Author Topic: MCT and the Six-Shooter Propellant Depot (Read 31475 times)

Ionmars · « **on:** 07/30/2015 06:34 am »

[Preface: the following series of 26 posts present an article about the design of a propellants depot in low Earth orbit. This is a serious proposal, but presented with a touch of tongue-in-cheek humor.]

Behold the Colt 45. It was a single action repeating pistol introduced to the US Army in 1873 as a standard issue firearm.

Ionmars · « **Reply #1 on:** 07/30/2015 06:36 am »

Six-shooter cylinder

This pistol featured a cylinder where six 45-caliber bullets were inserted, ready to fire at the pull of a trigger. Could this be the Earth analogue for a LEO propellant depot? ☺ ☺

Ionmars · « **Reply #2 on:** 07/30/2015 06:41 am »

LOLIPROP Depot

Transport yourself from 1873 forward 160 years to 2033, when a new company called Low Orbit Logistics International (LOLI) has created a propellant depot in space (LOLIPROP Depot). ☺ The purpose of this LEO depot is to provide LOX and LCH4 service to the SpaceX Mars Colonial Transporter (MCT) as the principal visiting vehicle.

The Depot is comprised of a titanium-alloy hollow framework of six berths with “saddles” to accommodate up to six MCTs at one time. The visiting vehicles lie together in parallel positions in a circle. The image below reveals the business end of the six-shooter Depot. ☺

Ionmars · « **Reply #3 on:** 07/30/2015 06:43 am »

The .45 caliber MCT

Here is the long tanker version of MCT to be accommodated at the depot. The difference between this MCT and a .45 caliber bullet is that this one has a diameter of 15 meters, landing legs, and a docking adapter under the nose cone. ☺

Ionmars · « **Reply #4 on:** 07/30/2015 06:44 am »

The docking interface pad

Each berth at the LOLIPROP Depot features a single beam “saddle” and an interface pad located at its forward end. The pad is the metallic circular element in front of each of the six MCTs shown in the previous front-end view of the Depot. When an MCT is docking, it moves nose first along the saddle toward the interface pad to connect to a NASA Low Impact Docking System (LIDS). Once docked, the vehicle is locked in place to the saddle beam below it by mechanical latches and locked to structural elements on one side of the vehicle.

Each docking interface pad is held in position by additional framework at the forward end of each saddle; however, the latch mechanism used to hold the vehicle in its saddle can be adjusted to accommodate different diameters of vehicles. This means that the Depot can accommodate a range of vehicle sizes.

Note that each vehicle that visits the Depot must have automated rendezvous and docking (ARD) capability.

Ionmars · « **Reply #5 on:** 07/30/2015 06:46 am »

MCT in the saddle

The image below shows a side view of a MCT docked at a depot berth and connected to the interface pad. Note that it is latched to the berth’s saddle beam and the latches can be extended or contracted for different size vehicles.

In this design, a tanker MCT that is docked at one or more of the six berths serves as a storage tank for the Depot.

Ionmars · « **Reply #6 on:** 07/30/2015 06:48 am »

Standard NASA interface

The docking system of the Cryogenic Propellant Storage and Transfer (CRYOSTAT) mission, as proposed by Stephan Davis of NASA/MSFC in 2011, provided the model for the LOLIPROP depot’s six LIDS; however, the RCS, CMG wheels, and avionics shown below were not required for each pad, but were placed elsewhere in the depot. An electrical power interface was added to provide electrical service while a vehicle is docked at the Depot

Ionmars · « **Reply #7 on:** 07/30/2015 06:50 am »

Depot plumbing

The cryogenic fluids that flow through the fluid coupler at each docking pad are combined with fluids at all other pads via a common plumbing system. A remote controlled flow valve at each pad regulates the flow of fluids so that a MCT at any saddle can either deliver propellant fluid to the depot system or receive fluid from the system. The image below indicates one circular system, but one system is required for each of the two propellants that are handled (LOX and CH4).

Ionmars · « **Reply #8 on:** 07/30/2015 06:51 am »

Depot systems

In addition to pad-to-pad plumbing, the other major depot equipment includes MCT propellant storage tanks, a cooling unit (cryocooler) for re-liquefying boil-off gases, pumps for LOX and LCH4, an overpressure device to prevent gaps in the liquid stream during pumping, radiators for excess heat, insulation and sun shields to reduce boil-off, and a management and control unit.

