Much of the work in the Artemis Project has been devoted to two main areas of consideration: one is the initial Reference Mission to take the first habitat module to the moon; the other is long-term plans for large vacation cruises to the Luna City Hotel aboard a Future Earth-Moon Transportation System. While both of these areas certainly deserve a lot of effort, there must also be effort put forth to plan a path between the initial missions and the tourist cruises. This paper will endeavor to lay out the author's vision of the first part of that path.
After the initial Artemis lunar base has been established, there will be a need for routine missions to provide supplies and exchange crew for the base. A logical method for doing that would be to use an evolved lunar transfer vehicle (LTV) from the Reference Mission. This method would use a similar LTV command module, but replace the service module with a new one, specifically suited for flying all the way to the lunar surface and back. This new vehicle, dubbed the lunar resupply vehicle (LRV), has been designed to carry a crew of three and 2000 lb of supplies (food, water, textiles, small machinery/parts, etc.) from low-Earth orbit (LEO) to the lunar surface. It has been designed to return with three different people and a cargo of 200 lb of moon rocks (or anything else of interest).
Reusable launch vehicles (RLVs) have been chosen as the Earth-LEO launch vehicles primarily for cost reasons. Two 25,000-lb payload RLVs can be used to launch all of the required propellants, cargo, and crew. If RLV keeps to its development goal of costing $1,000/lb, the launch services for each of these lunar resupply missions would cost only $50,000,000. RLVs also hold great promise for supplying the Earth-LEO launch services to unmanned resupply missions.
This paper will now move on to more detailed discussions. First, a list of the major assumptions that underlie this mission will be presented. Then, a more detailed description of the resupply mission will be given. Next, an overview of the basic LRV design will be presented. Then, a look at the RLV payload packaging and additional requirements imposed on the RLV to perform this mission will be taken. Finally, some conclusions will be drawn.
The Resupply Mission has been designed to take advantage of a lunar liquid oxygen manufacturing capability. The mission uses a Pratt & Whitney RL-10A-3-3A liquid hydrogen (LH2) / LOX engine to provide all of its major burns. The propellants brought from Earth include all of the LH2 and maneuvering thruster propellants needed for the round trip, but only the LOX needed for the outbound trip. The LOX needed for the return trip is added to the LRV's service module on the surface of the moon. This allows for the total amount of propellants brought from Earth to be considerably smaller than it otherwise would be.
Since the Resupply Mission has been conceived as a monthly steady-state type of mission, there will likely be two LRVs carrying out the mission. One LRV will carry out the outbound trip at the same time the other LRV is on its return trip. Each will make one leg of the trip per month, to allow both trips to take advantage of the moon's optimal orbital positioning that occurs once per month. The outbound and return trips will now be discussed individually.
The outbound trip has been designed assuming a velocity budget similar to the Apollo 15 mission (which is also the velocity budget for the Reference Mission). The schedule of burns and maneuvers for such a mission is listed elsewhere in the Artemis Data Book, and will not be repeated here. Using this velocity budget and the RL-10A-3-3A's specific impulse of 444.4 sec, the outbound trip will have an initial to final mass ratio of approximately 3.98.
The outbound trip (the trip from Earth to the moon) begins with the launch of two RLVs into LEO. Once in orbit, each of the RLVs will proceed to rendezvous and dock with a LEO staging platform, where an empty LRV is already docked. After docking is complete, the crew members will use the platform's robotic arm(s) to transfer full cryogenic propellant tanks from the RLV cargo bays to the LRV service module. The outbound tanks will then be connected to the propellant feed lines. Once the tank connections have been verified, the crew will transfer its supplies to the LRV and await the proper time for departure.
As the LRV approaches its landing site on the moon, it will be guided by some type of homing beacon, presumably similar to those that exist in airports today. The landing site itself will be located a short distance away from the main lunar base site, to protect the base from any dust and debris kicked up by the LRV's engine. After the LRV has landed, the crew will exit the vehicle, and its cargo will be unloaded. Then, each of the outbound propellant tanks will be disconnected from the vehicle and taken to a storage area, where they will be drained of any residual propellants. (Some type of gantry/forklift/crane system will probably be needed at the lunar landing site to facilitate crew egress/ingress, cargo handling, and propellant tank handling.) The last act of the outbound trip will be to secure the return LH2 tanks for their three-week wait on the lunar surface.
Like the outbound trip, the return trip has been designed assuming a velocity budget similar to that of Apollo 15. With an RL-10A-3-3A engine, the return trip will have an initial-to-final mass ratio of approximately 4.02. The departure of the return trip will be timed to allow for an easy rendezvous with the LEO staging platform when the LRV arrives in LEO.
The return trip begins with the tanking of the LOX needed for the return trip. Two of the outbound LOX tanks will be refilled with oxygen manufactured at the lunar base. The other two will remain at the lunar base. After the LOX tanks are filled, they will be installed in the LRV and the return LH2 tanks will be connected and checked out. The three returning crew members will then board the LRV, taking with them up to 200 lb of lunar samples or other items to be returned to Earth.
After the LRV has successfully docked with the staging platform, the returning crew members will transfer their return cargo to one of the RLVs docked at the platform. They will then disconnect the return propellant tanks and transfer them to the other RLV. The crew will prepare the LRV for its stay in LEO, then board one of the RLVs. Both RLVs will then return to the surface of the Earth.
Top and side views of the LRV are shown in the following two figures.
