dna@dsd.UUCP (02/25/84)
Posted: Wed Feb 8, 1984 7:15 AM GMT Msg: NGIE-1712-8217
From: JKING
To: DOCS
CC: OFFICE
Subj: WATER ROCKET REPORT
USING WATER AS A PRIMARY METHOD OF PROPULSION
FOR SPACERAFT MODIFYING STANDARD STS ORBITS
Jan A. King W3GEY
V.P. Engineering
The Radio Amateur Satellite Corporation
The Space Shuttle has modified the method by which space-bound
payloads enter orbit for the forseeable future. The STS offers
the promise of lower payload cost and the ability to carry large
payloads into orbit to mention but a few of its primary
objectives. For very low cost payloads such as those pioneered
by the radio amateur community (the OSCAR series), the Space
Shuttle poses, however, a number of severe engineering obstacles
which have become major stumbling blocks to the exploitation of
this valuable resource. Not unlike most free flying satellites,
the communications satellites launched by radio amateurs and used
in the Amateur Satellite Service are intended to meet long life
objectives. In addition, to meet mission objectives for the
communications service to be provided either a geostationary
transfer orbit or a sun synchronous polar orbit must be attained
by the spacecraft. Unfortunately, neither of these objectives
can be met by the standard provisions of a Space Shuttle mission.
STS orbits, typically 296 km in altitude and inclined from 25 to
57 degrees are unstable. A small spacecraft, with a low surface
area to mass ratio, will decay from such an orbit in a matter of
months. This class of orbits, with a few exceptions, is also
unsuitable for communications experiments of interest to the
Amateur Satellite Service. It is therefor a necessary
requirement for Amateur Satellites and other free flying
spacecraft seeking stable orbits to carry a propulsive capability
if launched by the Space Shuttle.
Having accepted the burden of a propulsive system as an added
spacecraft complexity, yet another problem becomes apparent.
Classical propulsion systems employed by satellites are
characterized as hazardous devices. Due to the manned presence
on board Shuttle safety considerations are necessarily more
stringent when using this method of launching a spacecraft. The
added complexities and paperwork resulting from the inclusion of
hazardous devices on board Shuttle launched satellites conflicts
with the low cost nature of these programs and may make such
payloads totally impractical or viable only if launched by
alternative methods. This problem is exascerbated by the fact
that most orbit alternatives can be reached from Shuttle orbits
only by multiple delta-V maneuvers. This requires multiple solid
rocket engines or a restartable engine on board the satellite,
further multiplying the safety hazard problem.
A solution is sought to the "Shuttle Dilema." The Shuttle
Dilemma may be defined as follows:
1/ A Shuttle payload is always two burns away from the
desired orbital elements when separated from the cargo bay.
2/ The cost of NASA safety approval for a propulsive device
used aboard Shuttle by a low cost user is approximately a factor
of three higher than the entire cost of the payload itself.
3/ As a rule of thumb, the mass of the paperwork necessary
for NASA approval of a hazardous device for a Shuttle flight is
greater than or equal to the mass of the payload.
While the above may seem humorous, these statements are all too
true and must be dealt with squarely by would-be Shuttle low cost
payload designers.
A propulsion system that would solve the Shuttle Dilema could be
expected to have the following characteristics:
1/ The propellant used should not be a chemical, pressue or
explosive hazard as defined by NASA or the USAF (ref. AFETRM-127-
1).
2/ The loading of propellant into the spacecraft should not
constitute a hazardous activity. No special safety equipment
should be required.
3/ No portion of the propulsion system should contain
hazardous devices of any kind. Certain exceptions to this rule
might be taken to include category B electro-explosive devices
such as pyrotechnically operated valves.
4/ No portion of the propulsion system should be pressurized
or become pressurized even remotely while the satellite is on the
ground, during powered flight or during astronaut activities in
orbit, including those conducted to separate the satellite from
the Shuttle.
5/ No portion of the propulsion system should be susceptable
to damage due to the environment of the Cargo Bay during powered
flight or in orbit prior to or during separation of the
satellite.
The Radio Amateur Satellite Corporation (AMSAT), having had
practical experience with both liquid and solid propulsion
systems on board low cost satellites believes that the above
requirements will prove to be virtually mandatory for low cost
payloads flown by the Space Transportation System. Two methods
have been considered for some time by AMSAT that appear to meet
the above five conditions and produce satisfactory performance
for space applications. Both involve using water as a fuel and
both have been considered by other groups from time to time as
methods of space propulsion.
PROPULSION VIA WATER ELECTROLYSIS:
The propulsion of a space vehicle via hydrogen/oxygen fuel
produced from the electrolysis of water is far from a novel idea.
Hughes Aircraft Company, Space Systems Division documented the
results of an internal research and development IR&D project
which developed a working model of a water electrolysis rocket
during the first half of 1964 (1,2,3,4). Using this technique a
single pressure vessle acts as storage for the water and
electolyte, as an electrolysis chamber and finally as a pressure
bottle for the combined electolyzed gases. The premixed gas may
be fed via a single line into the injection chamber of a small
rocket engine or thruster. In their final report on this
technology Hughes stated, "The Water Electrolysis Rocket has been
explored in sufficient depth to verify the feasibility of the
concept. Furthermore, it has been determined that this system
offers significant advantages over other presently available
reaction control systems. Among these advantages are:
1/ Higher specific impulse
2/ Lower system weight
3/ Lower power requirements
4/ Extended life in space
5/ Improved system reliablity
6/ System simplicity "
It is interesting to note that at the time of the writing Hughes
did not cosider the safety advantages of the system which are of
prime interest to AMSAT. The specific impulse of a small
electrolysis motor of the type required for a low cost satellite
mission is between 330 and 360 sec. This is considerably better
than either an equivalent solid or bipropellant liquid motor
system (270 and 305 sec. respectively). Preliminary
investigations by AMSAT suggest that the power required to
electolyze one kilogram of water is approximately 5,000 WH. This
may be related to delta-V for a specific satellite case as shown
in Table 1. A minimum system schematic is shown in Figure 1.
It is not known why Hughes did not continue to develope this
technology to the point of commercial introduction. Clearly,
however, monopropellant hydrazine systems replace other methods
for reaction control starting about the same time as the Hughes
research on water electrolysis motors. Since this work was done
there have been dramatic improvements in both electrolysis
electrode and thruster technologies. AMSAT has also conducted
preliminary studies on an advanced method of drying the hydrogen
and oxygen gas which should lead to improved thruster
performance. This was one problem reported by the Hughes
research team.
PROPULSION BY STEAM EXPULSION:
A second method of exploiting water as a safe propellant is by
means of a small steam engine integral to the thruster in a water
fed propulsion system. Water is allowed to superheat in a small
chamber adjacent to an expansion nozzle. Thrust is produced by
the acceleration of water molecules as they exit the nozzle. The
specific impulse of this technique is far poorer than the
electrolysis method (107 sec.), however, the system complexity is
very low indeed and the energy required to liberate a kilogram of
water into steam is only 750 WH, considerably less than with
electrolysis. The specific impulse for a motor of this type can
be shown to be governed by the equation:
| 2 C K Tb (1 - <Pexit/Pchamb.>**C-1/C)|
Isp = SQRT | ________ |
| (C-1)n m |
where:
C = Heat capacity of propellant (water = 1.3)
K = Boltzman Constant = 1.38E-23 J/K
Tb = Gas Temperature = (approx.) 400K
n = molecular weight of propellant (water = 18)
m = mass of a hydorgen atom = 1.66E-27 Kg
Pexit = Nozzle exit plane pressure (assumed = 0.01 Bar)
Pchamb. = Thruster chamber pressure (assumed = 5.0 Bar)
As can be seen, the specific impulse depends inversely on the
molecular weight of the fuel used, taken to the 1/2 power. This
favors the use of low molecular weight fuels. As can be seen
water is nearly optimum for an engine of this type particularly
when the other physical properties of this fluid are taken into
consideration in a practical system.
While, on balance, a system using steam as a propulsion technique
is far from optimum with respect to Isp it is simple enough to be
included on even GAS CAN mission and can solve a reasonable
number of propulsion problems for small spacecraft. Figure 1
reviews the performance of this system for a variety of missions
of interest to AMSAT and gives a comparison to the electrolysis
method.
ORBIT CORRECTION TECHNIQUE USING WATER PROPUSLION METHODS:
A salient characteristic of both propulsion techniques reviewed
is that they take electrical energy from the solar arrays of the
spacecraft and convery it into potential energy (either in the
form of stored gas to be burned or in the form of stored
electrical energy). Thrust is best produced in a burst mode
rather than with a continuous firing. This is a typical
operating mode for a reaction control system (RCS) but is
somewhat unusual for an orbit transfer maneuver. In effect, time
is traded against the burn duration (power production rate of
the satellite) so that a reasonable compromise for the total
duration of the propulsion phase of the mission is reached. An
important consideration for such a mode of operation is that the
total delta-V achieved per day during the maneuver must be
greater than the deceleration per day due to drag in the Shuttle
base orbit. The orbit transfer strategy for circular orbits with
a spinning spacecraft is shown in Figure 2. Two small thrusters
at either end of the satellite are employed. Firings always
occur at the line of apsidies. Alternate thrusters are used so
that a first firing occurs at perigee thus raising apogee and a
subsequent firing occurs at apogee now raising perigee. Mini-
Hohman transfer maneuvers are repeated until the desired circular
altitude is reached. Eliptical orbits can be achieved with a
single thruster fired at consecutive perigees thus continuously
raising apogee. Inclination changes with this technique are also
possible and are most efficiently applied when apogee also
coincides with the ascending node of the orbit.
SUMMARY:
The techniques reviewed have been considered in the past by space
research projects and by commercial spacecraft manufacturers.
While these propulsion technologies have never been reduced to
commercial practice, sufficient study has been done to verify
their applicability to space missions. If the STS is going to
fulfill its mission as a launcher for ALL space interests then
some acceptable methods of propulsion must be found for smaller
payloads. These methods must take into consideration the low
cost nature of such projects and the very stringent safety
constraints imposed by NASA on all STS users. In view of the
above AMSAT believes that water propulsion technologies should be
revisited because they have the potential of solving the "Shuttle
Dilema" for a class of users that can bring significant benefit
to the space program as a whole.
REFERENCES:
1. Newman, Daniel D., Study of the Water Electrolysis Propulsion
System- Final Report, Engineering Record No. 151, Hughes Aircraft
Co., S.S.D., Propulsion and Power Systems Laboratory, 5 June
1964.
2. Water Electrolysis Rocket, Hughes Aircraft Company
Proposal, 63H-7438/9419 (Dec. 1963).
3. Electrochemical Service Unit, Hughes Aircraft Company
Proposal, 64H-2115/A3951-001 (April 1964).
4. Water Electrolysis Rocket, Hughes Aircraft Company Proposal,
64H-2510/A4682-001 (May 1964). .