Author Topic: A Framework for the MCT Propellant Depot  (Read 44000 times)

Online Ionmars

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Re: A Framework for the MCT Propellantt Depot
« Reply #20 on: 08/24/2015 01:55 PM »
Questions

A number of engineering questions should be addressed. For example:

(1)   Depot materials: Titanium aluminate alloy (Ti-Al) has been proposed for the framework, but thus could be expensive. Could another material do the job equally well? What about composites? Aluminum alloy 7068?
(2)    Framework design: Is the proposed framework strong enough to handle a fully loaded MCT tanker in its berth? Any changes or adjustments?
(3)   Cold fusion: Can this be made to work to fuse together the framework of a real in-space project?
(4)   Robots: Could two Canadarm robotic arms or their equivalent grab a fully loaded MCT and pull it into a berth? If not, what would be the specs on another option?
(5)    Connectors: Would the proposed connectors work or would some alternative be preferred?

Your input is the most important part of this article.

« Last Edit: 08/30/2015 11:11 AM by Ionmars »
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Offline Paul451

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Re: A Framework for the MCT Propellantt Depot
« Reply #21 on: 08/27/2015 09:09 AM »
Cold metal welding

In spite of the name, this kind of enhanced surface stiction is not welding. The adhesive effect is strong enough to be an annoyance, but it's not strong enough to be used structurally to securely attach elements together.

[To add confusion, there is a form of genuine metal joining called "cold metal welding" which is pressure-fusing metals together. Typically used to join wires and thin rods end-to-end. And even more confusingly, a bad mig weld (too shallow) is also often called a "cold weld".]

Re: Robot-arm.
Any reason it is inside your structure? I would expect the arms to spend most of their time on the outer frame. Both during assembly and during operations.
« Last Edit: 08/27/2015 09:10 AM by Paul451 »

Online Ionmars

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Re: A Framework for the MCT Propellantt Depot
« Reply #22 on: 08/27/2015 08:45 PM »
Cold metal welding

In spite of the name, this kind of enhanced surface stiction is not welding. The adhesive effect is strong enough to be an annoyance, but it's not strong enough to be used structurally to securely attach elements together.

(To add confusion, there is a form of genuine metal joining called "cold metal welding" which is pressure-fusing metals together. Typically used to join wires and thin rods end-to-end. And even more confusingly, a bad mig weld (too shallow) is also often called a "cold weld".)
Quote
Thank you, Paul

Perhaps I was too taken with the Wikipedia entry on this subject and by a quote from Feynman:
"The reason for this unexpected behavior is that when the atoms in contact are all of the same kind, there is no way for the atoms to “know” that they are in different pieces of copper. When there are other atoms, in the oxides and greases and more complicated thin surface layers of contaminants in between, the atoms “know” when they are not on the same part.
—Richard Feynman, The Feynman Lectures, 12–2 Friction

These connectors are often joined with just the force of normal friction with a tight fit to hold parts together, and in this case with metal pegs. The "stiction" friction would add some more adhesion but it wouldn't hurt to return to these junctions after assembling the Depot to perform some hot metal welding too.
Re: Robot-arm.
Any reason it is inside your structure? I would expect the arms to spend most of their time on the outer frame. Both during assembly and during operations.

Yes , I agree the robotic arms will spend most of their time above the saddle, but the interior cavity provides a quick shortcut to pass from one saddle to another. Actually, passing through sidewalls may not even be feasible if there are any MCTs docked at intervening berths.
« Last Edit: 08/30/2015 11:13 AM by Ionmars »
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Online Ionmars

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Re: A Framework for the MCT Propellantt Depot
« Reply #23 on: 09/04/2015 09:09 PM »
Box tube assembly version 2

If we want to employ metal connectors for tubes that are part of a rigid framework, then we need a way to place a tube into the connector at a right angle rather than endwise. Therefore, one of the four sides of the connector sleeve will be removed so that the interior of the sleeve is exposed.

The method of assembling a box tube into a connector sleeve was originally presented on this thread in Replies 7 and 8 above. The new version 2 was developed (a) to improve the ease of connecting pieces so that a robot arm could more easily complete the necessary motions, and (b) to place more compression on the junction and increase the amount of striction (cold metal welding) that takes place.