The LRV command module is basically the same structure as the LTV command module: a single-length Spacehab module. This module is about 10 feet long, and is shaped like a cylinder 13.5 feet in diameter that has been sliced off on one side. This sliced-off side forms the top of the command module. The command module's weight has been estimated at 4,000 lb. The empty structural weight is about 2,500 lb, and the remaining 1,500 lb will be taken up with life-support, guidance, and communications equipment and the like. The astronauts riding in the LTV are assumed to weigh 275 lb, including their spacesuits.
The remainder of the LRV is basically a trusswork arrangement designed to hold the propellant tanks, engines, and supplies the vehicle is carrying. This trusswork has been designed in an open fashion so that it will be easy to install and remove propellant tanks. The tanks themselves are spherical, with stiffened points for attachment on either side and valves at the aft end. Propellant feed lines will run down the middle of the trusswork. At the aft end of the truss is a LOX/LH2 engine (assumed for now to be an RL-10A-3-3A), and a set of fixed landing struts.
The propellant tanks themselves deserve special mention. As seen in the LRV sketches above, there are three outbound LH2 tanks, three return LH2 tanks, and four LOX tanks. All of the tanks have been designed using aluminum with a yield strength of 70 ksi and a factor of safety of 1.5. The use of aluminum in the baseline configuration adds a good deal of conservatism to the design. The actual vehicle will almost certainly use a lighter, stiffer material such as aluminum-lithium, a graphite-epoxy composite, or titanium. This would result in considerable weight savings that could be applied to offset weight growth elsewhere on the vehicle.
The outbound LH2 tanks are each designed to carry about 2,000 lb of LH2 in a spherical volume of 475 cubic feet (an inner radius of about 4.85 ft). They have been designed to operate at 3 atmospheres of pressure, and are 0.028 inches thick. Each bare tank weighs about 119 lb, but 51 lb of additional weight for insulation, valves, baffles, and attachment stiffeners has been added. This increases the total weight to 170 lb/tank. The outbound tanks will be disconnected from the LRV after the completion of the outbound leg and left on the moon.
The return LH2 tanks are each designed to carry about 1,070 lb of LH2 in a spherical volume of 250 cubic feet (an inner radius of about 3.91 ft). They have also been designed to operate at 3 atmospheres of pressure, but this pressure may need to be increased to prevent excessive LH2 boiloff over its nearly month-long storage. The tanks are 0.023 inches thick. Each bare tank weighs about 63.6 lb, but 106.4 lb of additional weight for insulation, valves, baffles, attachment stiffeners, and a possible increase in operating pressure has been added. This increases the total weight to 170 lb/tank.
The LOX tanks will be used in two different configurations over the course of the trip. On the outbound leg, four tanks carrying 7,500 lb of LOX each will be used. On the return leg, two tanks carrying 8,000 lb of LOX each will suffice. The other two are disconnected and left on the moon. The LOX tanks each have a volume of 125 cubic feet (inner radius of 3.1 ft) and have been designed to operate at 55 psi of pressure, giving them a thickness of 0.022 inches. Each bare tanks weighs about 38.3 lb, but 41.7 lb of additional weight for baffles, insulation, valves, and attachment stiffeners has been added. This increases the total weight to 80 lb/tank.
A summary of the major LRV component and propellant masses for the outbound trip is shown in the following table:
Component Mass (lb) Number Total LRV Command Module 4,000 1 4,000 RL-10A-3-3A Engine 305 1 305 LRV Service Module 355 1 355 Misc Outbound Cargo 2,000 1 2,000 Astronauts 275 3 825 Outbound LH2 Tanks 170 3 510 Outbound LH2 2,000/tank 3 6,000 Return LH2 Tanks 170 3 510 Return LH2 1,070/tank 3 3,210 Outbound LOX Tanks 80 4 320 Outbound LOX 7,500/tank 4 30,000
Total outbound mass is 48,085 lb at LEO departure and 12,085 lb at lunar arrival.
A summary of the return trip component and propellant masses is shown in the table below:
Component Mass (lb) Number Total LRV Command Module 4,000 1 4,000 RL-10A-3-3A Engine 305 1 305 LRV Service Module 355 1 355 Misc Return Cargo 200 1 200 Astronauts 275 3 825 Return LH2 Tanks 170 3 510 Return LH2 1,070/tank 3 3,210 Return LOX Tanks 80 2 160 Return LOX 8,000/tank 4 16,000
Total return trip mass is 25,565 lb at lunar departure and 6,355 lb at LEO arrival.
Two RLV launches are needed to provide all of the propellant, cargo, and crew for this mission. RLV # 1 is unmanned and functions solely as a propellant carrier. It contains almost all of the LH2 and half of the LOX. Specifically, RLV # 1 contains:
The total payload weight in RLV # 1 is 24,150 lb. A preliminary arrangement of the cargo in the payload bay of RLV #1 is shown below.
The overlapping tank planforms shown above can be accommodated by staging the tanks at different heights in the cargo bay.
RLV #2 serves as the personnel and cargo carrier. It also contains half of the LOX and one return LH2 tank. Specifically, RLV #2 contains:
The total payload weight in RLV #2 is 23,650 lb. A preliminary arrangement of the cargo in payload bay of RLV #2 is shown below.
In addition to meeting the payload mass requirements that have been laid out in the paragraphs above, the RLVs will have to meet a number of other requirements to perform this mission:
A scenario for a Lunar Resupply Mission has been presented. This scenario uses 2 RLVs to provide all of the necessary propellants, cargo, and crew for the mission. A preliminary look at this scenario has touched on: