Author Topic: A Framework for the MCT Propellant Depot  (Read 55996 times)

Offline Ionmars

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A Framework for the MCT Propellant Depot
« on: 08/23/2015 06:19 pm »
A Framework for the MCT Propellant Depot

In the previous thread called “MCT and the Six-Shooter Depot,” I developed the idea that an efficient design for a LEO propellant depot could consist of six MCTs docked in a circle in close proximity and lying parallel to one another. Each vehicle will be a tanker MCT delivering LOX and LCH4 to the Depot, a tanker serving as a depot propellant tank, or a mission-oriented MCT to be fuelled up and sent to a BEO destination. The nominal appearance of this arrangement was humorously likened to the cylinder of a Colt .45 “six-shooter.”

The six vehicles will be latched to a hollow framework with no pressure vessels for humans. Robotic arms remotely controlled from Earth will perform most operations. The central cavity will become the robotic service area.

A plumbing system for each propellant will tie together all MCTs, both delivering and receiving propellants. A propellant pump, a cryocooler for boil-off, and an overpressure chamber will serve all vehicles.

In the following 20 posts I will spell out in more detail the framework elements of a MCT Propellant Depot.
« Last Edit: 09/06/2015 05:32 pm by Chris Bergin »

Offline Ionmars

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Re: A Framework for the MCT Propellantt Depot
« Reply #1 on: 08/23/2015 06:21 pm »
Advantages of the Six-Shooter Design

As pried out of me by sdsds in the previous thread, the Six-Shooter Depot will feature the following advantages:

A. The arrangement will enable the following routine depot functions in an efficient manner:

(1) Each vehicle will receive the service of a cryocooler and boil-off control so it doesn't need to bring its own.
(2) Each vehicle will have the option of contributing propellant to a common storage tank at the Depot for later use, rather than always delivering directly to a destination-bound vehicle. This will provide flexibility in planning missions.
(3) "Extra" propellant stored at the Depot can be used to "top off" a nearly full tank, thereby saving an extra trip from Earth.
(4) Each vehicle will receive the service of an overpressure chamber during a propellant transfer.
(5) The Depot will also provide electrical service to each vehicle through the interface pad.

B. The six MCTs lying in parallel around a central cavity means that all berths will lie next to the robotic services area.   The robotic arms can move quickly and efficiently between the various vehicles to service them. These arms can grab and dock an MCT. They can push a fully loaded vehicle away from the Depot before its engines are fired up. They can replace repair parts on the Depot and on MCTs, if required.

C. The six MCTs lying in close proximity around a central core enables an efficient design for solar radiation protection. With a moderate increase in the size of a protective shell that would serve just one vehicle, six vehicles can be provided the same protection under one protective shell. Some reviewers of this proposal have questioned whether extra radiation protection would be required because CH4 has a higher boiling point than H2, the propellant previously considered to require a depot for long-term storage. CH4 will be easier to handle.

 D. A prominent aspect of the depot design is that it minimizes development cost. No new pressure vessels will be required for propellants because MCT tanks themselves will serve as the depot tanks. For the MCTs that provide their tanks to the Depot for long-term service, their propulsion units will be returned to Earth, if the final design of the MCT allows it.  No new pressure vessels will be required for humans because robots will conduct depot operations. If humans must attend to a special problem at the Depot, their visit will be short-term and their temporary habitat will be the Dragon capsule that brought them.

Edited: phrasing
« Last Edit: 09/01/2015 11:39 pm by Ionmars »

Offline Ionmars

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Re: A Framework for the MCT Propellantt Depot
« Reply #2 on: 08/23/2015 06:26 pm »
Giving birth to one berth

In this article I will specify in more detail the framework of the depot. I will concentrate on just one berth with the understanding that this pattern will be repeated six times to complete the Depot.

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Re: A Framework for the MCT Propellantt Depot
« Reply #3 on: 08/23/2015 06:28 pm »
Principles of construction

Following a study of the structure of he Depot, certain design principles were chosen as follows:

1) The general appearance of the structure will be circular as described in the first thread, but straight struts will be employed for all the framework elements. Thus, the outer edges of the metal framework will form a hexagon rather than a circle as previously conceived. The outer protective shell, when employed, will be cylindrical in shape.

