Sunday, August 26, 2012

The Coming SSTO's: Applications to interplanetary flight.

Copyright 2012 Robert Clark

Credit: NASA image of an Orbital Transfer Vehicle with aerobrake. From David S.F. Portree's page: Shuttle-Era Manned Mars Flyby (1985).

 Note also a key fact about SSTO's is that the delta-V requirement for
a round-trip mission from LEO to the lunar surface is a little less
than that for flights from Earth's surface to LEO. Then if you could
do orbital refueling, you could have a single, reusable vehicle that
does lunar missions. This important capability about SSTO's is
mentioned in G. Harry Stine's very nice book Halfway to Anywhere:
Achieving America's Destiny in Space

" SSTO that is refueled in orbit has the capability to fly to the
Moon, land, lift off, and fly back without additional refueling."
Halfway to Anywhere: Achieving America's Destiny in Space, p. 220.

A table that gives the delta-V budget for trips in the Earth-Moon
system is given here:

Delta-V budget.
Earth–Moon space.

  From this you can calculate that the delta-V for a round trip from
LEO to the lunar surface is less than that for getting to LEO.
It has been argued that SSTO's are not economical. But that such a
vehicle with orbital refueling could also be used for lunar missions
changes the economic equations significantly.
 Surprisingly such SSTO's could also be used for Mars missions.
Elon Musk has argued in favor of promoting creating a self-sustaining
colony on Mars:
Elon Musk "Mars Pioneer Award" Acceptance Speech - 15th Annual
International Mars Society Convention.

 For such a colony he proposes reusable vehicles and getting propellant for
return trips from Mars. Musk proposes cutting the costs to space by two
orders of magnitude by reusability. Then there would be also a dramatic drop
in the cost to lift the large amount of propellant to space.
 So let's suppose there are propellant depots at LEO. Since Musk proposes a
self-sustaining colony on Mars, lets also suppose propellant depots in low
Mars orbit for return trips.
Here's a map of delta-v's between Mars/Moon/Earth:

 If you add up the delta-v's from low Earth orbit to low Mars orbit you get
6.1 km/s. Now use the same specifications for the Falcon 9 v1.1 first stage
as estimated before, 13 mT dry mass and 375 mT propellant load. Then
you could transport 45 mT from LEO to low Mars orbit:

311*9.81ln(1 + 375/(13 + 45)) = 6,130 m/s.

   Bob Clark


  1. So how many weeks would it take an SSTO to reach Mars?

  2. The low propellant Hohmann transfer orbit would take 6 months to 8 months, the same as others.
    You could reduce the time with greater propellant load.

    Bob Clark

  3. Hi Robert
    Sorry to reply a year after the last comment, but I've been posting about Moon flights on SpaceX's Facebook page and mentioning possible Mars Semi-Direct architectures using the Falcon Heavy. In the latter scenario it computes out as about 4 launches - 3 Tankers, plus 1 payload - per Mars Vehicle delivered. Mars Semi-Direct uses a Mars Transfer Habitat, a Mars Ascent Vehicle and an Earth-Return Vehicle, thus requiring 12 launches per mission. At $135 million/Falcon Heavy, that's $1.62 billion for a Mars mission, in launch fees. The vehicles themselves probably add another ~$1.5 billion, so all up ~$3.5 billion to send ~6 people to Mars.

    If SpaceX can get the launch prices down to ~$500/kg, then the fees drop to ~$27 M per launch, so with enough flights of the Habitat/MAV/ERV combo, the prices of each vehicle drops to ~$50 M each, then it's under $0.5 billion a mission.

    One-Way colony transfers, using the (possible) induced torpor recently proposed by a NIAC study, could then launch 24 people at a time for ~$0.2 billion. But what's in it for the Nations of the World to support starting a Mars Colony? A New Creative Minority seems to be a good thing to foster and making Humanity multi-planetary will future-proof the species. A Frontier for the restless and restive might have a certain appeal for the wannabe People-Managers back on Earth.

    1. Thanks very much for that. Another possibility for getting the propellant for the return trip from Mars would be getting it from the abundant near surface ice now known to exist even in mid-latitudes on Mars.
      Also, I'm investigating a different architecture for a manned Mars mission that only would require two launches of the Falcon Heavy or a single launch of the SLS. It would require very aggressive weight optimization of the in-space stages that could get a 30 to 1 mass ratio for hydrolox stages.
      I discuss why such lightweighting may indeed be possible here:

      Budget Moon Flights: Ariane 5 as SLS upper stage.

      Bob Clark