The RS-68 engine is an interesting engine. An aerospike engine using
a single RS-68 pump set can easily be built, just as Rocketdyne built
their first aerospike engine using the J2 pump set back in 1966 (the
J2T-250K, J2T-200K)

http://www.friends-partners.org/part...es/j2t200k.htm

To build seven ET style elements that are 31.6 m (103.6 ft) tall, 6.7
m (18.6 ft) in diameter. That mass 31,670 kg (69,673 lb) empty and
263,912 kg (580,606 lb) fully loaded.

These elements are equipped with cross-feeding to allow them to
operate as three separate stages to carry 125,000 kg (275,000 lbs) to
LEO.

http://www.scribd.com/doc/30943696/ETDHLRLV
http://www.scribd.com/doc/31261680/Etdhlrlv-Addendum

The booster masses 1,973,282 kg at take off. Four of the seven
elements drain to feed all seven elements at lift off. Burning
928,969 kg of propellant the booster achieves 2,621 m/sec. After the
four elements fall away, and are recovered down range by their own tow
planes, three elements continue, massing a total 916,735 kg. Two of
the three elements drain feeding all three engines, burning through
464,484 kg of propellant. At the end of this burn 6,364 m/sec before
subtracting gravity and air drag. The two empty elements fall away,
also to be recovered downrange. Meanwhile, one element containing a
125,000 tonne payload in the stretched intertank region, continues
skyward - totaling 388,911 kg and carrying 232,242 kg propellant.
After completing this final burn, the idealized speed is 9,273.5 m/sec
- actual speed after air and gravity drag, 7,900 m/sec.

This system will cost $2.5 billion to develop, and five built (35
elements) for another $1.5 billion - for a total program cost for $4.0
billion. With complete reusability, less than $1.25 million per
launch - $10 per kg - recurring cost - providing another $1.2 billion
is spent automating launch procedures.

This is less than 1/3 of one year's NASA budget.

125,000 kg payload is comparable to the Saturn V rocket's capacity.

This stage consists of two sections. With three RD-0128 engines, each
with 474 sec Isp, 30,000 kgf thrust and 7% structural fraction. The
top section is a recoverable propellant tank carrying 58,827 kg of
propellant and the tank massing 4,118 kg - with no engines. This
propellant carries the 66,172 kg to 10.85 km/sec - on a lunar free
return to the moon.

The stage containing three engines burns through 26,861 kg of
propellant to land 39,311 kg on the moon. Of this total 15,958 kg is
propellant to return to Earth. 23,353 kg is payload that returns to
Earth. To re-enter the atmosphere, and landing at the launch center,
to be relaunched and reused on the next flight.

This is 10x the payload of the Lunar Ascent Module; due in part to
improved structure fraction and in part (and mostly) due to improved
performance of the LOX/LH2 engine.

Enough to take 20 people to the moon for a few days. Enough to
support 7 people for a few weeks.

Lunar Module Masses

Ascent stage: 10,024 lb (4,547 kg)
Descent stage: 22,375 lb (10,149 kg)
Total: 32,399 lb (14,696 kg)

Crew: 2
Crew cabin volume: 235 cu ft (6.7 m3)
Height: 9.29 ft (2.83 m)
Width: 14.08 ft (4.29 m)
Depth: 13.25 ft (4.04 m)
Mass including fuel: 10,300 lb (4,700 kg)
Atmosphe 100% oxygen at 4.8 psi (33 kPa)
Water: two 42.5 lb (19.3 kg) storage tanks
Coolant: 25 pounds (11 kg) of ethylene glycol/water solution
Thermal Control: one active water-ice sublimator
RCS propellant mass: 633 lb (287 kg)
RCS thrusters: sixteen x 100 lbf (440 N) in four quads
RCS propellants: Aerozine 50 fuel / nitrogen tetroxide(N2O4)
oxidizer
RCS specific impulse: 290 s (2,840 N·s/kg)
APS propellant mass: 5,187 lb (2,353 kg)
APS engine: Rocketdyne RS-18[10]
APS thrust: 3,500 lbf (16,000 N)
APS propellants: Aerozine 50 fuel / nitrogen tetroxide oxidizer
APS pressurant: two 6.4 lb (2.9 kg) helium tanks at 3,000 pounds
per square inch (21 MPa)
APS specific impulse: 311 s (3,050 N·s/kg)
APS delta-V: 7,280 ft/s (2,220 m/s)
Thrust-to-weight ratio at liftoff: 2.124 (in lunar gravity)
Batteries: two 28–32 volt, 296 ampere-hour silver-zinc batteries;
125 lb (57 kg) each
Power: 28 V DC, 115 V 400 Hz AC