Ionmars · « **Reply #9 on:** 07/30/2015 06:53 am »

Cryocooler

The image below shows a high-efficiency cryocooler developed by Northrup Grumman Space Technologies for the James Webb Space Telescope. A larger version was placed in a Dragon capsule and sent to the LOLIPROP Depot as a permanent fixture. The Dragon was docked at a berth and the cryocooler was connected to the fluids plumbing system for re-liquefying boil-off gases.

Ionmars · « **Reply #10 on:** 07/30/2015 06:54 am »

Robotic services

Note that a circular cavity lies at the center of the depot. This cavity provides the working space for two remote controlled mechanical arms. These robots operate from any one of a number of fixtures located along the “bottom” of each saddle. Canadarm 2 at the ISS provided the model for these robots. Working in tandem, they can move around inside the cavity by transferring from one fixture to another. They have the ability to move a MCT from one saddle to another. They can help to dock a MCT to its interface pad if the ARD system in the MCT is not working properly. They can construct sections of the Depot, repair certain parts of a MCT, and perform depot maintenance..

[It was rumored that when ISS was closed down in 2025, LOLI grabbed up the Canadarms at bargain prices and transferred them to the LOLIPROP Depot.] ☺

Ionmars · « **Reply #11 on:** 07/30/2015 06:56 am »

Management and control

As shown below, each end of the Depot features a conic shape projecting from the inner core, a feature that adds structural strength. The center of the fore end cone features a blunt end that contains a seventh interface pad, which provides a central location for a management and control unit. The unit consists of a Dragon capsule containing a computer control system for robotic motors, a cryogenic fluids control system (including boil-off control), and an electric power management system.

In addition to solar panels, a methalox internal combustion engine generates electricity during periods when a higher than average power level is required. The engine draws propellants from the Depot’s fluids plumbing system.

From this central location, the Dragon’s (methalox) thrusters provide guidance and altitude control for the Depot. These thrusters also draw propellants from the depot as required.

The depot systems are managed by remote control from Earth, but work through the on-depot computers. When humans are required at the Depot, their habitation module is docked at an eighth central interface pad located at the aft end of the Depot.

Ionmars · « **Reply #12 on:** 07/30/2015 06:58 am »

Air chamber versus overpressure chamber

A second Dragon was sent to the Depot with two fluid pumps (one for each plumbing system) and two overpressure chambers. The design of the chamber was inspired by the air chamber commonly installed in household water plumbing systems. The image below shows a vertical copper tube that traps a small column of air in the water plumbing system. When a nearby faucet is quickly turned off the sudden spike in pressure is transmitted to the air chamber where the compression of air dampens the impact and prevents a water hammer noise.

Ionmars · « **Reply #13 on:** 07/30/2015 06:59 am »

The Depot overpressure chamber

The principle of an air chamber was applied to the Depot to address two problems. First, it ameliorates a sudden spike in pressure that could damage the system. Second, it prevents gaps from forming in the flow of fluid, a phenomenon that can occur in outer space.

On Earth, when a fluid is accelerated through a pipe, some of the fluid may not be pulled into the stream, due to inertia, thus causing air bubbles. In space there is no air to form bubbles; thus the same acceleration will create gaps between fluid droplets. Instead of reducing flow volume the flow will stop altogether. To prevent gaps the fluid pressure must be high enough to maintain positive pressure while the accelerating liquid tends to reduce pressure to zero.

The overpressure chamber employed at the Depot employs propellant boil-off gas to exert pressure on fluid propellant that is being pumped. This device is shown in the sketch below. It features a a long tube that joins the main plumbing system at a right angle, similar to the air chamber shown above.

Liquefaction and gasification are controlled by controlling the temperature and pressure on two sides of a diaphragm located inside the chamber. On the fluid side the temperature is maintained below the boiling point of the propellant, while temperature on the gaseous side is maintained a few degrees above boiling. At equilibrium the pressures on each side are equal.

As indicated below, the diaphragm consists of a ball made of cryogenic sealant material. It forms a seal against the inside wall of the chamber that is tight enough to prevent almost all of the liquid from passing around the diaphragm, but loose enough to allow the diaphragm to move when significant pressure is applied from either side.

Ionmars · « **Reply #14 on:** 07/30/2015 07:01 am »

Space-saving chamber

To save space, the overpressure chamber was designed as a coil rather than a straight pipe. For better efficiency, it was coiled around the cryocooler.

Ionmars · « **Reply #15 on:** 07/30/2015 07:02 am »

Fluids transfer

The image below shows three saddles at the Depot that are occupied. One saddle holds the storage tanks of a MCT that serve as storage tanks for the Depot. The second saddle holds a tanker MCT that is delivering propellants to the Depot storage tank. A vehicle containing the cryocooler, overpressure chamber, and depot pump is docked at the third saddle..

