17-07-2013, 03:02 PM
RFI Focus Area: Crosscutting Design Drivers and Architecture Elements
Crosscutting Design.pdf (Size: 304.53 KB / Downloads: 93)
Relevance to RFI
In order to be adaptable, the CEV needs to have flexibility to use pre-positioned propellant tanks or drop
tanks, and these should be low-pressure tanks to avoid the mass and complexity of high-pressure systems.
The CEV needs to be safe, and low-pressure propellant systems are safer. This drives the system design
toward a higher performance pumped propulsion system. CEV systems need to be scalable and the
pistonless pump is scalable. CEV components need to be sustainable, and the pistonless pump is a robust
design that could be built by a wide range of manufacturers using standard machining and welding
techniques. The CEV needs to be affordable, and the pistonless pump is inexpensive.
Pump description
The pistonless pump is similar to a pressure fed system, but instead of having the a main tank at high
pressure (typically 300-500 psi) the proposed pump system has a low pressure tank (5 -50 psi) which
delivers propellant at low pressure into a pump chamber, where it is then pressurized to high pressure and
delivered to the engine. A diagram of the pump operation is shown in Figure 1. Two pumping chambers are
used in each pump, each one being alternately refilled and pressurized. The pump starts with both chambers
filled (Step 0, not shown). One chamber is pressurized, and fluid is delivered to the rocket engine from that
chamber(Step 1). Once the level gets low in one chamber, (Step 1a) the other chamber is pressurized; and
flow is thereby established from both sides during a short transient period(Step 2) until full flow is
established from the other chamber. Then the nearly empty chamber is vented and refilled. (Step 3) Finally
the cycle repeats. This results in steady flow and pressure. The pump is powered by pressurized gas which
acts directly on the fluid. Initial tests showed pressure spikes as the pump transitioned from one chamber to
the other, but these have since been eliminated by adjusting the valve timing. For more details on the pump
and a discussion of the second-generation design see reference 1 or 8. This pump is more robust than a
piston pump in that it has no high pressure sliding seals, and it is much less expensive and time consuming
to design than a turbopump and a system which uses the pump has far lower dry mass and unusable
residuals than turbopumps do. For more info, see www.rocketfuelpump.com
Spacecraft Applications of the Pump.
This pump offers substantial performance and flexibility improvements for a space vehicle such as the
Crew Exploration Vehicle. Space vehicles currently use spheroidal tanks pressurized to 200-300 psi. These
tanks are somewhat heavy, are very expensive and require propellant management devices to keep liquid
propellant at the tank outlet for engine starting in a zero gee environment. The pump allows for
lightweight, low-pressure tanks and the pump can be stopped with one chamber full of fuel so that when the
spacecraft starts, the fuel will settle to the bottom of the tank and no PMDs are required in the tank. The
spacecraft tanks need not be spheroidal, and options such as low pressure drop tanks, flexible composite
tanks etc. become feasible. The low-pressure tanks can be lifted to LEO empty and then filled from the
upper stage, thereby limiting the structural loads on the tanks. Low-pressure tanks can also be more easily
jettisoned or connected, and low-pressure plumbing, valves and fittings are lighter, less expensive and more
reliable. For lunar and mars missions, fuel can be pre-positioned by robotic spacecraft at the destination for
the return trip. These tanks can be more easily integrated with the spacecraft, and the dangers associated
with handling propellant tanks and transferring propellant are lower at low pressures. We imagine a system
that utilizes aircraft drop tank style operations. Since ascent stages from the moon or Mars need not be
streamlined, concepts for use of propellant produced locally on the moon or Mars may benefit from fiber
reinforced external flexible bladder tanks. This will reduce delivered vehicle size and mass.
Disadvantages:
• The pistonless pump uses about 10-15% more pressurant than pressure fed systems. However, the
pressurant can be heated to save 30% on pressurant mass.
• The pistonless pump system uses more valves and is more complex than pressure fed systems,
however, spacecraft valve design is a mature technology, so this is not expected to be a problem
If a spacecraft were designed as a pressure fed vehicle, the pump could be a straightforward upgrade, with
no major system changes. Alternatively, the pump could be coupled to a high-pressure engine for increased
performance as described below
Vapor Cycle Pumped Spacecraft.
For a spacecraft which uses the pump, the pressurant weight becomes an issue. If we can recover the
pressurant used to run the pumps, then the burnout mass of the propulsion system would be very low. The
ideal solution would be to use a closed cycle system which recovers the pressurant used to drive the pump,
condenses it in radiators, pumps it up to the required pressure, and vaporizes it in a heat exchanger mounted
to the combustion chamber to be used in the pumps again. This adds mass to the spacecraft for radiators,
Conclusions
The gas powered pistonless pump has been shown to offer substantial performance and flexibility increases
for space vehicles. The pump design is not complex and the pump can be developed using low cost
materials and upgraded to spacecraft quality materials late in the design process to save costs.