IEEE - Aerospace and Electronic Systems - September 2019 - 4

Feature Article:

DOI. No. 10.1109/MAES.2019.2923312

Water Electrolysis Propulsion as a Case Study
in Resource-Based Spacecraft Architecture
(February 2020)
Kyle P. Doyle, Mason A. Peck, Cornell University, Ithaca, NY, USA

INTRODUCTION
The ability to replenish expendables fundamentally
changes the paradigm of spacecraft design. One of the
most significant impacts derives from the rocket equation,
which motivates the use of high specific-impulse propellant when there is no prospect for in-orbit refueling. The
situation is quite different when a plentiful resource, such
as water, is available. Water in particular can be used as a
green propellant either in a thermal rocket, or in an electrolysis propulsion system, as proposed in this paper [1],
[4], [5], [17]. Electrolysis propulsion can be thought of as
a hybrid between electric and chemical propulsion. Power,
typically from solar cells, is used to decompose water into
its constituent elements, producing hydrogen and oxygen
gas. These products can be combusted together on demand
to produce thrust. Rather than being converted directly
into kinetic energy as in ion thrusters, the electrical energy
is converted to chemical energy as an intermediate step.
Although the specific impulse of electrolysis propulsion is only about a tenth that of ion thrusters, its DV can
be competitive with most other forms of propulsion.
Advantages include greater thrust per unit power than ion
thrusters because of the high efficiency of electrolysis,
chemical energy storage capability inherent to the propulsion system, and dense propellant storage compared to
storage of hydrogen and oxygen separately in cryogenic
liquid form. The result is green, nontoxic, dense storage of
a propellant capable of providing substantial DV, without
the additional cryogenic tankage and complex pumping
apparatus required for LH2/LOX.
Authors' current addresses: K. P. Doyle, M. A. Peck,
Cornell University, Ithaca, NY 14850, USA, (E-mail:
kpd43@cornell.edu).
Manuscript received September 30, 2018, revised
February 5, 2019, and ready for publication June 13,
2019.
Review handled by F. Davarian.
0885-8985/19/$26.00 ß 2019 IEEE
4

This paper considers the process by which to design a
spacecraft around the availability of a certain resource.
Water electrolysis propulsion is the chosen example, considered both as a subsystem technology and in terms of
secondary benefits that the presence of water can have for
spacecraft architecture. Many opportunities to save mass
result from using the same water for multiple purposes,
such as for crew consumption and oxygen production, in
addition to use as propellant.
In the future, even more mass could be saved through
ISRU of water collected from propellant depots or celestial bodies. Due to the exponential nature of the rocket
equation, doubling the DV for a trajectory-such as for a
round trip-more than doubles the propellant required for
the same payload. Therefore, gathering propellant for the
return trip in situ reduces the amount of propellant to be
launched from Earth by more than half. This leads to substantial mass savings for any round-trip mission, such as a
sample return from a near-Earth asteroid, or NASA's
Mars Design Reference Architecture.
This paper summarizes the design of the two CisLunar
Explorers 3U CubeSats, a technology demonstration mission for water-electrolysis propulsion, among other technologies. The design centers on the use of water,
leveraging it for attitude stabilization of a spinning spacecraft design, sinking waste heat, and radiation shielding.
The water propellant tank itself forms the structural core
of the spacecraft. Other subsystems include optical navigation and a CO2 cold gas thruster for attitude control. We
use the example of the CisLunar Explorers design to
develop a framework for designing future spacecraft
around ISRU in general and water in particular.

WATER ELECTROLYSIS PROPULSION BACKGROUND
Although there are several innovative technologies on
board, the CisLunar Explorers mission primarily serves
as a technology demonstration for water electrolysis
propulsion. Some of the advantages of this propulsion

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SEPTEMBER 2019

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