Version 2 of a box tube connection assembly is shown below. It features the major components labeled as follows:
A.   A 12-inch metal box tube to be inserted “downward” into the sleeve of a box tube connector.
B.    A joiner plate with alignment rods attached to each side and four bolts pre-set loosely into four holes.
C.    A 12-inch connector sleeve to receive the box tube.
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Online Ionmars

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Re: A Framework for the MCT Propellantt Depot
« Reply #24 on: 09/04/2015 09:23 PM »
Employing the joiner assembly version 2.

The joining of pieces will begin on Earth where each tube and connector will be produced and partially assembled. Joiner plates will be attached to the ends of box tubes. Two alignment rods will be attached to each plate, one on each side as shown in the above sketch. Four holes will be drilled for the four bolts to be pre-inserted in the holes loosely, ready to be screwed into the corresponding threaded hole in the rim of the connector sleeve.

To join pieces, one robotic arm will hold the connector in place. A second arm will grasp the box tube and place it above the connector sleeve. Then it will lower the tube into the sleeve, using the alignment rods to obtain accurate positioning within the sleeve. Then an end effector drill will screw down the four bolts, first with light pressure and then with the predetermined foot-pounds of pressure.

A cargo MCT will bring these tubes and connectors to LEO. If cargo space is tight, it may be desirable to remove a section of one face of the tube below the joiner plate. The top surface of the joiner plate would then be even with the outer surface of the tube, thus saving some space when the tubes are packed together.

If the parts are designed to fit tightly, then additional steps may be needed. Taking advantage of the expansion coefficient of metal, the connector sleeve can be exposed to direct sunlight. This will raise its temperature by 100 +/- degrees C and cause the metal to expand. The tube can be kept in the shade, which will lower its temperature by 100 degrees +/- and cause it to contract. The two parts will slide together easily. Before the final tightening of bolts the two parts should be allowed to come to the same temperature.

To maximize the effect of cold metal welding all the metal contact parts should be cleaned by abrasion prior to joining.

The joining procedures recommended here should be tested by means of an experiment carried out at the ISS. The joining process should be conducted in outside space, employing the Canadarms and the appropriate end effectors.

Edited: grammar
« Last Edit: 09/04/2015 09:40 PM by Ionmars »
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Offline lamontagne

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Re: A Framework for the MCT Propellantt Depot
« Reply #25 on: 09/06/2015 01:22 PM »
How do you address the question of pumping out the fuel? In free fall the propellant will float around in gobs, and pumps may vary from pumping fluids to gases in an unplanned manner. I've seen rotation proposed as a solution, expansion membranes in the tanks, squeezable tank and internal paddles wheels.  What would be used for this?

And how much power might be needed?

Offline jg

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Re: A Framework for the MCT Propellantt Depot
« Reply #26 on: 09/06/2015 02:47 PM »
How do you address the question of pumping out the fuel? In free fall the propellant will float around in gobs, and pumps may vary from pumping fluids to gases in an unplanned manner. I've seen rotation proposed as a solution, expansion membranes in the tanks, squeezable tank and internal paddles wheels.  What would be used for this?

And how much power might be needed?
See ULA's IVF work. One side effect is a very small thrust that keeps propellants settled which also reduces losses due to boiloff.

Offline muomega0

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Re: A Framework for the MCT Propellantt Depot
« Reply #27 on: 09/06/2015 03:53 PM »
How do you address the question of pumping out the fuel? In free fall the propellant will float around in gobs, and pumps may vary from pumping fluids to gases in an unplanned manner. I've seen rotation proposed as a solution, expansion membranes in the tanks, squeezable tank and internal paddles wheels.  What would be used for this?

And how much power might be needed?
See ULA's IVF work. One side effect is a very small thrust that keeps propellants settled which also reduces losses due to boiloff.
Boiloff is continuous. So the stage with distributed tanks must always have an axial trust and cannot loiter for IVF to function?

Here is another image of the Dual MTV with sunshield, which suggests that the propellant is indeed a benefit to the crew, although it ignores loiter times.

Unfortunately, Propellant shielding does not address GCR, so there are many advantages of keeping the crew and cargo/supplies separate, and the supplies can improve the cost and performance of the science missions as well (?).   Send the crew directly to shorten the trip time and send the supplies ahead of time with EP to minimize IMLEO.  Tanks rotating about the center line is a very practical demonstrated solution.

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Re: A Framework for the MCT Propellantt Depot
« Reply #28 on: 09/06/2015 10:31 PM »
How do you address the question of pumping out the fuel? In free fall the propellant will float around in gobs, and pumps may vary from pumping fluids to gases in an unplanned manner. I've seen rotation proposed as a solution, expansion membranes in the tanks, squeezable tank and internal paddles wheels.  What would be used for this?

And how much power might be needed?
I think you are pointing out a real problem. Any type of propellant depot is unlikely to work if propellant cannot be reliably pumped from one tank to another while the vehicles are lying together quietly.

You have listed a number of ideas that might work or might not. I have my favorites, but ISTM what is lacking is a program to systematically test the proposed ideas. For example, experiments could be set up on the ISS to test which procedures, if any, will work. If we don't find a reliable method for pumping fluids then many architectures that involve a propellant depot must be tossed out the window.
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Offline guckyfan

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Re: A Framework for the MCT Propellantt Depot
« Reply #29 on: 09/07/2015 05:13 AM »
If we don't find a reliable method for pumping fluids then many architectures that involve a propellant depot must be tossed out the window.

Propellant depots are possible using a very small acceleration. Very large depots might be a problem because a very large mass of propellant needs to be accelerated for every tanking event. Worst case a depot would fill up just one MCT.

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Re: A Framework for the MCT Propellantt Depot
« Reply #30 on: 09/07/2015 08:59 AM »
If we don't find a reliable method for pumping fluids then many architectures that involve a propellant depot must be tossed out the window.

Propellant depots are possible using a very small acceleration. Very large depots might be a problem because a very large mass of propellant needs to be accelerated for every tanking event. Worst case a depot would fill up just one MCT.
The comments from you, lamontagne and others are inspiring me to dig further into this issue. Maybe it deserves its own thread?

The problem was was succinctly stated by lamontagne:

“How do you address the question of pumping out the fuel? In free fall the propellant will float around in gobs, and pumps may vary from pumping fluids to gases in an unplanned manner. I've seen rotation proposed as a solution, expansion membranes in the tanks, squeezable tank and internal paddles wheels.”

Rotation: The whole tank or vehicle rotates to provide a force to push the gobs together while pumping takes place. How do we male this work in a practical way? Does the propellant delivery vehicle have to rotate in concert with the receiving vehicle?

Expansion membranes in the tank: A very desirable approach, but does there exist a membrane that will maintain flexibility under cryogenic temperatures? Every material that I know of becomes brittle.

Squeezable tanks. This is new to me. The problem may be similar to that of a flexible membrane, only now you have a whole tank that has to remain flexible under cryogenic conditions. (?)

Internal paddle wheels: Provides a force similar to a rotating tank but less mass is being rotated.  May be a leading candidate.

Controlled rapid boil-off: My addition to the list.  The liquid propellant from the delivery tank would be deliberately warmed to convert liquid to a cold gas. The gas would provide its own force of expansion. It is the gas that would be pumped, first through a cryocooler to re-liquefy it and then into the second tank.  Drawbacks: requires a large crycooler and lots of energy. Also, CRAB is not an attractive acronym.  :)
« Last Edit: 09/07/2015 09:00 AM by Ionmars »
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Offline guckyfan

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Re: A Framework for the MCT Propellant Depot
« Reply #31 on: 09/07/2015 12:14 PM »
A small linear acceleration is my personal favorite. It is simple, I like simple. :)

They need a LOX/methane RCS engine for MCS anyway. Such an engine would be suitable for the required small acceleration. It does limit the depot size. I imagine you don't want to accelerate 10,000t of fuel to transfer 500t to one MCT. But I don't see that as a drawback. A depot for ~5 MCT is fine IMO.

A large rotating structure? You need to spin it up for tanking and then back down. Otherwise you have to dock MCT to a rotating structure. Then fuel MCT while the balance of the structure shifts or you have to maintain balance while pumping the fuel.

Online Ionmars

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Re: A Framework for the MCT Propellant Depot
« Reply #32 on: 09/07/2015 03:53 PM »
A small linear acceleration is my personal favorite. It is simple, I like simple. :)
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I like simple too. Simple is probably more reliable.

How's this: Coordinate the RCS maneuvers with pumping of propellants to give a linear acceleration to all MCTs parked together. When this is repeated the orbit might get too high; so turn the RCS around and lower the orbit during the following pumping episodes.
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Offline Burninate

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Re: A Framework for the MCT Propellant Depot
« Reply #33 on: 09/07/2015 06:17 PM »
A small linear acceleration requires constant fuel expenditure per time for pumping, and modifies the orbit.  Pumping needs to be very fast.

A rotation only requires fuel expenditure while spinning up or spinning down, and can be done without modifying the orbit.  Pumping can be done at leisure.

Offline Semmel

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Re: A Framework for the MCT Propellant Depot
« Reply #34 on: 09/07/2015 07:38 PM »
If the depod is in LEO, it requires regular lift of its orbit anyway because of the atmospheric drag. Similar to ISS. Tanking operations would than be limited to these occasions. It requires timing, nothing more. And maybe small RCS engines of the docked ship, the depot it self does not require the capability. Much like the ISS.

Offline guckyfan

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Re: A Framework for the MCT Propellant Depot
« Reply #35 on: 09/07/2015 08:59 PM »
The depot will need that capability. It must be able to do debris avoidance, when no MCT is docked. But that's not hard. It has the fuel and can easily have a few methane RCS engines.

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Re: A Framework for the MCT Propellant Depot
« Reply #36 on: 09/07/2015 10:38 PM »
A small linear acceleration requires constant fuel expenditure per time for pumping, and modifies the orbit.  Pumping needs to be very fast.

A rotation only requires fuel expenditure while spinning up or spinning down, and can be done without modifying the orbit.  Pumping can be done at leisure.
If we employ rotation at the 6-berth Depot, what should be the center of rotation? Should the propellant flow toward the forward end, the aft end, or the sides of the MCT tanks lying in berths?

Would it be necessary to spin down or terminate rotation to accommodate a vehicle arriving or leaving the Depot?

Thank you for your insights.

Edited: grammar.
« Last Edit: 09/08/2015 01:17 AM by Ionmars »
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Offline CuddlyRocket

Re: A Framework for the MCT Propellant Depot
« Reply #37 on: 09/08/2015 12:42 AM »
The problem was was succinctly stated by lamontagne:

“How do you address the question of pumping out the fuel? In free fall the propellant will float around in gobs, and pumps may vary from pumping fluids to gases in an unplanned manner. I've seen rotation proposed as a solution, expansion membranes in the tanks, squeezable tank and internal paddles wheels.”

How about a piston internal to the tank taking up the whole cross-section? The rod of the piston, which will be on the opposite side to the propellant, can run through that end of the tank and have a ratchet and gear mechanism so that an electric motor can drive it forwards and backwards etc.

However, although that would drive the contents of the tank out, pressure by itself will not separate liquids from gases. That requires weight and in free fall that either means rotation or thrust. But do you need to separate liquids from gases if you're just transferring propellant from the depot's tanks to those of some vehicle? Once in that vehicles tanks, it can carry out the usual ullage procedures etc.

Online Ionmars

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Re: A Framework for the MCT Propellant Depot
« Reply #38 on: 09/08/2015 01:44 AM »
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How about a piston internal to the tank taking up the whole cross-section? The rod of the piston, which will be on the opposite side to the propellant, can run through that end of the tank and have a ratchet and gear mechanism so that an electric motor can drive it forwards and backwards etc.

However, although that would drive the contents of the tank out, pressure by itself will not separate liquids from gases. That requires weight and in free fall that either means rotation or thrust. But do you need to separate liquids from gases if you're just transferring propellant from the depot's tanks to those of some vehicle? Once in that vehicles tanks, it can carry out the usual ullage procedures etc.
Yeah, that might work.
I recently ran across a message from a company extolling the virtues of their new cryogenic sealants, saying they are better than ever. Highly effective seals will be needed if the piston is to fill up the cross section of a whole fuel vessel. You will want a small amount of of gas leakage around the seals or a gas membrane in the face of the piston to pass gas to the non-liquid side of the piston. The pressure and temperature should be controlled to  let the gas pressure equalize the pressure on the liquid side where fuel is being pumped.

However, no matter how well you refine your idea, the critics will say that it involves more mass and more gadgets to be hauled up to orbit inside every propellant tank.

Another problem is that propellant tanks are not just cylinders but have rounded ends; sometimes they are just a globe. Definitely a limit on how much of the fuel could be pumped out.
« Last Edit: 09/08/2015 01:49 AM by Ionmars »
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Offline Semmel

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Re: A Framework for the MCT Propellant Depot
« Reply #39 on: 09/08/2015 06:06 AM »
The depot will need that capability. It must be able to do debris avoidance, when no MCT is docked. But that's not hard. It has the fuel and can easily have a few methane RCS engines.

Well, the engines on the depot cant use the large tanks because they cant get the fuel to the engines. Its the same problem as with tanking. They need pressure fed engines, which must draw fuel from different tanks. Why would you need Methane if you need extra tanks anyway? Cant it be any fuel, Hydrazine for instance?

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