Author Topic: Assembling the MCT Propellant Depot (Read 9113 times)

Ionmars · « **on:** 10/21/2015 10:48 am »

If SpaceX successfully develops the Mars Colonial Transporter, then the colonization of Mars will be close at hand. When the MCT comes out of development and into production, the number of vehicles will accelerate. Many MCT missions will involve propellant transfers in LEO. For the first few missions these transfers can be carried out employing a small depot or even no depot. But once the number of vehicles rises to about 6 or 12, then the MCT Propellant Depot, or one like it, will be required.

In this article, I will follow up two previous articles on this subject to show how the depot will be assembled. See the following 20 posts in sequence:

Ionmars · « **Reply #1 on:** 10/21/2015 10:51 am »

Topic one

In the first article 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 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 operate the depot. 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 control, and an overpressure chamber will serve all vehicles.

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

Ionmars · « **Reply #2 on:** 10/21/2015 10:57 am »

Topic two

In the second article regarding the MCT Propellant Depot, the framework for the Depot was described in more detail. The arrangement of struts and beams for one berth was specified. Slots in the framework provided for the movement of robotic arms through the interior core below the berths. When completed, the framework for six berths will be fused together into one in-space depot.

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

Ionmars · « **Reply #3 on:** 10/21/2015 11:01 am »

Robotics 101 learned at the ISS

Based on our experience with the Space Shuttle and the ISS we know that humans can assemble engineering works in space, but keeping human beings alive in space is expensive. Fortunately we have also learned that remote-controlled robots can carry out many tasks that humans were previously required to perform.

In the image below we see Canadarm1 from the Shuttle handing off the P-5 truss section to Canadarm2, which was located on the partially constructed ISS.

Ionmars · « **Reply #4 on:** 10/21/2015 11:03 am »

Introduction to Robotics 201

To build the MCT Depot in LEO we will employ robots with even better capabilities. Two robotic arms with specialized end effectors will be carried to LEO in a cargo MCT along with building materials and tools. Operators on Earth will remotely control these robotic arms to assemble the entire Depot from the open hatch of the cargo bay, the “front porch”, as indicated below These arms will be employed to construct the Depot and then operate the Depot when construction is completed

Ionmars · « **Reply #5 on:** 10/21/2015 11:04 am »

An automated warehouse

Once the MCT arrives at its LEO destination, the cargo bay doors will open and the bay will become an automated warehouse. I expect a cargo handling system featuring a robotic arm (MCT arm) to be installed in the ceiling of the cargo bay of each cargo MCT. This system will handle the struts and beams for this project.

Automated warehouse on Earth

Ionmars · « **Reply #6 on:** 10/21/2015 11:06 am »

Warehouse End effector

For warehouse operations where the box tubes are stacked end upwards, a specialized end effector, the box tube end gripper (BTEG), will be an extensor for the robotic arm. This device will grab a tube by placing the gripper inside the end of one tube and applying pressure outward, similar to the operation of a brake shoe inside an automobile wheel. The BTEG could be made-to-order by a company that produces custom end effectors, such as Positech Corporation

Ionmars · « **Reply #7 on:** 10/21/2015 11:08 am »

Packing the parts (1)

Besides using robots, other techniques will also be employed to reduce costs. One cost-cutting strategy will be to reduce the number of flights of the MCT required to deliver building materials for construction. Considering that 41 flights of the Space Shuttle were required to construct the ISS, there is considerable leeway for building a large structure with fewer flights.

One approach to reducing the number of launches is to bundle materials compactly. The MCT will have considerable mass-lifting capacity but the volume that can be lifted in one trip may be constrained. The square box tubes to be employed as the main construction elements can be bundled compactly. We will not assemble large sections on Earth as was done to construct the ISS. In that case whole truss sections were built on Earth. Rather, the individual struts and beams will be sent to LEO and assembled there. Assembly in LEO can be accomplished because the robots of 2025 will be more capable than the robots of1985.

To appreciate the compactness of handling individual parts, compare the volume of the middle frame in the sketch below with the volume of the box tubes and connectors that comprise the frame. Assembled, this one hollow frame occupies 168 m^3, while the individual tubes and connectors stacked as individual components comprise just 22.4 m^3.

