Restore-L Intern Final Report

Student Name Abhay NegiAcademic Level 4th Year Undergraduate StudentAcademic Major Aerospace Engineering, Mechanical Engineering

Academic Institution University of California, DavisInternship Date Summer 2019Mentor Name Brian Nufer

Mentor Job Title Propellant Transfer Subsystem Fluids LeadOrg Code/Branch NE-L5, Exploration Payloads Branch

Division Laboratories, Development & Testing DivisonDirectorate Engineering Directorate

https://ntrs.nasa.gov/search.jsp?R=20190030299 2020-07-12T19:55:36+00:00Z

Abhay NegiFinal Report

1 Restore-L Mission

Every year, approximately 20 satellites are retiredbecause their propellants are exhausted [1]. Primarilylimited by fuel, the satellites are usually fully func-tioning otherwise. The lifetimes of satellites can beextended with the capability of refueling. Not onlywould refueling extend satellite lifetime, it would alsoprovide future satellites flexibility to prioritize otherelements of their missions.

Restore-L is a Goddard Space Flight Center Satel-lite Servicing Projects Division mission with the pri-mary objectives of developing tools, technologies,and techniques to extend the lifetime of satellites -even if they were not designed to be serviced in or-bit. Equipped with five key servicing technology el-ements, the Restore-L Servicing Vehicle will demon-strate industry-changing satellite servicing technolo-gies. The primary goal of the Restore-L missionis to rendezvous with, grasp, refuel, and relocatea government-owned satellite to extend its lifetime.The Propellant Transfer Subsystem (PTS) is requiredto deliver measured amounts of fuel to the client satel-lite in a safe manner.

Figure 1: Servicing Technology Elements [2]

2 Water Hammer

Water Hammer is a surge in pressure that occurswhen a fluid is forced to suddenly change velocity.This phenomena occurs during priming - when thelines are initially being filled with propellant. If suffi-ciently large, the pressure surge could cause damageto components in the Restore-L or client spacecraft.

Pressure surges occur in the Restore-L PropellantTransfer Subsystem when the latch valve downstreamof the Restore-L propellant tank is opened for prim-ing. It is mission critical to sufficiently mitigate theeffects of water hammer such that components ofthe Restore-L and client spacecrafts are not dam-aged. Water Hammer may be estimated using theJoukowsky equation shown in Equation 1, where ρ isthe liquid density, uf is the change in liquid velocity,and c is the comperssional wave speed.

∆PWH = ρufc (1)

3 Testing

To characterize the expected pressure surge, em-pirical testing of water hammer is performed on asystem representative of the Propellant Transfer Sub-system. For safety reasons, deionized water is used insubstitution of the toxic Hydrazine monopropellant.A pressure scaling law is used to scale the measuredpressure surges of water to hydrazine. Several Pres-sure transducers are used to measure pressure alongthe propellant lines. High speed pressure transduc-ers are used to ensure data capture of the pressuresurge peaks. Multiple trials of different configura-tions are run where hardware such as filters and ori-fices is changed. Additionally, different staging pro-cedures are tested to determine the best propellantpriming operation.

4 Analysis & Results

The pressure scaling law shown in Equation 2 isused to scale the measured pressure from water toHydrazine [2]. The speed of sound in the fluid, c isfound using Equation 3, where E is the elastic modu-lus, D is the pipe inner diameter, tw is the tube wallthickness, and µ is Poisson’s ratio. It is found thatthe pressure surge in Hydrazine is expected to be 30%higher than that of water.

(Psurge)N2H4

(Psurge)H2O

=

(c√ρ)N2H4(

c√ρ)H2O

(2)

c =c0√

1 +ρc20E

(Dtw

)(5− 4µ)

(3)

Abhay NegiFinal Report

Figure 2: Sample Water Hammer from Priming

When an orifice or cavitating venturi is used andthe valve is initially opened a sudden drop in pres-sure is measured, as shown by PGT08 in Figure 2.The pressure remains at that level until the line fillsand there is a spike in pressure that eventually equili-brates to the tank pressure. The pressure transducerat the end of the line, PT08, also measures a spikein pressure. However, the pressure surge may occurin such a short amount of time that its peak value isnot captured by the relatively low sampling rates ofthe PT08 and PGT08 sensors. A high-speed pressuretransducer, HPT5, is used at the end of the line tomeasure the true peak value of the pressure surge.

For the shown configuration of priming from SLV1to the Client Interface, the pressure surge occurs overa duration long enough such that the low-speed pres-sure transducers are able to capture the peak. This isconfirmed by the high-speed and low-speed pressuretransducers giving similar results. The pressure oscil-lation that occurs due to the pressure wave sloshingbetween the ends of the pipe can be observed in Fig-ure 3. Note that Trial 1 and 2 are very close in waveamplitude and frequency. Trial 3 is run with H2O de-saturated of dissolved Helium; it is hypothesized thatsaturated water dampens the pressure wave. Thus, ahigher pressure peak is observed in Trial 3.

Tests are currently being run to determine the bestpriming configuration. There are four possible oper-ations as shown in Figure 4. The best priming oper-ation will be determined based on the peak pressuresobserved in the tests and the components at the endsof each line.

Figure 3: Sample Water Hammer Oscillation

Figure 4: Priming Configurations

5 Future Work

The Restore-L spacecraft is scheduled for launchin late 2023. The PTS passed a subsystem CDR inmid-July of 2019. Future work will include furthertesting and analysis of the PTS. Based on the results,modifications may be made to the system design orpriming operation to reduce pressure surge.

References

[1] NASA Goddard Space Flight Center, “On-OrbitSatellite Servicing Study Project Report,” NP-2010-08-162-GSFC, October 2010

[2] NASA, “Restore-L - Proving Satellite Servicing,”NP-2016-2-392-GSFC, September 2017

[3] Harbaugh, J., “Satellite Servicing TDMProject Overview,” NASA Available:https://www.nasa.gov/mission pages/tdm/satellite-servicing.html.

[4] Kandula, Nufer, ”RESTORE-L-EM-002025PTSPressure Surge,” RESTORE-L-EM-002025PTS,August 2016