Author Topic: A Framework for the MCT Propellant Depot  (Read 43803 times)

Offline guckyfan

  • Senior Member
  • *****
  • Posts: 6604
  • Germany
  • Liked: 1698
  • Likes Given: 1633
Re: A Framework for the MCT Propellant Depot
« Reply #40 on: 09/08/2015 06:24 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?

IMO they will want a fuel for the RCS that can be sourced at Mars and want to go with one fuel, getting rid of hypergols for that purpose. They can always refill the pressurized tanks from the main tanks while accelerating. Morpheus is using a LOX/methane RCS so it is already proven.

Offline Burninate

  • Full Member
  • ****
  • Posts: 1130
  • Liked: 346
  • Likes Given: 72
Re: A Framework for the MCT Propellant Depot
« Reply #41 on: 09/08/2015 07:05 AM »
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.
So let's say we use a deviation from the 6-shooter depot: We cant the tanks outwards by just 2 degrees or so, so one end of each tank is sticking out at a slightly larger diameter.  Then place the pumps on the greatest-diameter section on the outer sidewall.

Now, when the vehicle rotates, the propellant flows towards the side of the tank, and then slowly rolls "downhill" to a spot near where the hemispherical endcap curvature begins.  The slight cant guarantees you can get the thing 99.9% empty instead of only 99% empty; Just placing it on the outermost sidewall of parallel tanks should work most of the way.

Offline lamontagne

  • Full Member
  • ****
  • Posts: 1091
  • Liked: 1548
  • Likes Given: 241
Re: A Framework for the MCT Propellant Depot
« Reply #42 on: 09/08/2015 02:29 PM »
A whole series of questions  ;-)

What might be the acceleration required to settle down the fluid?  I expect the fuel transfer operation might take many hours and I wonder what might be the final deltaV change.  As someone pointed out, though, we can accelerate and then decelerate.  That would be the simplest, if the acceleration is low.

A full MCT requires something like 5-700 tons of fuel depending on how much of a hurry you are.  However, most BFR designs can only deliver 250 tons to orbit.  So the depot would hold the fuel for six Mars transfers, lifted up by 17 or 18 tanker flights? 

Would there be some tanks dedicated to ion propulsion for different types of missions, or SEP assist for Mars transfer?  and what might the Ion propellant be?  Argon would be nice, since it can be replaced on Mars.

Regarding rotation, what might be the best solution?

1- We can rotate the MCT and depot assembly around a common center of gravity on the short axis (this is particularly useful for fuel transfer from tankers)

2- Rotate the tanks as a group along the long axis, and rotate the MCT, with a very precise rotation system the MCT might even dock at the center point of a rotating tank array, like the 2001 shuttle.  that way we wouldn't need to accelerate and decelerate the depot,

3- We can have a non rotating dock, or six contra rotating tanks in a fixed frame, although there may be some unsolvable seal problems with that.

The angle for the tanks might be quite small?  Liquids flow very well with just 1 or 2% slope, so this might not affect construction much.

On the question of flexible membranes, NASA had a study with Thin Red Line Aerospace and Bigelow on inflatable cryogenic fuel tanks.  these could be used either as tanks directly, or as expansion membranes in rigid tanks.

Regards,

Michel Lamontagne

Offline lamontagne

  • Full Member
  • ****
  • Posts: 1091
  • Liked: 1548
  • Likes Given: 241
Re: A Framework for the MCT Propellantt Depot
« Reply #43 on: 09/08/2015 02:36 PM »
Forward frame

Given the geometrical layout of a berth, the following is the layout of the struts of the most forward of the frames:

It would be interesting if we could spin individual tanks around the point C.  Using contra rotating tanks would cancel out angular momentum, as long as the tanks were loaded more or less evenly?

I've joined a depot paper on cooling.  Although the hydrogen question is not really a problem for this thread.

