You can get really large payloads with the 8.4 meter wide super
"Evolved Atlas" stage by using parallel, "trimese", staging with cross-
feed fueling. This would use now three copies of the lower stages
mated together in parallel with the fueling for all the engines coming
sequentially from only a single stage, and with that stage being
jettisoned when its fuel is expended.
Again we'll calculate first the case where we use the standard
performance parameters of the RD-180, i.e., without altitude
compensation methods. I'll use the average Isp of 329 s given in the
Kyle article for the first leg of the trip, and for the required delta-
V, again the 8,900 m/s often given for kerosene fueled vehicles when
you take into account the reduction of the gravity drag using dense
propellants. Estimate the payload as 200 mT. Then the delta-V for the
first leg with all three super Evolved Atlas's attached will be
329*9.8ln(1+1,323/(3*70 + 2*1,323 + 200)) = 1,160 m/s. For the second
leg we'll use the vacuum Isp of 338 s, then the delta-V will be
338*9.8ln(1 + 1,323/(2*70 + 1*1,323 + 200)) = 1,940 m/s. And for the
final leg 338*9.8ln(1 + 1,323/(70 +200)) = 5,880 m/s. So the total
delta-V is 8,980 m/s, sufficient for orbit with the 200,000 kg
payload.
Now let's estimate it assuming we can use altitude compensation
methods. We'll use performance numbers given in this report:

Alternate Propellants for SSTO Launchers.
Dr. Bruce Dunn
Adapted from a Presentation at:
Space Access 96
Phoenix Arizona
April 25 - 27, 1996
http://www.dunnspace.com/alternate_ssto_propellants.htm

In table 2 is given the estimated average Isp for a high performance
kerolox engine with altitude compensation as 338.3 s. We'll take the
vacuum Isp as that reached by high performance vacuum optimized
kerolox engines as 360 s. Estimate the payload now as 250 metric tons.
Then the delta-V during the first leg will be 338.3*9.8ln(1+1,323/
(3*70 + 2*1,323 + 250)) = 1,180 m/s. For the second leg the delta-V
will be 360*9.8ln(1 + 1,323/(2*70 + 1*1,323 + 250)) = 2,020 m/s. For
the third leg the delta-V will 360*9.8ln(1 + 1,323/(70 + 250)) = 5,770
m/s. So the total will be 8,970 m/s, sufficient for orbit with the
250,000 kg payload.
Now we'll estimate the payload using the higher energy
methylacetylene. The average Isp is given as 352 s in Dunn's report.
The theoretical vacuum Isp is given as 391 s. High performance engines
can get quite close to the theoretical value, at 97% and above. So
we'll take the vacuum Isp as 380 s. Estimate the payload now as 300
mT. The first leg delta-V will now be 352*9.8ln(1 + 1,323/(3*70 +
2*1,323 +300)) =1,210 m/s. For the second leg 380*9.8ln(1 + 1,323/
(2*70 + 1*1,323 + 300)) = 2,080 m/s. For the third leg 380*9.8ln(1 +
1,323/(70 + 300)) = 5,660 m/s. So the total is 8,950 m/s, sufficient
for orbit with the 300,000 kg payload.

This trimese version of the vehicle would be huge however. For
instance it would weigh more than the Saturn V. One of the big cost
factors for the development of some of the super heavy lift launchers
is that they are so heavy they would require the construction of new
and expensive launch platforms. Undoubtedly, the bimese version would
be the one to be built first if this launch system is selected.



Bob Clark