An Outer Shell for the MCT Propellant Depot

[Doesitfloat]

Propellant transfers can be accomplished with little or no depot as long as there are only a few missions during each synod. But the question arises: when will the volume of traffic to Mars require a larger depot?

To address this question I have been developing a worksheet - see the file attachment below.

The number of launches to LEO and the number of Mars trips will depend on the volume of MCTs of the various types that are produced, the current inventory of each type on Earth, and the rate of return of vehicles from Mars. In table 1 below is a conservative projection over 4 synods, beginning with the first introduction of a prototype during “Synod 0.” To do this projection I assumed a constant production rate of 2 MCTs per year or 4 per 26-month synod with no increase in the production rate. I also did not account for any contribution from preliminary Dragon missions that could set up small ISRU propellant facilities on Mars.

 If this is a "low-ball" estimate what would an optimistic estimate look like?

I agree with the theory of the "non-depot" propellant transfer but for other reasons. If filling a tanker stage in orbit then using that as a "depot" IMO there are other benefits:
1) Less Docking events- ie Tank1- Tank2 then Tank1-3 then Tank1-MCT  => 3 docking events Vs
                                        Tanlk1-depot then Tank2-depot then Tank3-depot then MCT-depot=> 4 docking events
2) Less Propellant at risk per docking event and exposure to MOD. Simply put if all the gas is in one place one bad event can get rid of it all.

3) More launch windows- To me this is the big one. Having only one target in space means only one or 2 launch windows per day.  Having more targets in space  means more opportunities to launch and dock with them. (until they are full then it's the same as a single depot.)

4) Cheaper- Hard to dispute  the cost of nothing vs the cost of a depot.

5) Scaleable more MCT missions means more tankers in orbit. As I see the Orbcomm-2 deliver satellites in an array all covering the same section of earth, at 15 minutes apart. Can they do the same thing with a tanker array? That would give hundreds of orbital slots for tankers. Maybe even similar to the plan of 4000 internet satellites.

[nadreck]

4) Cheaper- Hard to dispute  the cost of nothing vs the cost of a depot.

I do dispute that not having a depot costs you nothing - without a depot each tanker needs to have the fuel transfer system and if that system is not complicated, it has its simplicity based in waste of propellant meaning an increase in mass being sent to orbit, if it is complicated it has significant mass that offsets the amount of propellant that can go up on each load.  If we presume that the mass inefficiencies from going without a depot are 5t, then in 20 to 25 tanker flights you could have saved one whole tanker flight (including the depot which with the extra 5t still arrived with 95% propellant load).

Now personally I believe that you will not have anywhere near that efficiency on the transfer system without using some sort of regenerative system to capture overflow from the craft being refueled and recycle it into the depot supply. The faster you transfer the propellant, the more energy you are adding to it, that energy needs to be managed. Also you either provide a small amount of spin(roll most likely) for ulage OR your overflow rate increases as you approach full tanks and your overflow is a mix of liquid and gas which is more complicated to deal with.

[Ionmars]

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I agree with the theory of the "non-depot" propellant transfer but for other reasons. If filling a tanker stage in orbit then using that as a "depot" IMO there are other benefits:
1) Less Docking events- ie Tank1- Tank2 then Tank1-3 then Tank1-MCT  => 3 docking events Vs
Tanlk1-depot then Tank2-depot then Tank3-depot then MCT-depot => 4 docking events

At this proposed depot the sequence is: MCT cargo 1 is docked at depot =>Tanker 1- cargo 1 => tanker 1 (second launch) - cargo 1 => tanker 1 (third launch)  - cargo1 => cargo 1 to TMI. Same as no depot except facilities at the depot facilitate operations. There is no tank per se permanently located at the depot, just those within the visiting MCTs.

2) Less Propellant at risk per docking event and exposure to MOD. Simply put if all the gas is in one place one bad event can get rid of it all.

You are right. Scattering the targets into smaller bundles in space will surely lessen the extent of damage when a target MCT is hit. OTOH one the facilities at the depot is the ability to steer away from IMOD, just like is done at the ISS.

3) More launch windows- To me this is the big one. Having only one target in space means only one or 2 launch windows per day.  Having more targets in space  means more opportunities to launch and dock with them. (until they are full then it's the same as a single depot.)

You are right. The same problem as connecting with the ISS. We will try to make the tradeoff worthwhile for the visiting vehicles.

4) Cheaper- Hard to dispute  the cost of nothing vs the cost of a depot.

You are right.

5) Scaleable more MCT missions means more tankers in orbit. As I see the Orbcomm-2 deliver satellites in an array all covering the same section of earth, at 15 minutes apart. Can they do the same thing with a tanker array? That would give hundreds of orbital slots for tankers. Maybe even similar to the plan of 4000 internet satellites.

You are right. However, the difference between mini-sat launches and MCT launches is that you can launch hundreds of mini-sats in one MCT launch. To place 4000 MCTs into orbit you will have to launch 4000 MCTs.

[Jim]

You are right. However, the difference between mini-sat launches and MCT launches is that you can launch hundreds of mini-sats in one MCT launch. To place 4000 MCTs into orbit you will have to launch 4000 MCTs.

There are other threads for scifi.   Even 6 MCTs on orbit at the same time is not realistic.

[Ionmars]

There are other threads for scifi.   Even 6 MCTs on orbit at the same time is not realistic.

Even to consider the implications of what E.M. says seems unrealistic. IMHO it may still be worth a try.