch, and a launch rate of once or
twice a week should make a profit with this vehicle.

10,000 pounds of payload should be capable of lifting 25 passengers
and 4 crew (its larger than the DC-3 in terms of payload) At $1
million per flight, this is $40,000 per passenger or on a per pound
basis $100 per pound.

A plug in cargo section allows cargoes to be prepared preflight and
quickly added to the launcher once its made ready for flight. Seven
cargo sections would allow up to 4 weeks to prepare a cargo for launch
that is then plugged in once its wrapped up.

At $1 million launch cost per flight - 50 launches per year would be
cargo - and 50 launches per year would be passenger - carrying 1,250
tourists into space at $40,000 each.

Cargo preparation and handling would add another $4 million per
flight.
Tourist training and gear (including personal spacesuits, videos
etc.,) would provide another $60,000 per passenger in revenue.

There are 10 million millionaires in the world, and 1,250 of them
could be identified each year to pay $100,000 to spend three days in
space with two weeks training - to return with a documentary of their
flight and their own spacesuit and certificate.

A satellite network of 144 satellites each 10,000 pounds - would
consume another 30 per year for 5 years - to build a telecom network
that provides substantial continuing revenue.

The remaining 20 per year cargo flights would be to traditional space
launch buyers.

50 x $5 = $250 million - tourists
20 x $5 = $100 million - space launch
30 x $5 = $150 million - telecom (internally billed)

The comsat network provides wireless broadband worldwide and earns
several billion dollars per year - which is sufficient to enlarge this
program with an RD-68 sized aerospike engine of the same design (AMPS-
X) - and carry significant payloads to the moon and mars, and large
numbers people to orbit and beyond.