Offline Ionmars

  • Full Member
  • ****
  • Posts: 791
  • North Carolina, USA
  • Liked: 224
  • Likes Given: 498
Re: A Framework for the MCT Propellant Depot
« Reply #44 on: 09/08/2015 07:38 PM »
...
...
...
So let's say we use a deviation from the 6-shooter depot: We cant the tanks outwards by just 2 degrees or so, so one end of each tank is sticking out at a slightly larger diameter.  Then place the pumps on the greatest-diameter section on the outer sidewall.

Now, when the vehicle rotates, the propellant flows towards the side of the tank, and then slowly rolls "downhill" to a spot near where the hemispherical endcap curvature begins.  The slight cant guarantees you can get the thing 99.9% empty instead of only 99% empty; Just placing it on the outermost sidewall of parallel tanks should work most of the way.
What a great idea!

With your permission I would like to incorporate this approach into the next iteration of the Depot. I already have a sketch in mind.
Mars Pioneers will require our continued support.

Offline Ionmars

  • Full Member
  • ****
  • Posts: 791
  • North Carolina, USA
  • Liked: 224
  • Likes Given: 498
Re: A Framework for the MCT Propellant Depot
« Reply #45 on: 09/08/2015 08:04 PM »
A whole series of questions  ;-)

What might be the acceleration required to settle down the fluid?  I expect the fuel transfer operation might take many hours and I wonder what might be the final deltaV change.  As someone pointed out, though, we can accelerate and then decelerate.  That would be the simplest, if the acceleration is low.
...
...
...
Regards,

Michel Lamontagne
A whole series of GREAT questions.I will try to do them justice.

The acceleration required to settle the fluid and the thrust to attain that acceleration will require a mathematical model of the Depot. The model must consider current mass distribution within the Depot and take into account the number and location of MCTs currently in berth, which berths, and the amount propellant in each tank. It will also require knowledge of the types and locations of RCS thrusters on the Depot.

I must acknowledge right now that I am not the best person to set up this model. There lies 55 years between me and my last physics class as well as age-related disadvantages. I am best to stick to thinking up new ideas to be tested, but I would be eager to assist someone who could do it.
Mars Pioneers will require our continued support.

Offline Burninate

  • Full Member
  • ****
  • Posts: 1130
  • Liked: 346
  • Likes Given: 72
Re: A Framework for the MCT Propellant Depot
« Reply #46 on: 09/08/2015 08:41 PM »
...
...
...
So let's say we use a deviation from the 6-shooter depot: We cant the tanks outwards by just 2 degrees or so, so one end of each tank is sticking out at a slightly larger diameter.  Then place the pumps on the greatest-diameter section on the outer sidewall.

Now, when the vehicle rotates, the propellant flows towards the side of the tank, and then slowly rolls "downhill" to a spot near where the hemispherical endcap curvature begins.  The slight cant guarantees you can get the thing 99.9% empty instead of only 99% empty; Just placing it on the outermost sidewall of parallel tanks should work most of the way.
What a great idea!

With your permission I would like to incorporate this approach into the next iteration of the Depot. I already have a sketch in mind.

Sure!

Tossing out an abortive idea I had for something fairly similar:
Instead of canting the tanks outwards at 2 degrees, keep the tanks parallel, and put some sort of lightweight closed-cell foam in place on the interior of each cylinder as a ramp at a 2 degree cant.  The pressure vessel still needs those cylindrical walls to hold together efficiently;  This foam ramp would not be resillient under bending loads, it would simply transfer pressure to the outer cylindrical pressure vessel.

The problem is in the materials.

Things that I would prefer to use:
*A solid form, shaped like a ramp insert;  But too heavy.
*An inflated / sealed composite ballast tank, shaped like a ramp insert and with plenty of internal pillar supports, like an inflatable mattress;  But then you need to mess with ideal gas expansion of the internal reservoir over varying pressures, and worse you would need to plausibly deal with phase change behavior over a thermal gradient.
*A foam of some sort.  Can you make a closed-cell aerogel strong enough to hold pressure of a gas with lower boiling point than liquid oxygen, while still being lightweight?  Probably not.

