Update 10-25-2022: a version of this was presented at the 2022 Mars Society convention at ASU in Tempe, AZ. It was well-received.
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This article describes a concept for an on-orbit
propellant depot capable of refilling visiting craft, which presumes the visiting craft have
rendezvous and docking capabilities. This
is only a concept, which has not had any
design analysis.
Such a depot needs orbit adjustment, reboost,
and debris avoidance propulsion capability. There is the option to propel the depot
sufficiently that it might “go where the job is”, instead of having the visiting craft come to
it, which could assist with some aspects
of orbital debris removal. But because
the facility is so large, a separate
space tug is a far better option for such missions.
Classes of Propellants
There are fundamentally two completely different
classes of propellants to handle. These
are the more-or-less room-temperature storable propellants, now most commonly used in thruster and
attitude-control systems, and there are
the cryogenic “main stage” propellants,
which do in fact include the storable material kerosene. The thruster storables need to be supplied in
relatively small quantities, while the main
stage cryogens (and the kerosene) need to be handled in very large quantities.
Zero-gravity propellant ullage problems have been thoroughly
discussed in Ref. 1. The
solution for the storable propellants (and the storable kerosene) is the
bladdered tank. This could be a
free bladder, a side-everted
bladder, or an axially-everted
bladder. The exact bladder geometry to
be selected is not the real issue here, in a concept design description. Those geometries are discussed in Ref. 1.
The solution for the cryogenic propellants has been rather difficult
to find. As discussed in Ref. 1, the historical solution for various rocket
stages requiring free-fall restart has been application of ullage thrust. The same is said in Ref. 2. There have been some ideas tested and flown
that take advantage of surface tension effects,
as discussed in Ref. 2. Those
same references point out that application of ullage thrust can be expensive in
terms of ullage motor propellant quantities,
if you have to do it many times. The
solution selected for this quandary is the spinning tank technology described
in Ref. 1.
The other piece of this puzzle is the facility occupancy by
a crew. There is no need to continuously-man
this facility! You only
need to send up a crew when there is a propellant transfer to be made. Once done,
the crew returns to Earth. There
are already standard docking ports of the necessary types (plural) on the
International Space Station. You just
equip this facility’s crew module with those same docking ports, plus some room for a few future new designs.
What Propellants to Have In Stock
The small-quantity thruster propellants under consideration
here are the four common hydrazine variants,
and hypergolic nitrogen tetroxide (NTO) as the oxidizer for all of
them. Those hydrazine variants are plain
hydrazine (C2H4),
monomethyl hydrazine (MMH),
unsymmetrical dimethyl hydrazine (UDMH),
and the brand-name product Aerozine-50,
(a 50-50 blend of UDMH and plain hydrazine). These are commonly stored at significant
pressure, for pressure-fed thruster
systems.
The mostly-cryogenic combinations are (storable)
rocket-grade kerosene (RP-1), cryogenic
liquid methane (LCH4, which
is not the same as liquified natural gas), and cryogenic liquid hydrogen (LH2), with cryogenic liquid oxygen (LOX) as the
oxidizer for all of them. These are
commonly stored at rather low pressures,
only such that boiloff is slowed or prevented. Such engines are usually fed by
turbopumps. List follows:
Fuel oxidizer application
C2H4 NTO small-quantity thrusters
MMH NTO small-quantity
thrusters
UDMH NTO small-quantity
thrusters
Aerozine-50 NTO small-quantity
thrusters
RP1 LOX main
stage propellant
LCH4 LOX main stage propellant
LH2 LOX main stage propellant
There are two other storable oxidizers, although neither has been used in spacecraft
for some years now. Those are inhibited
red fuming nitric acid (IRFNA) and high-test hydrogen peroxide (H2O2). If such should ever be demanded again, you just add extra storable oxidizer modules
to the facility for them.
Properly-Applying Spinning-Tank Technology to Solve Cryogen
Ullage Problems
The “trick” is not to spin the cryogenic tanks while
they are attached to the depot! That
introduces severe sealing problems and the resulting logistical problems.
Instead, the craft to be refueled docks to a
tank, detaches it from the depot, and moves off some distance, then spins up with the tank docked to the
craft, to effect the transfer
tank-to-spacecraft with pumps. Then it
de-spins and returns the tank to the depot. You do this twice: once for the fuel, then again for oxidizer. See Figure 1. Vapor exchange is required along with liquid
transfer, per Ref. 1.
Figure 1 – Spinning-Tank Technology Applied to Cryogenic
Refill On-Orbit
Transfers to the depot from tanker craft work exactly the
same way: detach the empty tank to be
filled, move off and spin up the docked
tank-and-spacecraft, transfer the liquid
(and vapor), then de-spin and return the
filled tank to the depot. See again
Figure 1.
