Cryogenic Electro-Mechanical Actuator for Lander Control Valves

The proposed work seeks to build upon previous developments in the area of Electro-Mechanical Actuators (EMA) for cryogenic lander control valves. Existing EMA and valve hardware was developed under the CATALYST program, specifically for throttling a liquid oxygen/liquid methane 4klbf thruster. The target outcome of this project is to provide a much more robust EMA solution for performing deep throttling of lander class engines. The new EMA design will not only improve throttle capability, but will also provide mass savings to the system. The end product of this work will ultimately be provided for hot fire testing with actual lander engine hardware.

Although the existing EMAs and valves have been designed for oxygen and methane propellants, the successful outcome of this project will be relevant to most other liquid propellant combinations. Therefore, the proposed work presents an opportunity to enhance Marshall’s role in lander technology development as a whole. In addition, considering the number of companies that exist in the lander technology space, this project could serve to open avenues for collaboration with industry in the very near future.

Marshall Space Flight Center is currently involved in an array of lander technology development activities, including development and testing of liquid oxygen, liquid methane engines. Considering the typically constrained nature of a lander’s mass budget, emphasis has been placed on producing systems that provide ample capability with low mass. An example of such is the development of Electro-Mechanical Actuators (EMAs) for control valves, which can provide benefits in terms of mass and capability. The use of EMAs can eliminate the need for hydraulic or pneumatic plumbing and pressure control systems, providing significant mass savings. In addition, the use of EMAs can deliver precise valve control for deep throttling capability, which is very beneficial for lander propulsion systems.

The Valves, Actuators, and Ducts Design & Development Branch (ER33) recently developed a ¾” ball valve designed specifically for throttling a liquid oxygen, liquid methane 4klbf thruster. This design implemented a trunnion type ball, as opposed to a floating ball, which greatly improved the valve’s throttling capability. Some of the realized benefits included more precise flow control, better cryogenic sealing, and lower seat friction leading to lower mass actuators.

The ball valve design included a relatively low cost commercial off-the-shelf (COTS) stepper motor for valve actuation. Hardware was designed for integrating the motor onto the valve and initial testing was completed in ambient and cryogenic conditions. After modifications to the actuators and attach hardware, further bench testing was completed and the valves were ultimately prepared for hot fire testing. However, the lead-up to this testing illustrated the challenges of implementing EMAs on cryogenic control valves. The proposed work aims to overcome these challenges by building upon lessons learned in order to produce an EMA and gearbox that will provide mass savings and deep throttling capability for small cryogenic engines.

In order to illustrate how the proposed work will succeed, the challenges faced while operating the existing EMA will be addressed. These challenges include the following:

1. Torque required increases significantly when valve is chilled and pressurized
2. Motor/gearbox require thermal standoff and heated purge to maintain acceptable operating temperature
3. Accurate positioning and position feedback are required for deep throttling
4. Lack of desired failure characteristics for off-nominal scenarios

Issue number 1 was the primary reason for the addition of a gearbox into the existing EMA. The gearbox allowed the motor to meet and exceed the torque required to turn the valve at temperature and pressure. However, due to the fact that the motor and gearbox were sized separately, the combination was off nominal in terms of mass and volume. In addition, the gearbox contributed greatly to issues 2 and 3. In order to alleviate the first issue, the proposed work will size the motor and gearbox combination together. After testing the existing EMA, the required torque values for the ball valve are well known. Therefore, the motor and gearbox combination can be optimized to meet this requirement without incurring a mass or volume penalty.

Simultaneous sizing of the motor and gearbox will also address the second issue of operating temperature. The initial EMA was built up using a standard COTS motor and gearbox, both of which were not designed for operating at very low temperatures. Due to this, a heated purge was added, which actually caused an overheat failure of the motor during an attempted hot fire test. Considering this experience, the new EMA will be designed with a much wider operating temperature range in mind. Therefore, the vendor survey will focus exclusively on vendors that produce motors and/or gearboxes capable of operating at very low temperatures.

The third challenge was introduced primarily due to the addition of a gearbox into the existing EMA. As discussed previously, this gearbox was not sized in tandem with the motor, which meant that the total backlash of the system increased. Sizing of the motor and gearbox simultaneously will address this issue in multiple ways. First, the motor size and gear ratio can be optimized, which will allow for the use of a lower backlash gearbox. Second, the new gearbox can be selected to accept a zero backlash coupler to link it to the valve stem, ultimately reducing the overall backlash of the EMA.

In the existing system, the backlash introduced by the gearbox and coupler lead to difficulty in both repeatable positioning and accurate position measurement. An external sensor was added in an attempt to provide position indication, however, options for sensors were very limited due to the existing design of the valve attach hardware. Therefore, the proposed work will first identify the desired sensor for position feedback, as well as a zero-backlash coupler. The valve attach hardware will then be designed to accommodate said sensor and coupler, which will greatly improve the position repeatability and feedback accuracy. With these additional features, the new EMA will have a much greater capability to provide deep throttling to the engine.

The final challenge to be addressed by this project is the failure behavior of the EMA. As mentioned above, a motor overheat failure was encountered during an attempted test. When the overheat condition occurred the motor went into a safe mode, which did not allow for movement of the valve. This behavior is consistent with the other failure states of the existing motor. If the motor encounters an off nominal condition, the valve will fail at its current position. For hot fire testing, this behavior is undesirable because it could lead to loss of control of the propellant flow. Therefore, the desired failure characteristic of the new EMA will be fail-closed, which will be considered as a requirement at the beginning of the design process.

The end goal of this work is to produce an EMA that can successfully provide deep throttling capability to a 4klbf cryogenic thruster. 

This is early/mid-stage (TRL 4) — the most realistic path in is NASA SBIR/STTR, which funds small businesses and research institutions to develop technology aligned with NASA's needs (equity-free, phased funding). Check whether a current SBIR/STTR solicitation topic overlaps with this project's technology area, or contact the project directly (above) to ask.

None of these are guaranteed paths for this specific project — TechPort itself doesn't have an "apply" button. Reaching out to the contact(s) above with a specific question is usually the fastest way to find out what's actually open.