The estimates for the payload of the Falcon 9 v1.1 first stage as SSTO I'm getting from Dr. John Schilling's Launch Vehicle Performance Calculator:
http://www.silverbirdastronautics.com/LVperform.html
SpaceX, irritatingly, has not released the propellant loads and dry masses for the individual stages of either version of the F9, just the total gross masses. This is in contrast to the other launch providers at least among the Western ones. Perhaps these numbers will be released when SpaceX is launching commercial payloads on a regular basis.
The propellant mass for the F9 v.1.1 has been estimated in the range of 400 metric tons (mT). If the 96% propellant fraction is right that would put the dry mass at 16.6 mT. If the 95.5% value is right, the dry mass would be 18.8 mT.
Enter the selected numbers for the propellant load and dry mass into the Calculator. For the thrust and Isp values, Schilling says use the vacuum values as the Calculator takes into account the reduction at sea level. The SpaceX page on the F9 v1.1 gives the total vacuum thrust of the first stage as 6,672 kN. The vacuum Isp for the Merlin 1D has been reported as 311 s.
Among the options, select "No" for the "Restartable Upper Stage" option, otherwise the payload will be reduced. Use the default altitude of 185 km. And select Cape Canaveral as the launch site, at a 28.5 degree orbital inclination to match the site latitude. Then I got in the range of 5,000 kg for the 96% propellant fraction case, and in the range of 3,000 kg for the 95.5% case.
But for an economical SSTO, you really should use altitude compensation. This will greatly increase the payload for the SSTO, but its usefulness goes beyond that. Even for a two stage vehicle using altitude compensation on the first stage can increase the payload. The increase will not be as pronounced as for the SSTO case but it will be significant.
Try using the Schilling Calculator where you use the Merlin Vacuum's Isp of 340 s. I think you'll be surprised by the answer. 
Bob Clark