Spin-up/spin-down thruster propellants are minimized by the
use of “rifle-bullet spin”, and also
very much by being limited to only the inertia of spacecraft plus tank, not the entire depot or its entire assemblage
of tanks! It is end-to-end docking plus
that “rifle-bullet spin”, of only the
tank and visiting craft, so the moment
of inertia, and total impulse required
of the spin thrusters, is minimized. This was well-discussed in Ref. 1. At about 1 rpm and near 2 m radii, the spin induces about 0.0022 gee (corrected value 2-4-22).
That's for the cryogenic materials only. The storables are in the same sort of
bladdered tanks as the spacecraft that use them. You just dock with the depot module and
pump, from bladdered tank to bladdered
tank. Pressurant gases are involved to
push on the bladders. There is no spin
needed.
The only design requirements imposed on the craft
transferring-out cryogenic liquids, are
the need for propellant suction points out on the tank periphery, as well as on the aft domes, plus the presence of perforated radial
baffles in the tank that speed the spin-up of the liquid globules inside. See again Ref. 1. Craft that only receive liquids do
not need periphery drains or baffles,
spin or not.
Depot Station Layout
Excepting kerosene,
the bladdered storables are primarily for attitude/maneuvering
thrusters. Those quantities are far
lower, no matter which combination you
desire. So there are hydrazine tanks for
around a 1 cubic meter each of hydrazine,
MMH, UDMH, and Aerozine-50, plus tanks for about 5 cubic meters of NTO. Those are 1-ton-class quantities of each fuel
material, and a 5-ton-class quantity of
NTO to support any-and-all of them.
The storable kerosene,
and the cryogenics LOX, LCH4, and LH2 are needed in far, far larger quantities, being main stage propellants. The kerosene transfers just like the other
storables, except the tanks are far
larger, around 300 cubic meters for a
nominal 250 ton quantity. The LCH4
is near twice that volume, and the LH2
around 11-12 times that volume. This assumes
very large receiving craft, and
roughly-equal frequency of demand for each fuel type, which may or may not be actually true. The shared LOX volume to support each of these
is simply huge (at around 2300 cubic meters),
that being the largest volume stored on the depot station.
See Figure 2 for some crudely-estimated quantities. These were done in an Excel spreadsheet.
Figure 2 – Crudely-Estimated Quantities for Storables and
for Cryogens
Reboost and attitude control could be a combination of the
storable propellants plus electric propulsion powered by some solar panels, all located on (or in) a “power and
propulsion module”. This power and
propulsion module would be at one end of the depot station. The crew compartment would be at the other
end. You want the NTO tanks near one
end, as physically far from the 4 types
of hydrazine tanks as possible, to avoid
any possibilities of explosions-upon-contact,
if there are ever any leaks.
These materials are hypergolic,
even in vacuum.
The large kerosene bladdered tanks should probably be at the
same end as the hydrazine tanks, just
closer than the hydrazines to the center of mass. The largest and heaviest item is the LOX
tankage, which should be near center of
mass. The LH2 tanks can be at
the same end as the NTO storage, just
closer to the center of mass than the NTO.
I show the LCH4 tanks forward of the LOX, just to better center the LOX on the center
of gravity. Because of the toxicity of
NTO residues from any spills, the crew
compartment should be far away from the NTO storage. That puts the propulsion and power module at
the NTO end of the facility. See
Figures 3, 4, and 5.
Figure 3 – The Bladdered-Tank Storable Modules
Those modules associated with bladdered tanks need an
adjacent docking adapter for the visiting spacecraft, with the tanks situated such that there is
clearance to come in and dock. Once
docked, you just hook up the transfer
lines, and do the fluid transfer. This would also apply to tankers refilling
the facility. There is no spin, and the pressurant gas supplies plus the
plumbing and electrical, are within the
depot module truss. The visiting craft
undocks and then redocks elsewhere, to
switch from fuel to oxidizer. There are
manipulator arms on the modules to assist with the docking operations.
Figure 4 – The Cryogenic Tank Modules
Figure 5 – Recommended Facility Layout
The cryogenics are a different situation. These large tanks must be perpendicular to
the axis of the depot facility, and must
have appropriate docking gear on both ends of the tanks. The visiting craft docks directly to the end
of the tank, and makes the fluid and
vapor connections. Then it detaches the
tank from the depot, and moves off a
short distance (for safety). The docked
vehicle-and-tank gets spun up for the fluid transfer, then de-spun once completed. It redocks the tank with the station, undoes the connections, and finally undocks from the re-docked
tank.
If both fuel and oxidizer are cryogenic, then one repeats this procedure with the
appropriate other tanks. There are a few
relocatable manipulator arms that attach to the cryogenic tank docking
structures, which assist with all the
docking operations. Not every tank needs
a manipulator arm.
Preventing Incorrect Connections
If the visiting vehicle is to be refilled from the
facility, its tanks need the plumbing
connections to make the indicated hook-ups.
This is pretty simple for the storables,
including the kerosene. For the
cryogens, the visiting vehicle does not
necessarily need the perforated spin baffles or periphery fluid drains that are
needed in the depot tanks. But, any tankers attempting to refill facility tanks
will need these extra plumbing features,
as was indicated in Figure 1.
Since the oxidizer quantities are nearly always larger than
the fuel quantities, I recommend making
the hose/plumbing connections and fittings for oxidizers about 1.5 to 2 times
physically larger than the fuel fittings.
That prevents incorrect hook-ups,
something particularly important with the hypergolic hydrazines and NTO.
However, there also
needs to be some sort of keying feature on all of the fuel connections, so that only the correct fittings can be
coupled, even though they are of equal
size. You do not want to mix species
among the hydrazines, or get kerosene
when you wanted a hydrazine, or vice
versa.
You also need to key the oxidizer fittings by species, just in case IRFNA or high-test H2O2
ever get added to the facility. It would
be harder, but not impossible, to mix up NTO and LOX, just because of the difference in bladdered
versus spinning-tank technologies. But keying
the fittings absolutely stops that.
These
fitting sizes and keying features are a critical safety requirement, and should be standardized “up front”, so that everyone is using the same equipment!
Power and Propulsion Module
This item (also shown in Figure 5) has the thrusters
and the solar panels required to power the entire facility, and to move it as needed. Those thrust applications would include
reboost for orbital decay, deorbit at
end-of-life, debris avoidance
maneuvers, and general attitude control. I’d recommend something similar to the Space
Shuttle “OMS” and attitude-thruster systems,
which were a hydrazine variant-NTO thruster design, plus some sort of electric thruster, for its far-higher specific impulse, for the reboost operations and any other
orbit changes.
My own preference for the electric thruster would be an
iodine thruster, for the easy fuel
storage and handling, even though those
are not yet very common, or even in wide
use. The bigger the electric
system, the more solar panels would be
needed on this module, and there are
geometric limits to that.
Crew Habitat and Docking Module
The layout in Figure 5 also shows a crew habitation
module at the other end from the power and propulsion module. There is attached to it a multi-port docking
module for crew transfer vehicles. Crews
would be required during any refilling operations for visiting vessels, or for tankers re-supplying the
facility. Otherwise, there is no need for crew aboard, not even during transits to other
orbits. Crews would be aboard relatively
short-term for the necessary fluid transfer operations. Long term occupancy (like the International
Space Station) is not an issue here!
Options For Spinning-Up (and De-Spinning) Cryo-Tank and
Vessel
The concepts selected here do minimize the thruster
propellant quantities required for spin-up and de-spin, assuming thrusters are used for this
purpose. That is the historically-proven
way to do it.
However, adding a
flywheel for electric torque spin-up and de-spin is another viable (if
undemonstrated) option. This flywheel
should probably be within the docking adapter on the end of the tank adjacent
to the vessel, so as to be nearest the
docked center of gravity.
The flywheel will need to be rather massive, and will spin quite fast, so that its angular momentum magnitude equals
the angular momentum magnitude of the docked cryo tank and vessel.
Adding a “Go-To-The-Job” Capability Is Unattractive
If you want this depot facility to take on the role of
scavenging abandoned spent stages for their propellants, before de-orbiting them, then you need to add very considerable extra propulsive
capability to the depot station. It would have to go and rendezvous
with every target spent stage.
You do that with very much larger fuel tanks for the
electric propulsion and for the storable-powered thruster, both located in the power and propulsion
module. That option is also shown in Figure
5, as extra tanks to be added
surrounding the power and propulsion module.
The orbital changes to do this are made unmanned, controlled from the ground. You send up crews temporarily, only when propellant transfers are to be
made.
Otherwise, there is just
no need for a facility propulsive capacity that large! That is why the option takes the form
of added tanks. Everything else is
the same. Although, choosing instead to create a specialized
vehicle to operate as a “space tug” to go get the spent stage, and transport it to the facility, is probably the better concept, and by far. Such a space tug is just not that
massive, while this propellant depot
facility is simply enormous.
Thrust Sizing
But in any event, the
thrust-induced accelerations need to be quite low, in order to avoid large bending loads in the
cryogen tank attachments. Accelerations
on the order of 0.01 gee or less should be fine, with the storable thruster. It is quite likely the electric propulsion
would provide accelerations more in the ballpark of 0.0001 gee. While very low indeed, that’s all right: the electric propulsion would only be used
while unmanned, to make significant
orbital changes that are not time critical.
More time-critical adjustments (such as debris avoidance), would use the storable-propellant thruster.
References
#1. “Propellant Ullage Problem and Solutions”, Gary W. Johnson, posted 18 August 2021 to http://exrocketman.blogspot.com.
#2. “A Detailed Historical Review of Propellant Management
Devices for Low Gravity Propellant Acquisition”, Jason W. Hartwig, NASA Glenn Research Center, Cleveland, OH,
44135, USA; undated AIAA paper but ca. 2015;
(pretty much the same information was also in the Hartwig Ph.D.
dissertation).
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