2) The framework elements will consist of a network of large (12 -inch) box tubes composed of Ti-Al alloy or an optional material. Some other choices could be steel I-beams, round aluminum alloy tubes, or composites. This choice was based on good structural strength, moderate mass density, heat conductivity, four flat faces for joining flat metal struts and beams, and a method of joining parts that can be executed by remote-controlled robotic arms.

3) Seven, six, five, and four-way tube connectors will be used to join together the tubular struts and beams.

4) The plumbing and electrical pipes and their insulation will be located inside the hollow tubes for protection.
Edited: grammar
« Last Edit: 09/01/2015 11:46 pm by Ionmars »

Offline Ionmars

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Re: A Framework for the MCT Propellantt Depot
« Reply #4 on: 08/23/2015 06:29 pm »
Heat Radiation

The outer shell of the Depot will feature a light-reflecting outer layer to help prevent excessive solar heat gain. In addition, the box tube construction of struts and beams will also play a role in controlling heat. They will be composed of heat reflecting bright Ti-Al metal alloy, which will also have a desirable heat-conducting feature as compared to say, composites. Each beam will be perforated on two sides to allow heat to better radiate into space. Because the entire framework is composed of metal, it will conduct heat from the sunny side of the Depot to the shady side.

The whole depot should operate to minimize heat gain. One way to achieve this is to continually maintain one side of the Depot facing the  sun.The berths located on the sunny side are likely to require the reflective outer shell for sun protection, while berths on the shady side of the Depot might be left uncovered to better radiate heat into space.

Edited: phrasing
« Last Edit: 09/02/2015 12:07 am by Ionmars »

Offline Ionmars

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Re: A Framework for the MCT Propellantt Depot
« Reply #5 on: 08/23/2015 06:30 pm »
Box tube connectors

A common type of connector for hollow metal box tubes is composed of solid black nylon with or without a metal core. It can be designed to join tubes from multiple directions as shown below. To join connectors to tubes, one arm of a connector is pushed into one end of a tube, which produces a permanent joint without further welding or gluing. However, it cannot be employed where each end of a tube will be connected in a rigid framework because the framework would have to to be flexible enough to stretch and allow both ends of a tube to be inserted onto two connectors. A solid connector also blocks the otherwise hollow tubes so they cannot carry electrical cables or propellant plumbing lines. For these reasons, another type of connector will be employed.

Efited: grammar
« Last Edit: 09/02/2015 01:07 am by Ionmars »

Offline Ionmars

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Re: A Framework for the MCT Propellantt Depot
« Reply #6 on: 08/23/2015 06:32 pm »
Metal connectors for metal box tubes

Another type of connector is composed of square metal sleeves that slip around a square metal tube rather than inside it. Thus the whole framework is hollow and can carry electrical ad propellant lines. However, In order to work inside a rigid framework the method of joining must be modified.

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Re: A Framework for the MCT Propellantt Depot
« Reply #7 on: 08/23/2015 06:34 pm »
A lidded connector system

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. One end of a metal box tube tube can be placed into the connector sleeve horizontally, and a lid that is a little wider than the sleeve replaced over it. To make this procedure work the connectors will be redesigned for this purpose. A channel on each side of the fourth (lid) side of the connector sleeve will accommodate the lid, as shown in the sketch below.

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Re: A Framework for the MCT Propellantt Depot
« Reply #8 on: 08/23/2015 06:36 pm »
Inserting tube into connector

In the side view below a metal box tube is lowered “downward” into the metal connector. Holes drilled in the bottom of the tube exactly match a set of metal pegs in the bottom of the connector sleeve. When the holes and pegs are joined the tube is correctly aligned in the sleeve. Then the lid (not shown) is slid into the two channels at a slight incline to compress the metal faces together.

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Re: A Framework for the MCT Propellantt Depot
« Reply #9 on: 08/23/2015 06:37 pm »
Cold metal welding

An interesting phenomenon has been discovered in outer space. When two flat surfaces of identical material are pressed together in a vacuum the atoms at the interface of the two objects do not discern which object they “belong to” and act as one. In effect, the two objects become fused. This phenomenon has been called “cold metal welding.”

To ensure that cold welding takes place between the connectors and tubes, both will be composed of the identical metal alloy. Note that it is the bottom of the connector sleeve (the side with pegs) that is to fuse together with the end of the box tube. On Earth, such surfaces would not join together because of oils, films, dirt and a protective layer of metal oxide. To ensure the joining of surfaces in space, they will be abrasively cleaned with a diamond-faced rotary tool just before joining. With no atmospheric oxygen present, the oxide cannot re-form. Also, a high level of cleanliness will be maintained during launch preparations, launch, and in-space construction.

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Re: A Framework for the MCT Propellantt Depot
« Reply #10 on: 08/23/2015 06:39 pm »
Lessons from the ISS

One of the prominent features of the International Space Station is the 109-meter truss structure shown below. A track was built onto the truss as part of a mobile base system.  The large robotic arm (Canadarm) can attach itself to a mobile base mounted on the track and transport itself fairly quickly from one end to the other. Then it can unload itself onto a fixed base to begin operations. Should this track feature be employed at the Six-Shooter Depot? If so, it would be a circular track to travel around inside the circular cavity at the central core, which has a circumference of only 27 meters. This distance can be cut even shorter if the Depot robot could take a shortcut across the interior cavity, a distance of just 17 meters at longest.

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Re: A Framework for the MCT Propellantt Depot
« Reply #11 on: 08/23/2015 06:44 pm »
Point-to-point travel

An updated version of the ISS robotic arm featured a second means of locomotion. A device known as a Power Data Grapple Fixture (PDGF) was installed at strategic locations around the station. Using a Latching End Effector (LEE) the Canadarm2 can insert itself into one of these fixtures and immediately begin operating from that spot. The PDGF serves as a base with built-in electricity and data transfer capability. With multiple PSGFs the Canadarm2 can move from fixture to fixture like an inchworm.

This system of pre-located PDGFs will be employed at the MCT Propellant Depot. One PDGF will be positioned on each of the six saddle beams, which means the robotic arm will lie directly under a MCT when it is in berth. To move to another berth, the arm will fold up, swing through a slot in the framework until it is positioned underneath the saddle beam and inside the central cavity. Then it will stretch out to the desired berth and insert itself into the PDGF under that saddle. It will then fold up again, revolve upward through the slot for that berth and begin operations above that saddle beam. For example, the fully stretched Canadarm2 is 37.3 m long and could easily stretch from one fixture directly to any other.

The CDM2 will not be swinging freely like a spider monkey; however, because the 1600 kg mass of the robotic arm still require proceeding at an inchworm’s pace.

PDGF:

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Re: A Framework for the MCT Propellantt Depot
« Reply #12 on: 08/23/2015 06:47 pm »
The Geometry of a Berth

One berth will utilize one-sixth of an imaginary circle, as shown in the geometric end view below. It represents space allocated for the forward construction frame of one berth. The drawing exhibits the following elements:

Circle S1:  A circular safety zone with a diameter of 17 m and a center located at C. It touches the sides of the berth at tangent points E and H and at point B, the top edge of the saddle beam. A MCT with diameter 15 m (assumed) will dock at the berth with a safety margin of 1 m between it and points E, H and B. Variable-length latching devices will bridge these gaps and lock the MCT to the frames.
 
Point A: A point in space representing the geometric center of the Depot and the starting point for imaginary projection lines defining one berth.

Line ADEF and AGHI: Imaginary projection lines from point A defining the sides of the berth.

Angles DAG, EFC, and HIC: 60 degrees

Lines DE, EF, GH, and HI: Struts that comprise real, visible structural elements that lie directly on the projection lines.

Circle S2: The circular outline of one interface pad that contains the LIDS connections. The centers of both circles S1 and S2 are at point C.

Lines CF, CE, CB, CH, and CI: Struts that support the interface pad.

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Re: A Framework for the MCT Propellantt Depot
« Reply #13 on: 08/23/2015 06:48 pm »
Geometric points of Interest

In the preceding geometric diagram, the following are points of special interest:

Point B was described as the top edge of the saddle beam. This depot terminology is such that all struts shown in the frame lie on the X-Y plane of a 3-dimensional figure. Beams lie on the Z-axis and join together a series of frames. A direction pointing toward the origin A will be called “down” and if it points away from A it will be called “up.” If one of the berths appears on a drawing in “upside down” position we will mentally spin the Depot around like a six-shooter cylinder until the berth we want is on “top.”

Points F and I are the topmost structural points on the drawing because the whole outer shell is left out. If the extra protection of a shield is needed, the two-panel doors that would comprise the shell segment will be hinged at points F and I. Two quarter-circle door panels hinged at these points will fit neatly over the safety zone and encompass a vehicle docked inside the berth.

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Re: A Framework for the MCT Propellantt Depot
« Reply #14 on: 08/23/2015 06:50 pm »
Length of struts

In the geometric diagram of the forward frame of the depot the triangles ADB, AEC, AGB, AHC, HCI, and EFC are right triangles. Acute angles are 30 or 60 degrees. This reduces the calculation of strut lengths to a trigonometry problem. Here are the nominal lengths of struts and some projection lines, in meters. The lengths of connectors are included as part of the strut lengths.

CE, CB, and CH = 8.50
CF and CI = 9.82
EF and HI = 4.91
DE and GH = 7.32
DB and BG = 4.25
DH = 7.36
Projection lines: AB = 8.50; AF and AI = 19.63

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Re: A Framework for the MCT Propellantt Depot
« Reply #15 on: 08/23/2015 06:52 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:

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Re: A Framework for the MCT Propellantt Depot
« Reply #16 on: 08/23/2015 06:54 pm »
Depot framework

The overall appearance for one berth of the Depot framework is shown below. It consists of a parallel series of frames beginning with the forward frame, which contains the LIDS connection as well as the plumbing system and the electrical lines located inside the struts. . Behind the forward frame extend six beams of length 5.5 m, which are attached to at points D, E, F, G, H, and I and extend aft-ward. The saddle beam is attached at point B and is also 5.5 m long and pointing in the same direction.

The image below also shows cross-members on one side of the berth. These are flat metal braces attached to beams for triangular rigidity. (Braces and beams are not shown on the other side to maintain simplicity of the image.)

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Re: A Framework for the MCT Propellantt Depot
« Reply #17 on: 08/23/2015 06:55 pm »

A place for a robot

After the forward frame and attached beams, the next frame in sequence is a “middle” frame. It is the same as the forward frame except that the centrally located struts were removed to allow a vehicle to reside within in this space. From here, short 1-m beams will extend to the next frame, which is also a middle frame. This configuration provides a 1 m wide slot in the framework for the operation of a large robotic arm, as suggested in the image below. A robotic arm working above the saddle beam can fold itself up and pass through the slot to operate below the saddle.

The two frames forming the slot for a robotic arm are followed by another set of 5.5-m beams, which extend to another middle frame. This frame represents the midpoint between the forward and aft sections of the berth. These two sections provide the operating spaces for the two robotic arms that will maintain the depot.

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Re: A Framework for the MCT Propellantt Depot
« Reply #18 on: 08/23/2015 06:57 pm »
Attaching the aft end

The aft one-half of the berth is a repetition of the first one-half. The aft frame is similar to the forward frame except that there is no LIDS connection. In this version of the Depot the aft frame closes off the rear of the berth and a docking MCT cannot enter the berth from the rear. Thus the two robotic arms will be employed to grab and pull a MCT into its berth horizontally. This procedure will allow the movement of a docking MCT to be controlled by robots; it should lessen the hazard of a MCT hitting the forward frame too hard and causing damage.

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Re: A Framework for the MCT Propellantt Depot
« Reply #19 on: 08/23/2015 06:58 pm »
Summary

In this article the framework for the MCT Propellant Depot was described in more detail. The struts and beams for one berth were specified. Slots in the framework were provided for the movement of robotic arms between the berth above the saddle beam and the interior core below the saddle beam. When completed, the framework for six berths would be linked together into one in-space propellant depot.
« Last Edit: 08/24/2015 01:54 pm by Ionmars »

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