Offline Burninate

  • Full Member
  • ****
  • Posts: 1130
  • Liked: 346
  • Likes Given: 72
Re: A Framework for the MCT Propellant Depot
« Reply #47 on: 09/08/2015 08:44 PM »
Oh, another note:
How are you going to handle LOX vs CH4?

You could go with common sidewall tanks, but they're fairly close to needing a 2:1 volume ratio at Musk's specified 3.8:1 mass ratio, which needs one less sidewall if you're starting out with a number of tanks divisible by 3.  Six tanks gives you this, and gives you a balanced center of mass.
« Last Edit: 09/08/2015 08:48 PM by Burninate »

Offline Paul451

  • Full Member
  • ****
  • Posts: 1339
  • Australia
  • Liked: 670
  • Likes Given: 565
Re: A Framework for the MCT Propellant Depot
« Reply #48 on: 09/08/2015 09:06 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.

I'm reading between the lines here: But do you realise that such a linear acceleration (a ullage burn) needs to be carried out for the entirety of the fuel transfer? You don't just fire a quick burst to "settle the fuel" and then go back to freefall.

Upper-stages, RCS, etc, can get away with short ullage burns because they only need to "settle the fuel" for long enough to get the main engines to start or restart. After that, the thrust from the main burn itself continues to do the job. But for a depot, the ullage burn would be needed for the entire fuel transfer.

Offline Ionmars

  • Full Member
  • ****
  • Posts: 791
  • North Carolina, USA
  • Liked: 224
  • Likes Given: 498
Re: A Framework for the MCT Propellant Depot
« Reply #49 on: 09/08/2015 09:17 PM »
A whole series of questions  ;-)
...
...
...
A full MCT requires something like 5-700 tons of fuel depending on how much of a hurry you are.  However, most BFR designs can only deliver 250 tons to orbit.  So the depot would hold the fuel for six Mars transfers, lifted up by 17 or 18 tanker flights? 
...
...
...
Basically yes. This question will involve another mathematical program, this time addressing scheduling and berth allocations.

The way that I see this LEO Depot coming into play is one synod after the first MCT landing on Mars. It will be bringing the first shipment of tools and supplies in advance of the first human landing. This means that the MCT is finally coming out of its development stage and coming into service. During the next 26 months, activity will begin to accelerate. The next 10-12 MCTs will take less time to produce than the first one took to develop. So it would behoove us to be ready for this event with a LEO rendezvous point of departure, which is the MCT Propellant Depot.

The Depot seems too large right now but when activity accelerates it may become too small. Scheduling the berths may go something like this: I want to load up 3 cargo MCTs that will form a small armada to Mars. Should I place all three into three berths at the same time and use the three empty berth to dock nine tanker MCTs in sequence to fill up their propellant tanks? Or should I dock three tanker MCTs in three berths and have one out-bound cargo MCT dock in one of the empty berths and load up all at once?

On the next synod we may load a passenger MCT to land on Mars. Using Depot resources we could send the passengers on a fast trip to Mars using an all-propellant approach. As described in the previous thread, this entails using one of the tanker MCTs as a booster. The fully fueled booster would be attached to the fully loaded passenger vehicle while they are still docked at the Depot. To begin the trip,  the Depot's robotic arms will push off  the duo from the Depot. The booster's raptor engines will fire up and accelerate them to HEO. At HEO the Passenger MCT will separate and fire up its raptors for TMI at a high speed. The booster then will return to the Depot or directly to Earth (depending on how much fuel is left).

To expedite this approach we will need to modify the depot design. The framework divider between two adjacent berths will need to be removed so that two MCTs can lie close together and be physically linked (think of Falcon H). A series of tanker MCTs will dock at the Depot and pump their propellant load into these two out-bound vehicles. Only then will they be ready for a  BEO destination.