To begin a propellant fluids transfer the propellant flow valves at the respective pads are opened and the depot pump is turned on. The pump has reversible flow so that it can direct flow toward the MCT to be filled and away from the MCT that is delivering fuel to the system. All other flow valves at interface pads remain off.

When the pump turns on the pressure throughout the plumbing system increases. The pressure inside the propellant delivery MCT tends to decrease, but still remains positive. The empty receiving tank has little or no pressure, which allows propellant to flow into it. When the transfer is complete the valves are closed, which may cause a spike in pressure in the plumbing system.

Ionmars · « **Reply #16 on:** 07/30/2015 07:03 am »

Overpressure during fluids transfer

When fluid pressure spikes in the overpressure chamber the force on the fluid side of the diaphragm exceeds the pressure on the gaseous side by a delta-p that is greater than the force of friction. Thus the diaphragm moves towards the gaseous side where the pressure is lower. This movement compresses the gas and increases its pressure and temperature until the spike in liquid pressure relaxes and the diaphragm rebounds in the reverse direction.

The rise in temperature on the gaseous side of the chamber needs to be ameliorated so that the temperature remains close to the propellant boiling point and the diaphragm settles near the center of the chamber. A manometer on the gaseous side of the diaphragm detects the pressure increase and switches on the cryocooler pump. The pump draws in higher-pressure, warmer gas from the chamber and pumps re-liquefied propellant back into it. This contact with “warmer” gas re-gasifies the incoming liquid, lowers the gas temperature and pressure on the gaseous side of the chamber, and pushes the diaphragm towards a new equilibrium. At a pre-defined midpoint the pump turns off and the diaphragm stabilizes.

One question may arise: How can fluid propellant be sucked out of a MCT delivering propellant to the depot without forming gaps? According to the CEO of LOLI the answer is this: “ Not my problem. Every MCT visiting LOLIPROP Depot must have the capability to maintain positive pressure on the propellant flowing into the depot without forming gaps in the fluid flow. Once we receive it, then it becomes our problem

Ionmars · « **Reply #17 on:** 07/30/2015 07:05 am »

Protecting the Depot

One of the functions of the LOLIPROP Depot layout is to provide insulation, reflectance, heat radiation and solar electric power for the depot and the vehicles docked within it. This is provided with a minimum of materials because the layout requires vehicles to be grouped in close proximity. In this design the entire group of vehicles is enclosed in one protective shell. The outer surface of the cylinder-shaped shell consists of reflective material to shield the Depot from the Sun’s radiation. It also protects the Depot from the lesser, reflected radiation from Earth.

Despite the reflecting surface, some heat penetrates to the inside of the shield. For this reason the inside surface features a blanket of multi-layer insulation (MLI) with 40 layers, a common thickness for space applications. These protections add to the shielding and insulation that is an integral part of each MCT that uses the Depot.

On some sections of the shell the reflective material is replaced by solar panels to generate power for the Depot.

Ionmars · « **Reply #18 on:** 07/30/2015 07:07 am »

Depot Doors

The protective outer shell of the Depot is subdivided into six double-panels located directly over each of the six saddles. The titanium alloy framework of the Depot extends outward from the central cavity; thereby creating a see-through partition that separates each saddle from its neighbors. Each double-panel is attached at the outermost edge of each partition on a set of hinges; these hinged panels form a double-door directly over each saddle space. Each double-door is opened and closed by remote control. A door is opened whenever a vehicle is approaching the Depot for docking and whenever a vehicle is exiting from the Depot. The doors are otherwise kept closed for protection, except when the doors are facing away from the Sun and Earth. In this case the open doors allow excess heat to radiate away into space. This procedure is similar to the operation of bay doors on the Space Shuttle, as suggested below:

Ionmars · « **Reply #19 on:** 07/30/2015 07:09 am »

What architectures are supported by the LOLIPROP Depot?

This Depot supports a number of different vehicles and flight architectures. Once it is docked at an interface pad, each vehicle is locked into place by a latch device that varies in length according to the diameter of the vehicle. Thus each berth can accommodate a Dragon, Orion, or Zvesda capsule as an alternative to MCT.

The fill-up process for a mission entails one or more tanker trips as required to complete the fill-up for the MCT that will leave for the final destination. As indicated below, three additional saddles could potentially harbor supply tankers simultaneously. Such a launch campaign could fill up the departing vehicle more quickly to avoid excessive boil-off. After receiving propellants, the mission vehicle will be ready for trans-orbital injection to GEO, L1, L2, the Moon, or Mars.