Tuesday, March 25, 2014

"Golden Spike" Circumlunar Fights, Page 2.

Copyright 2014 Robert Clark

 In the blog post "Golden Spike" Circumlunar Flights, I suggested that the Falcon 9 1.1/Dragon could do a circumlunar mission with a half-sized Centaur, of ca. 10 mT propellant load, to do the translunar injection.
 Interestingly it might be possible to do without even needing the extra in-space stage. Elon Musk has said through his Twitter account that the 13 mT payload capability was actually a reduction of the F9 V1.1's true capability due to reusability considerations. Gwynne Shotwell confirmed this on a TheSpaceShow interview on Friday, Mar. 21 at about the 9 minute mark. She said the quoted payload on their web site for the F9 v1.1 is about 30% less than that of a one-use version.
 This would put the expendable version in the range of the 16.6 metric tons to LEO given on NASA's site:

NASA Launch Services Program's
Launch Vehicle Performance Web Site.

 The point is this would be just about at the payload capability to do translunar with the Dragon using just its onboard Draco, or upgraded SuperDraco, thrusters. On the "NASA Launch Services Program" site, click the link for the Performance Query Tool and select the Falcon 9 and "elliptical" orbit option. Enter in 36000 km for the altitude corresponding to geosynchronous transfer orbit (GTO) and 28.5 degrees for the orbital inclination corresponding to launch from Cape Canaveral. Then the calculator gives the payload to GTO as 5745 kg.

 As shown here the delta-v to GTO is 2,500 m/s:

 Then translunar injection (TLI) at 3,100 m/s would only require an additional 600 m/s delta-v. The Dragon has a dry mass of 4,200 kg and a propellant mass of 1,290 kg. SpaceX has not released the Isp of the hypergolic thrusters on the Dragon, but they typically are in the 320 s range in vacuum. Then it could carry 1,800 kg payload to the 600 m/s needed to reach TLI:

320*9.81ln(1 + 1290/(4200 + 1800)) = 610 m/s.

  Actually that 1,800 kg payload would put the total mass beyond the 5745 kg capacity to GTO. Smaller payload say in the 250 kg range would be doable.
 Such missions would be important to do since at a perhaps $120 million total launch price for the Falcon and Dragon it would show lunar missions are possible without requiring huge launchers such as the Saturn V, Ares V or SLS.

   Bob Clark

UPDATE, April 1, 2014:

 On forum, DocM informed me via PM that in an environmental impact statement SpaceX gave the propellant load for the Dragon as 1,388 kg. This would raise the max payload to reach 600 m/s delta-v to 2300 kg. Again though this would put the total mass outside the range that could be lofted to GTO. Likely you would still have to limit the payload to ca. 250 kg or so.



1 comment:

  1. The pictures I saw a couple of years ago said the Super Dracos fire through the capsule sidewall just above the joint to the heat shield, inclined about 45 degrees off the axis. The nozzle exits get scarf-cut to do that. A cosine factor of near 0.7 should be used to knock-down thrust and Isp data for Super Dracos alone.