During the same year other space projects may take place, such as space hotels, trips to the moon, or other robotic exploration trips that want to utilize the Depot. In addition, other countries may want to get in on the action. So the schedule at the Depot could soon become as busy as Cape Canaveral.
Mars Pioneers will require our continued support.

Offline Ionmars

  • Full Member
  • ****
  • Posts: 791
  • North Carolina, USA
  • Liked: 224
  • Likes Given: 498
Re: A Framework for the MCT Propellant Depot
« Reply #50 on: 09/08/2015 09:34 PM »
...
...
...
Sure!

Tossing out an abortive idea I had for something fairly similar:
Instead of canting the tanks outwards at 2 degrees, keep the tanks parallel, and put some sort of lightweight closed-cell foam in place on the interior of each cylinder as a ramp at a 2 degree cant.  The pressure vessel still needs those cylindrical walls to hold together efficiently;  This foam ramp would not be resillient under bending loads, it would simply transfer pressure to the outer cylindrical pressure vessel.

The problem is in the materials.

Things that I would prefer to use:
*A solid form, shaped like a ramp insert;  But too heavy.
*An inflated / sealed composite ballast tank, shaped like a ramp insert and with plenty of internal pillar supports, like an inflatable mattress;  But then you need to mess with ideal gas expansion of the internal reservoir over varying pressures, and worse you would need to plausibly deal with phase change behavior over a thermal gradient.
*A foam of some sort.  Can you make a closed-cell aerogel strong enough to hold pressure of a gas with lower boiling point than liquid oxygen, while still being lightweight?  Probably not.
I hope you don't burn out your brain cells all in one day!
Yes, if we could get a real test program to try this out. I must admit my predilection for your first idea because it is profoundly simple and requires no change in the MCT tank design.
Mars Pioneers will require our continued support.

Offline Ionmars

  • Full Member
  • ****
  • Posts: 791
  • North Carolina, USA
  • Liked: 224
  • Likes Given: 498
Re: A Framework for the MCT Propellant Depot
« Reply #51 on: 09/08/2015 10:05 PM »
Oh, another note:
How are you going to handle LOX vs CH4?

You could go with common sidewall tanks, but they're fairly close to needing a 2:1 volume ratio at Musk's specified 3.8:1 mass ratio, which needs one less sidewall if you're starting out with a number of tanks divisible by 3.  Six tanks gives you this, and gives you a balanced center of mass.
The Depot has two plumbing systems, one for LOX and one for CH4. The plumbing ties together the MCT tank propellants through the respective interface pads and LIDS system.

The depot[s tanks will be one tanker MCT with the same tanks as other MCTs, so the volume and mass ratios will be the same for all visiting vehicles, at least in the beginning. As propellants are pumped around and back and forth, the boil-off and other losses may cause unbalance. But considering that propellant is moving around frequently, I don't know if an accurate mass balance could be maintained. We may have to just compensate. (?)
Mars Pioneers will require our continued support.

Offline Ionmars

  • Full Member
  • ****
  • Posts: 791
  • North Carolina, USA
  • Liked: 224
  • Likes Given: 498
Re: A Framework for the MCT Propellant Depot
« Reply #52 on: 09/08/2015 10:23 PM »
A whole series of questions  ;-)
...
...
...
Could there be some tanks dedicated to ion propulsion for different types of missions, or SEP assist for Mars transfer?  and what might the Ion propellant be?  Argon would be nice, since it can be replaced on Mars.
...
...
...
I know that many people favor SEP as a fuel-efficient architecture to Mars and back. And Spacex has said it is "considering" SEP in their plans.

But I am primarily considering the higher cost of developing new systems, the relatively low cost of methane, and the high delta-V that can be reached with just Raptors. Plus simplicity.

So I am not incorporating SEP at this time, But i will return to the subject if I must.
Mars Pioneers will require our continued support.