Ionmars · « **Reply #8 on:** 10/21/2015 11:09 am »

Packing the parts (2)

Expert sources are currently estimating that the cargo volume of the MCT may be about 1000 m^3 with an outer mold line that is capsule shaped like an enlarged Dragon 2. The maximum interior height in the middle of the bay may be 12 m. Additionally, my personal expectation is that the cargo hatch doors will be large, perhaps one third of one side of the cargo section. Thus the plan will be to stack the tubes, connectors and other components vertically, as suggested in the image below, and to exploit the area at the hatch opening as a working platform for construction. This location may be called the “front porch.”

Ionmars · « **Reply #9 on:** 10/21/2015 11:10 am »

Packing the parts (3)

To create an efficient arrangement for packing the cargo MCT, consider the following assumptions:

(1) The longest struts (9.8 m) will fit into the cargo bay with one end pointed “vertically” toward the interior of the nose cone. Lesser length tubes will be stacked around the longest tubes that are stowed vertically in the middle of the bay.

(2) A remote-controlled cargo handling system will be built into the ceiling of the cargo bay. A robotic arm extensor from that system will grasp the top end of one tube at a time and deliver it to the front porch as needed.

(3) The tubes will be arranged so that each length that is required at a certain time in the construction sequence will be available to the handling system at that time. The headroom in the cargo bay will allow the tubes to be raised above the floor. The cargo handler will translate each box tube to the front porch while held in a vertical position.

Ionmars · « **Reply #10 on:** 10/21/2015 11:11 am »

Packing the parts (4)

Given the above assumptions, the following sketch shows how a portion of the cargo bay of the MCT could be packed, including two large robotic arms and all the materials required to build one berth. This packing plan assumes that the maximum interior diameter of the bay is 14 m and that the height in the middle is 12 m; this allows headroom for the 9.8 m long tubes to be stored vertically with extra 2.2 m overhead for the the cargo handling system.

The first allocation of space is given to the two robotic arms. The floor area allowance is 12.6 m^2. The next allocation is a total of 116.6 m^2 for the metal connectors, metal braces, miscellaneous materials, and contingency space for assembling one berth.

In this packing plan a floor space 2 m wide by 6.3 m long is allocated for two large, folded, robot arms. The dimensions of Canadarm 2 were used for planning purposes, although the arms required for the Depot could be smaller. The longest (forearm) segment is 5.8 m long with a (round) tube diameter of 33 cm. Additional space is provided in the plan for the extra widths and lengths of joints. It is assumed that the multi-jointed arms can be folded into a compact rectangular box shape.

Ionmars · « **Reply #11 on:** 10/21/2015 11:12 am »

Packing the parts (5)

The robotic arms extend less than 7 m into the cargo bay, which is only halfway into this large space. The longest struts, coded 98 and 85 in the sketch, are stacked vertically near the end of the robotic arms to take advantage of the high middle ceiling. The shorter struts and beams are grouped around the long struts, as shown in the sketch.

The floor space allocated for robotic arms and the materials to build one berth is 29.2 m^2, whereas the 14 m interior diameter cargo bay area provides 154 m^2. The extra space is sufficient for packing the materials for 7.5 more berths. This means that the materials for the whole depot (disregarding mass) could fit inside the cargo bay of one MCT, even when packed loosely as in this plan.

Note that two Power Data Gripper Fixtures (PDGFs) are shown in the packing plan. They are installed in the bay floor at the door hatch side of the cargo bay. The two robotic arms are stowed pre-attached to their respective PDGFs.

Ionmars · « **Reply #12 on:** 10/21/2015 11:12 am »

Robots in action

To begin unpacking and assembling parts, the first arm (CA1) will rise from its folded position while still attached to its PDGF and remain vertical in its position on the front porch. During construction CA1 will place beams and struts into position as needed. CA2 will also rise from its folded position and assume its working post at the front porch.

Moving both robotic arms out of the way will open up an aisle down the middle of the bay. The MCT cargo arm will search for the first beam and then employ a box tube end gripper (BTEG) to grab it, raise it and carry it down the aisle to the front porch. This first beam will be the saddle beam for the 4-way connector at point B as shown in the previous diagram of the forward frame (in article 2).

