An Outer Shell for the MCT Propellant Depot

[Ionmars]

While we are awaiting SpaceX’ return to flight and, hopefully, a major announcement concerning their Mars architecture, here are some more sketches of the proposed MCT Propellant Depot.

One of the several reasons for choosing the six-berth design for the MCT Propellant Depot was that it offers an efficient way of surrounding a compact group of spacecraft with a protective outer shell. Such a design requires a minimal amount of material to perform its functions. Panels to reflect solar heat, solar panels to generate power, and radiators to dissipate solar heat, or a combination of these three may cover the outer surfaces of the Depot.

The hyperlinks below are the presentations and discussions that preceded this thread. Then, in the next 9 posts I will present some options for the design of the outer shell protecting the sides of the vehicles and a design for the end-cap.

http://forum.nasaspaceflight.com/index.php?topic=38146.0
http://forum.nasaspaceflight.com/index.php?topic=38308.0
http://forum.nasaspaceflight.com/index.php?topic=38686.0

[Ionmars]

Option 1

You may recall that the stepwise development of the Depot framework resulted in an end view of the Depot reminiscent of a crystal or snowflake.  In the figure below we observe six safety zones, which have a diameter of 17 m each. This is based on a15 m presumed diameter of the MCT and a 1 m safety zone around it. The vehicle never comes in direct contact with any part of the depot framework; each vehicle is locked into its berth by latches on arm extensions.

Option 1 for the outer shell is the bare framework with no shell around the outside of the Depot. This could happen if further analysis showed that no protection is required for MCT other than the insulation and reflective materials that already surround the propellant tanks as components of each vehicle.

[Ionmars]

Option 2

Option 2 is an open-ended cylinder that surrounds all six berths and the MCTs within them. The cylinder may be surfaced with powdered aluminum, a highly reflective material commonly applied on spacecraft for this purpose. The shell is divided into six sets of double doors that lie directly over each berth. These doors will open to allow entry into or exit from a berth.

[Ionmars]

Option 3

If further analysis shows that solar panels or heat radiators should be mounted on the sides of the depot doors, then flat surfaces may be needed. In option 2 a flat-sided (faceted) outer shell will replace the cylindrical outer surface, as shown below. The doors will be flat so that flat solar panels or flat radiators can be attached easily.

Edit: Substituted the figure for the correct one.

[Ionmars]

Additional struts

To implement option 2 or 3 it will be necessary to add struts to support the door hinges located between berths, as indicated below. These struts will be attached onto the forward and aft frames.

[Ionmars]

Option 4

Option 4 will employ multifaceted door panels such as shown below. This approach obviates the requirement for additional struts because the hinges are attached to struts that already exist. Multifaceted door panels also provide additional surface area for mounting solar panels or radiators. IMO it also offers a “coolness” factor. 

[Ionmars]

End cap shield

Regardless of which option is chosen for the outer shell to protect the sides of the Depot, an end cap will be required on the forward end. One type of end cap is the multiple use shield shown below. It will consist of flat heat reflective materials alternating with solar panels on its outer surface.

[Ionmars]

Sol-facing shield

The multipurpose end cap shield will be effective only when directly facing the sun. This means the forward face must always face Sol at all times during its orbital transit and never toward planet Earth. The reflective surfaces and the solar panels will be effective except when the Depot transits through Earth’s shadow. 

Note that the aft end of the depot will always be in the shadow of the forward end cap. If further analysis of heat accumulation shows that additional radiators are required, the aft end frame might be a good place to attach them, trailing away from the forward shield and always in its shadow.

A further analysis of heat generation and dissipation as well as a study of power requirements will determine the amount of depot surface area that should be composed of reflective material, solar panels, and radiators, and where they should be placed.

[Ionmars]

Unfolding end cap shield

This end cap will be subdivided into radial segments (not shown on the previous sketch)) that are unfolded from a compact storage space and then attached onto the forward frame of the depot. This circular shield does not unfold like a standard circular solar panel, Due to its large size (diameter nominally set to 61 m) it is larger than the diameter of the rest of the depot which is 54 meters. Instead, it unfolds like a Japanese fan with additional struts to support the radiating panels.

This shield is still too large to be folded up and stowed in one piece. All panels will be further subdivided into 10-meter wide circular bands. Beginning at the center the subdivisions will occur at 11, 21, 31, and 51 meters radii. Thus each folded segment will be no longer than 11 meters in length and will fit within the ceiling height of a MCT cargo vehicle. The radial segments may be further subdivided into radial sub-segments, e.g. one-quarter of the shield for each sub-segment, depending on the resulting thickness of the package and stowage requirements of the MCT cargo hold.

[Jim]

Why is a shell needed and why would there be more than one MCT's at the depot at a time?

[Ionmars]

Summary

In this thread four different types of outer coverings and one type of multipurpose end cap were presented as options for the MCT Propellant Depot. Further analyses will determine the specific surface areas allocated for reflective shield material, solar panels and heat radiators.

[Jim]

Summary

In this thread four different types of outer coverings and one type of multipurpose end cap were presented as options for the MCT Propellant Depot. Further analyses will determine the specific surface areas allocated for reflective shield material, solar panels and heat radiators.

Again, why do the MCT's need go be enclosed much less covered?

[Ionmars]

Why is a shell needed and why would there be more than one MCT's at the depot at a time?

Good query. My original thoughts were for a depot that would be entirely covered to protect both from solar radiation and reflected radiation from Earth, as in other depot proposals. Now that I have examined the front end cap reflector I realize that complete coverage probably will not be necessary. So now I would choose option 1 (no outer shell) but with a front end cap/shield.

As to why more than one vehicle at a time might dock at the depot, the premise for a large depot is that there will eventually be a large volume of traffic to Mars, as indicated by SpaceX. I have said at the outset that in-space transfers can be accomplished with no depot or a small depot, such as the one proposed by ULA/NASA as long as there are only a few trips to Mars each synod.

There are also some advantages to sharing facilities at the depot.

[docmordrid]

Or KISS by using modded BFS's as reusable tankers.

[Jim]

As to why more than one vehicle at a time might dock at the depot, the premise for a large depot is that there will eventually be a large volume of traffic to Mars, as indicated by SpaceX.

They haven't said anything close to that volume. You realize that would be around 600 people per day and associated depot support launches.

[Burninate]

*Note that all of this is predicated on practical microgravity pumping or cryostorage of methalox requiring some rotation.  Furthermore, it's predicated on a dedicated depot having some comparative advantage (eg, in insulation, active cryocooling, a reusable thermal shade, or a half dozen other things) over using an MCT spacecraft itself... but the MCT spacecraft must still be a good enough storage vessel to keep things topped up on the Martian surface, which is a bit of a pickle.

Using one berth complicates things substantially.  You have to worry about the behavior of sloshing liquids in an asymmetrically rotating body with a shifting center of mass and rotation rate.  I think two berths makes for a lot simpler project than one berth.

If you've made the decision to use a radially symmetric / cylindrical depot with an outer shell, though, once you have two berths, you basically have the option of using four berths or six berths for 'free' - the dimensions don't increase.

A berth doesn't necessarily need to serve visiting vehicles.  It can also serve as the actual storage tank of the depot.

Even if all six berths are for visiting tugs and there's a separate tank, if pumping requires rotation, a single spin-up / spin-down (transferring six tugs worth of propellant) would be easier than three or six spin-up / spin-down maneuvers for the same amount of propellant.  It's a matter of how many rotations the average kilogram of propellant is subjected to.

Actual pumping operations are not likely to take very long relative to the time spent waiting for the next tug, and would not be compatible with spinning down to dock new vehicles while still pumping.

One level of complexity up, I will note that if you tie two of these 4-6 berth vehicles together (with a very heavy rigid truss system), you can spin them up and down without resorting to propulsive thrust.

[Ionmars]

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?

[Ionmars]

As to why more than one vehicle at a time might dock at the depot, the premise for a large depot is that there will eventually be a large volume of traffic to Mars, as indicated by SpaceX.

They haven't said anything close to that volume. You realize that would be around 600 people per day and associated depot support launches.

My guess is that a depot will simplify operations for SpaceX or any other company or country using MCT-like vehicles. It just requires enough vehicle traffic to LEO whereby a structure in space will be convenient and will speed up their operation.

This structure is pretty simple compared to the ISS. No pressure vessels because it is remote controlled from Earth. Just a framework and an end cap heat shield. As indicated in the "assembly" thread, the framework can be put together with materials lifted to LEO inside one cargo MCT. Probably an efficient project.

[Jim]

My guess is that a depot will simplify operations for SpaceX or any other company or country using MCT-like vehicles. It just requires enough vehicle traffic to LEO whereby a structure in space will be convenient and will speed up their operation.

This structure is pretty simple compared to the ISS. No pressure vessels because it is remote controlled from Earth. Just a framework and an end cap heat shield. As indicated in the "assembly" thread, the framework can be put together with materials lifted to LEO inside one cargo MCT. Probably an efficient project.

That is a grossly optimistic (or even bad assumption) that there will be other companies or countries using MCT type vehicles.

Also, it would be constructed using an MCT, that is putting the cart before the horse. If there is going to be traffic going to Mars, it will grow incrementally and so will depots. 

[Jim]

Why are they docked axially and not radially?

[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]

...
...
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.