Offline Ionmars

  • Full Member
  • ****
  • Posts: 791
  • North Carolina, USA
  • Liked: 224
  • Likes Given: 498
Re: A Framework for the MCT Propellant Depot
« Reply #53 on: 09/08/2015 10:38 PM »
A whole series of questions  ;-)
...
...
...
Regarding rotation, what might be the best solution?

1- We can rotate the MCT and depot assembly around a common center of gravity on the short axis (this is particularly useful for fuel transfer from tankers)

2- Rotate the tanks as a group along the long axis, and rotate the MCT, with a very precise rotation system the MCT might even dock at the center point of a rotating tank array, like the 2001 shuttle.  that way we wouldn't need to accelerate and decelerate the depot,

3- We can have a non rotating dock, or six contra rotating tanks in a fixed frame, although there may be some unsolvable seal problems with that.

The angle for the tanks might be quite small?  Liquids flow very well with just 1 or 2% slope, so this might not affect construction much.
...
...
...
After you posed the question, I received much discussion on this question, Burninate has a proposal I am going to incorporate. Basically using your sloped tanks suggestion and rotation on a lengthwise axis.
« Last Edit: 09/09/2015 12:41 AM by Ionmars »
Mars Pioneers will require our continued support.

Offline Ionmars

  • Full Member
  • ****
  • Posts: 791
  • North Carolina, USA
  • Liked: 224
  • Likes Given: 498
Re: A Framework for the MCT Propellant Depot
« Reply #54 on: 09/09/2015 03:54 AM »
Update to The Depot (1/2)

Information and ideas originating on this thread have led to an update to the MCT Propellant Depot.

Guckyfan (Reply #29) “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.”

Ionmars (Reply #32) “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.

Guckyfan (reply #40) “IMO they will want a fuel for the RCS that can be sourced at Mars and want to go with one fuel, getting rid of hypergols for that purpose. They can always refill the pressurized tanks from the main tanks while accelerating. Morpheus is using a LOX/methane RCS so it is already proven.”

Burninate (Reply #41) “So let's say we use a deviation from the 6-shooter depot: We cant the tanks outwards by just 2 degrees or so, so one end of each tank is sticking out at a slightly larger diameter.  Then place the pumps on the greatest-diameter section on the outer sidewall.

Now, when the vehicle rotates, the propellant flows towards the side of the tank, and then slowly rolls "downhill" to a spot near where the hemispherical endcap curvature begins.  The slight cant guarantees you can get the thing 99.9% empty instead of only 99% empty; Just placing it on the outermost sidewall of parallel tanks should work most of the way.”


Lamontagne (Reply # 42) “Rotate the tanks as a group along the long axis, and rotate the MCT, with a very precise rotation system the MCT might even dock at the center point of a rotating tank array, like the 2001 shuttle.  that way we wouldn't need to accelerate and decelerate the depot.”

“The angle for the tanks might be quite small?  Liquids flow very well with just 1 or 2% slope, so this might not affect construction much.”
Mars Pioneers will require our continued support.

Offline Ionmars

  • Full Member
  • ****
  • Posts: 791
  • North Carolina, USA
  • Liked: 224
  • Likes Given: 498
Re: A Framework for the MCT Propellant Depot
« Reply #55 on: 09/09/2015 03:59 AM »
Update to the Depot (2)

The sketch below shows the forward construction frame of the MCT Propellant Depot with the propellant plumbing lines for LOX and CH4 linking all the interface pads. Please recall that each MCT is assumed to have a LIDS connector under its nosecone that connects to a male LIDS connector at the interface pad. Thus fuels can flow into or out of any vessel docked at a berth depending on which valves are turned on or off and provided that pressure is applied to the fuel to prevent isolated “globs” from forming.

Six RCS thrusters units have been placed at strategic points on the frame. Each unit consists of a Methalox combustion chamber with three nozzles pointing clockwise, counter-clockwise, and “up,” the z-axis toward the viewer. LOX and CH4 are fed to the units through a branch line from each propellant plumbing system. The directions of thrust are fixed (no gimbals), but the rate of fuel flow to the chamber will control the amount of thrust.

To spin up the Depot, one pair of thrusters located on opposite sides of the frame will fire up. For example, the pair could numbers 2 and 5 units shown in the sketch. The Depot will spin on an imaginary axis in the z direction running through the central core. The spin may be either clockwise or counter clockwise, which is opposite to the direction of the nozzle thrusts. For fast spin-up all three pairs of thrusters could be employed.

Note that the RCS thrusters are located on the outermost points of the forward frame. This allows small thrusters to impart more leverage to spin up the Depot.

As the whole depot is spun up, the propellants in each tank will migrate toward its outward sidewall. Each tank will be canted inward at the aft end by 2 degrees toward the center of the Depot, so that fuel will slide along the outer wall toward the forward frame. Please recall that each MCT will be held in its berth by a pair of latches, one near the forward end and one near the aft end, To achieve the cant, the rear latch will hold the MCT slightly toward the center and the interface pad will be tilted slightly downward. No other change in design will be required.

The interface pad is not located at the outermost wall of the fuel tank. Thus a discharge pipe will be required inside the tank that picks up the fuel at the outer sidewall and follows the curve of the end-cap to the interface pad. This will allow pressure on the fuel to move it through the interface pad to the Depot plumbing system as it is pumped out.

To accelerate the Depot linearly, the pairs of thrusters will be fired in the z-axis to push the Depot away from the viewer. The principal purpose will be to move the Depot to a higher or lower orbit or to steer it away from debris. The Depot can be steered by letting one of the pair of thrusters to apply more force than the other one.
Mars Pioneers will require our continued support.

Offline Burninate

  • Full Member
  • ****
  • Posts: 1130
  • Liked: 346
  • Likes Given: 72
Re: A Framework for the MCT Propellant Depot
« Reply #56 on: 09/09/2015 06:31 AM »
Oh, another note:
How are you going to handle LOX vs CH4?

You could go with common sidewall tanks, but they're fairly close to needing a 2:1 volume ratio at Musk's specified 3.8:1 mass ratio, which needs one less sidewall if you're starting out with a number of tanks divisible by 3.  Six tanks gives you this, and gives you a balanced center of mass.
Seems I was using the wrong estimate.

Correction:
Musk said O:F mass ratio was 3.8:1
Density of liquid methane is 422.4kg/m3
Density of liquid oxygen is 1141kg/m3
Therefore -
A measurement unit of 1 ton methane will require 1000/422.4 = 2.367m3
A measurement unit of 3.8 tons oxygen will require 3800/1141 = 3.333m3
2.367+3.333 = 5.700m3 total for the measurement unit of 4.8 tons total
2.367/5.700 = 41.53% methane by volume
3.333/5.700 = 58.47% oxygen by volume
Density of methalox combination: 4800/5.700 = 842.1kg/m3
« Last Edit: 09/09/2015 06:31 AM by Burninate »

Offline Ionmars

  • Full Member
  • ****
  • Posts: 791
  • North Carolina, USA
  • Liked: 224
  • Likes Given: 498
Re: A Framework for the MCT Propellant Depot
« Reply #57 on: 09/09/2015 11:45 AM »
...
...
...
Seems I was using the wrong estimate.

Correction:
Musk said O:F mass ratio was 3.8:1
Density of liquid methane is 422.4kg/m3
Density of liquid oxygen is 1141kg/m3
Therefore -
A measurement unit of 1 ton methane will require 1000/422.4 = 2.367m3
A measurement unit of 3.8 tons oxygen will require 3800/1141 = 3.333m3
2.367+3.333 = 5.700m3 total for the measurement unit of 4.8 tons total
2.367/5.700 = 41.53% methane by volume
3.333/5.700 = 58.47% oxygen by volume
Density of methalox combination: 4800/5.700 = 842.1kg/m3
Burninate, I see you will be analyzing the mass imbalances of the Depot using the appropriate data.

We have introduced a method for spinning the Depot to place a force on the fuel while pumping it out of a tank. However, the various MCTs docked at the Depot will have propellant tanks in various conditions of fullness, from empty to full and states in between. Furthermore, vehicles will be coming and going frequently, thus changing the mass balance of the Depot profoundly while doing so. Spinning the station will inevitably introduce wobbles in the deportment of the Depot.

What can be done about Depot wobble, if anything? As usual I will refer to an Earth analog.

Dynamic On-the-Car Spin Balancing (from Autos.com)

“This is the most accurate, yet most dangerous method of tire balancing. The tire and wheel are left on the car, and a ‘hat’ is placed on the wheel and secured. A motor with a wheel is placed against the tire, causing it to spin. Balance mechanisms contained within the hat determine where the unbalanced condition is occurring. There are six friction rings on a snout protruding from the hat. These friction rings are held lightly by the technician, while he rests a hand against the fender, moving weights inside the hat. When the most ideal combination of inner and outer weights is achieved, perceived vibrations will be minimized. The technician then reads the display, which reads graphically, and applies the required weights in the specified positions. He will then usually spin the tire up once again in order to verify his work. If the proper weights and positioning were used, the least amount of vibrations will occur with both inner and outer indicators reading zero or close to it. This type of balancing is highly accurate because the tire and wheel are spun on the car, allowing any vehicle specific issues to be accounted for, such as an off center lug pattern or hub.”

The point is that a force is applied at key location(s) on the rim of the wheel in the form of small weights to counterbalance the wobble. In the case of the MCT Propellant Depot, could the RCS thrusters, using variable thrusts in variable directions instead of weights, counterbalance a wobble in the spinning Depot? 

« Last Edit: 09/09/2015 01:11 PM by Ionmars »
Mars Pioneers will require our continued support.

Offline Burninate

  • Full Member
  • ****
  • Posts: 1130
  • Liked: 346
  • Likes Given: 72
Re: A Framework for the MCT Propellant Depot
« Reply #58 on: 09/09/2015 01:43 PM »
...
...
...
Seems I was using the wrong estimate.

Correction:
Musk said O:F mass ratio was 3.8:1
Density of liquid methane is 422.4kg/m3
Density of liquid oxygen is 1141kg/m3
Therefore -
A measurement unit of 1 ton methane will require 1000/422.4 = 2.367m3
A measurement unit of 3.8 tons oxygen will require 3800/1141 = 3.333m3
2.367+3.333 = 5.700m3 total for the measurement unit of 4.8 tons total
2.367/5.700 = 41.53% methane by volume
3.333/5.700 = 58.47% oxygen by volume
Density of methalox combination: 4800/5.700 = 842.1kg/m3
Burninate, I see you will be analyzing the mass imbalances of the Depot using the appropriate data.

We have introduced a method for spinning the Depot to place a force on the fuel while pumping it out of a tank. However, the various MCTs docked at the Depot will have propellant tanks in various conditions of fullness, from empty to full and states in between. Furthermore, vehicles will be coming and going frequently, thus changing the mass balance of the Depot profoundly while doing so. Spinning the station will inevitably introduce wobbles in the deportment of the Depot.

What can be done about Depot wobble, if anything? As usual I will refer to an Earth analog.