Ionmars · « **Reply #13 on:** 10/21/2015 11:14 am »

Exterior box tube grippers

Once a tube is moved to the front porch, it will be passed over to one of the robotic arms operating from that location. These two arms will require end effectors that grip the tubes on the outside of the tube rather than from inside, because it will grab the tube in the middle rather than from the end. The arm will then move the tube to the area outside the MCT to place it into the appropriate location in 3-D space. The exterior box tube gripper could operate like the parallel gripper shown below, except that the area of contact will need to be wider to enhance the grip.

In addition to the gripper end effector, the robotic arm will sport remote controlled live TV eyes mounted around the arm and work lamps to aid the controllers back on Earth.

The earth-bound human controllers pf robotic arms working in outer space will lack frames of reference. It will be different from working on a factory floor where robot positions are well known and actions can be pre-programmed. When grabbing a tube that is already outside the MCT, the angle of the gripper’s contact surfaces may not be able to accurately match the angle of the flat surface of a box tube, which may lie in various positions. The angle of contact between the gripper and the box tube may vary, say, from 0 to 10 degrees in different directions. One solution may be to give the gripper “soft hands” that can adapt to various angles of contact. For example a flexible air bag at low pressure could be attached to the contact surfaces of each gripper. The pressure of contact would press the bag to adapt to the different surface angles.

Ionmars · « **Reply #14 on:** 10/21/2015 11:16 am »

Box tube connectors

The box tube connectors will be simply designed but specialized components for this project. The design was introduced in the prior article with a sketch and an explanation. The sketch is shown below.

Ionmars · « **Reply #15 on:** 10/21/2015 11:18 am »

Effectors for connectors

The parallel gripper end effector will be employed by each of the robotic arms. The first arm will employ a gripper to hold the connector and the second one will grip the tube and place it into the connector. Then the first arm will release the gripper attachment and mount a drill effector onto the arm (or switch to an attachment at a different location on the same arm). The drill will employ a screwdriver bit rather than a drill bit in order to screw down the joiner plate bolts.

Ionmars · « **Reply #16 on:** 10/21/2015 11:19 am »

Frame by frame

To assemble the depot framework, the MCT cargo arm and the two long arms at the front porch will work in tandem to assemble the struts that comprise each frame. Together they will join the struts for one frame, and attach the beams that join the forward frame to the next one in sequence and install braces in the sidewall. In the sketch below we see the components of the forward frame emerging from the front porch.

Ionmars · « **Reply #17 on:** 10/21/2015 11:20 am »

Giving birth to one berth

One way to construct the Depot is to assemble one berth at a time, as shown below. To begin the second berth, one robot arm will detach itself from the MCT front porch and translate to the first robot arm base on the berth framework. The second arm will reposition itself onto the second arm base near the hatch. From here it can receive a strut from the MCT warehouse arm and pass it on to the first arm. This strut will be placed into one of the connectors on one side of the completed berth. Because each connector in the first berth is firmly positioned, another arm is not required to hold it in place. An additional strut can be placed into the connector with just one (two-handed) arm.

Ionmars · « **Reply #18 on:** 10/21/2015 11:23 am »

Or one-half at a time

A second approach to construct the depot is to build just one-half of each berth at a time, as shown below. The advantage is that both robot arms can be kept closer to the front porch. One arm will be positioned at the first arm base in the first berth located near the front porch. The second arm will be located on the arm base of the berth under construction. The first robot arm will receive a strut from the MCT warehouse arm, fold itself into a compact form including the strut, swing down through the slot in the framework to the space under the saddle beam, unfold itself, and pass the strut on to the second robot arm that is beneath its respective saddle beam. This approach will be particularly useful when building berths 4, 5, and 6, which will be located across the central cavity from the first berth.

The Falcon Heavy and a cargo Dragon capsule may be employed in an exploration program preceding the debut of the MCT. In that case it may be doubly desirable to build the forward one-half of the whole depot as indicated here. To accommodate the Dragon capsule only the forward sections would be built; the beam lengths and berth-to-MCT latches would be adjusted to the length and diameter of Dragon. Then the aft sections would be constructed when the MCT comes on line. Thus the depot could serve two vehicles and two phases of Mars development.

Ionmars · « **Reply #19 on:** 10/21/2015 11:24 am »

Model needed

A 3-D computer model is needed for the next step in developing this depot, but the author is unable to provide it. It will be especially needed when the detailed specs for MCT and Dragon are modified in the near future, and the depot must change to accommodate them. If you would like to create the model, send a PM to IonMars at this forum. You will be recognized and appreciated.