Dynamic On-the-Car Spin Balancing (from Autos.com)

“This is the most accurate, yet most dangerous method of tire balancing. The tire and wheel are left on the car, and a ‘hat’ is placed on the wheel and secured. A motor with a wheel is placed against the tire, causing it to spin. Balance mechanisms contained within the hat determine where the unbalanced condition is occurring. There are six friction rings on a snout protruding from the hat. These friction rings are held lightly by the technician, while he rests a hand against the fender, moving weights inside the hat. When the most ideal combination of inner and outer weights is achieved, perceived vibrations will be minimized. The technician then reads the display, which reads graphically, and applies the required weights in the specified positions. He will then usually spin the tire up once again in order to verify his work. If the proper weights and positioning were used, the least amount of vibrations will occur with both inner and outer indicators reading zero or close to it. This type of balancing is highly accurate because the tire and wheel are spun on the car, allowing any vehicle specific issues to be accounted for, such as an off center lug pattern or hub.”

The point is that a force is applied at key location(s) on the rim of the wheel in the form of small weights to counterbalance the wobble. In the case of the MCT Propellant Depot, could the RCS thrusters, using variable thrusts in variable directions instead of weights, counterbalance a wobble in the spinning Depot?
Disclaimer: I am not an automotive engineer

A wobble is problematic on a wheel largely because it exerts dynamic sinewave forces against the axle and against the wheel, and there's already literally tons of pressure on that joint in a vehicle, with additional peaking pressure during bumps.  Dynamic forces from a wobble in the vertical axis scale as load * speed, and they will tend to affect only one wheel out of a matched pair, twisting the axle (mitigated by the differential, but additional high-freq diff motion is its own shade of bad news). The only thing you can put against that is the spring suspension, which is already under high tension, and have to be stiff enough to hold up a vehicle.  Lastly, the wobble load isn't only in the direction that the springs were designed to absorb motion in: it will push horizontally as well as vertically.  This puts the materials into a situation where they're constantly flexing (with good modern tolerances) or even striking (with early loose tolerances) one another, and these materials have to survive over something on the order of 100 million cycles of load.

But there is no axle on a spacecraft to push and pull against, so long as there are no nonrotating elements.  As such, unless I'm mistaken, nowhere in a vehicle undergoing an off-axis spin should see a sinewave force: it should be constant under the whole rotation, except during spin-up and spin-down events that induce chaotic semiperiodic slosh on top of a slow increase or decrease, or pumping events that induce a slow increase or decrease.

To answer your question more directly:
I think any significant wobble that could be corrected by the RCS would require inordinately high amounts of propellant.  I think expecting the RCS to spin up and spin down a depot every docking event is reasonable up to a certain N of launched tanks, but if it has to thrust linearly through a long pumping operation, it will also require unworkable amounts of propellant.  Spinning the vehicle up to 1G is not the issue: Some tiny fraction of a G plus some time should be enough to settle out bubbles in the liquid propellant.  It would likely require a lot of precision RCS work and some incremental propellant to dock a tanker with a pre-rotating depot and do away with spin-up and spin-down, but that becomes more practical above some eventual number N of fuel tanks.

So long as there are propellant tanks that can be plumbed to the tanker and to each other arbitrarily, on opposite sides of the center of mass (the CoM will shift towards the tanker at any docking), then propellant can be onloaded or offloaded from the depot.
« Last Edit: 09/09/2015 02:00 PM by Burninate »

Offline Ionmars

  • Full Member
  • ****
  • Posts: 791
  • North Carolina, USA
  • Liked: 224
  • Likes Given: 498
Re: A Framework for the MCT Propellant Depot
« Reply #59 on: 09/09/2015 02:48 PM »
Burninate, I think what you are telling me, AIUI, is good news. We should reserve any linear acceleration to those situations where it is unavoidable but otherwise spin the Depot during normal pumping operations.

I would request a calculation from your capable mind. What is the minimum rate of rotation (rpm) of the Depot to produce a force (g) that will drive each of the two propellants to the outer tank wall over time (t), presumably a reasonable period of pumping.

I apologize for making this simple minded request. Like a savant, I may understand some things but still have a mental block when running through equations to get the right answer. It is why I am here rather than woking for SpaceX.
« Last Edit: 09/09/2015 02:55 PM by Ionmars »
Mars Pioneers will require our continued support.

Tags: