| Summary |
|
The need for
an
orbital tug emerged from discussions on required logistics for asteroid
mining. A conceptual exploration and debate on an orbital tug in LEO
was enhanced by a CAD-based graphical representation that acted as a
sounding
board to test and explore structural and operational ideas. This graph
is presented as a design under process of continuous change. This paper
represents a "milestone" on the ongoing design process.
The tug now has two grappling/docking interfaces at the two ends of its
main axis, with a boosting capacity in both directions. Manipulator
arms include two types, ie. "graspers" for holding irregular debris and
space junk or spent satellites, and "twiddlers" that can apply
specialized tools or lock onto
predesigned sockets on cargo containers.
Propulsion is by Microwave Electrothermal Thrusters that use plain
water as reaction mass and solar electricity from PV panels as source
of power.
|
Tugs will catch
payloads launched from Earth into very low orbits (200-250km) and
transfer them to wherever they need to be, thus earning a first income.
Tugs will refuel satellites and bring replacements such as batteries -
another source of income. Tugs will also hunt space junk and collect it
in one spot near an orbital base, where it will be used later for other
purposes. Junk removal will also be a source of income.
The orbital base will be a crewed facility that has a "garage" where
assembly of space vehicles will happen. When working on building the
infrastructure for mining asteroids, the tugs will mainly service this
base.
The orbital tug is initially refuelled by cheap launches of "water
cartridges" from Earth, which it captures in imprecise low orbits, and
which it then sticks with its own arms into predesigned sockets on
itself. Empty
cartridges are collected at the orbital base. Later, water for
refuelling will come from asteroids. |
Situation
|
|
The need for
an
orbital tug in Low Earth Orbit arose from operational considerations
for asteroid mining. Work on the milestone "Rig
for Mining Asteroids" by the NEAmines group
(http://www.asteroidmines.net/documents/MiningRig/RigForMining.html)
surfaced the following operational concept:
- Asteroid
material is processed at the asteroid itself only to the point of
separating stuff that is of no use in LEO - such as plain rock - from
stuff that can be further processed at LEO, such as metal grains,
water, carbonaceous compounds and any other useful volatiles.
- Containers
of usefull stuff (metallic "grit" and volatile slurry, mostly water)
will be boosted to meet Earth and be aerocaptured into LEO orbits.
- These
containers will then be orbitally manoeuvered to a processing plant in
LEO, which then processes the semiprocessed asteroid material further
into bullion of rare metals, metal parts for constructions, fuel,
etc....
|
It was then
seen that
for these operations the following infrastructure would be needed in
LEO:
- A staging base.
This will
initially be a fuel depot and a "garage"
for assembling outgoing craft, later upgraded with the processing plant
itself.
- An orbital tug
to move
cargos and construction supplies and other items to and from the
orbital base.
Further backtracking down the steps to achieve this orbital capacity
showed that the orbital tug is the key for achieving it: The orbital
tug will be required to begin to assemble the staging base in orbit
because of large diameter structures that are too expensive to launch
from Earth. It also turns out that the capabilities of the orbital tug
will allow it to hunt and collect orbital junk, and to transfer
satellites launched into LEO to their final orbits and even to service
them there. Both these capacities will allow the tug to already tap a
first income stream long before asteroid material begins to arrive from
asteroids. |
| Envisaged Operations |
|
Minimal
capability for refueling with water:
1. Leave the base and drop to orbit at around 200km to pick up a can of
water boosted there with a cheap launch from Earth.
2. Rendezvous and grapple the water can.
3. Reorient and boost up to base orbit.
4. Rendezvous with fuel depot at base, deposit water can there.
5. Refuel (with water) for a second trip to bring a further can of
water, leaving some water in the depot to be accumulated.
Construction material:
Analog as with water can. Instead of water can it will be a can full of
cheap elements for building the base and/or cheap elements for
space-craft to be fully integrated at base (in the "garage").
Capture of incoming
1. Leave the base and match up with orbit of aerocaptured cargo from
asteroid
2. Rendezvous and grapple with cargo (and its drive assembly).
3. Reorient and boost to base (where the processing plant is)
4. Rendezvous with processing plant at base and hand over cargo to
grapples of the base. (Base grapples decouple interplanetary drive and
park it in predestined slots).
5. Rendezvous with fuel depot and refuel.
Placement of outgoing
1. Base grapples hand over outgoing cargo to tug (previously
cargo and its interplanetary drive have been mated by base).
2. Navigate the outgoing assembly to insertion orbit for transfer
trajectory.
3. Decouple, reorient and boost back to base. (Outgoing drive then
independently fires its engines to insert into transfer trajectory).
4. Rendezvous with fuel depot and refuel.
|
Satellite delivery:
1. Leave base and drop to LEO at around 200km to pick up a satellite
payload boosted there from Earth.
2. Rendezvous and grapple satellite
3. Reorient and boost to final orbit of satellite
4. Release satellite
5. Reorient and boost back to base
6. Rendezvous with fuel depot and refuel.
In-situ servicing:
1. Pick up supplies and fuel for satellite at base
2. Leave base and boost to orbit of satellite to be serviced
3. Rendezvous and grapple with satellite, refuel and service the
satellite, etc.
4. Disengage and return to base
5. Rendezvous with fuel depot and refuel.
On-base servicing:
1. Leave base and boost to satellite
2. Rendezvous and grapple with satellite.
3. Reorient and boost back to base
4. Rendezvous with "garage" at base and hand over satellite to grapples
of the base (The satellite will then be serviced in the "garage" of the
base).
5. Rendezvous with fuel depot and refuel.
Junk removal:
1. Leave the base and match up with orbit of junk.
2. Rendezvous and grapple space junk.
3. Reorient and boost to base.
4. Rendezvous with "junk pile" near base and deposit junk there.
5. Refuel at depot.
|
| Encountered
challenges and ways to deal with them |
|
Propulsion
Propulsion for the tug will be with Microwave Electrothermal Thrusters
MET, using water as fuel: Microwaves heat water to just beyond the
point where it cracks into Hydrogen and Oxygen, then releases this out
a nozzle to create thrust (actually "fuel" is the wrong word here, as
the water is just the reaction mass, and the energy comes from the sun
via photovoltaic panels as electricity to power the microwave
generators). METs are shown to achieve up to 800s of ISP which is
higher than most chemically fuelled motors including even the motors
operating on cryogenic liquid oxygen and hydrogen. Furthermore it has
been shown that METs can be built in a compact way, and that they are
scalable. The thrust levels are suitable for orbital manoeuvering, ie.
big enough to achieve fast response, and yet with a high ISP. The most
important advantage is that they run entirely on water alone. Water is
the best fuel option in terms of ease and risks of handling, ease of
stockage, and ease of processing from asteroid material.
Refuelling
Earlier attempts at conceiving orbital tugs have been shelved because
it turned out to be impractical and too costly to refuel them in orbit.
However, with water as "fuel" it is seen that very cheap launches will
put cans of water into very low orbits for the tugs to pick up.
Refuelling the tugs from Earth is
only a temporary measure. Once water
arrives from asteroids, this costly launch of plain simple water will
be replaced. For this reason tugs only have to pay for themselves until
asteroid processing has begun. First material to arrive from asteroids
in LEO is expected to be water.
How to avoid snaking lines of fuel, search for sockets on a moving tug,
pumping in zero-G, etc? The concept of "water cartridges" evolved:
These are cans of water that are specifically designed to be picked up
by grapples of the tug and stuck into predesigned sockets on the tug
where they feed into the fuel lines to the METs. Empty cartridges are
pulled out by the tug itself and stuck into refilling sockets on the
fuel depot. It is expected that future satellites will refuel also by
simple
exchange of cartridges.
|
Attitude control
Fuel mass is by far the most expensive operational item. Therefore all
must be done to increase fuel efficiency. Large gyroscopes in the tug
shall therefore take care of attitude control instead of directional
thrusters. This is not trivial due to the large masses attached to the
tug. In order to save on the need for turning the whole orientation,
the tug will be doublesided, with a back-and-forth capability of
boosting. This also allows some measure of redundancy and therefore
higher operational security.
Grappling
The operational requirements appear to demand two types of arms on the
tug:
- "Graspers":
Grapples that can grasp and reliably hold irregular
debris not designed for docking with the tug, ie. space junk, old
satellites, etc.
- "Twiddlers":
Arms reminiscent of industrial robot arms with
selectable toolheads. They are for doing specific work on whatever is
being grasped.
Control of these arms is expected to be via telerobotics from Earth,
ie. adaptations from "Robonautics".
Energy
Electricity is required for the manipulative operations, for spinning
up the gyroscopes for attitude control, and particularly for firing the
METs. Energy will be supplied through PV panels strategically placed
around the tug.
Batteries need to take care of operations during night side of orbits.
Boosting with the METs may require to always wait until the tug is
again in the sun.
|
| Overall
concept |
|
The below
illustration
LEOtug4_3Annotaed.jpg is the state of concept by the time this
synthesis was written. The figure does not depict what will finally fly
in orbit. It is just a sounding board to test ideas and identify
clashes and test possible solutions in configurations. It has also been
instrumental in identifying structural challenges. The figure is
therefore in continuous change and thereby reflects the ongoing debate
on searching for solutions.
The figure depicts the tug with one "hopper" firmly docked on one side
and approaching another, with a robotic "twiddler" arm extended to lock
with sockets on the target. "Hopper" here refers to containers arriving
from asteroids with semiprocessed mixed asteroid material for further
processing at
the base. Instead of hoppers also think of cargo cans from Earth, space
junk, or satellites.
The tug is two-sided with identical interfaces on both ends.
|
Graspers are here depicted
using the template of hydraulic arms used on
construction machines on Earth. However, here they have a "thumb" that
can help in clasping an irregular object. Hydraulics may become useful
in case force has to be applied to really grasp well, or to crush some
debris.
Twiddlers are here adapted from industrial robots and also using some
ideas from the CanadArm on the International Space Station, in
particular the folding back of unused tools.
All the rest can be understood when reading the annotations on the
figure. The anaglyph (next figure) is for viewing with blue-red
glasses. This helps to understand the 3D relationships in the
structure.
Click on the figures to get a full sized
view. |
|
|
| Further
encountered challenges |
|
Cheap rockets
Initially the tugs will have to be refuelled by water cartridges
launched from Earth. This refuelling process will be cost-critical.
However it was recognized that launches of such cans of water into LEO
should be possible with rockets that can been designed to be
particularly cheap:
- They could be small, just big enough for whatever a
full cartridge weighs, but they can also be large.
- They only need to reach a very low orbit, such as
aiming for 220km by 350km.
- They may be unreliable, ie. a failed launch does not
have to pay for the lost payload, since the payload is easily replaced.
- They can be quite imprecise, ie. undershoot the
insertion-apse by 20km - achieving 200km minimal height, and underboost
on the non-insertion apse by 100km, achieving 250km. This double
imprecision would result in a 200km by 250km orbit which is still
stable for several days and therefore good enough for a tug to pick up.
This cuts down on costs for avionics.
- Fairings or shrouds are not needed, as the can itself
is the fairing.
It is now clear that a tug only makes sense in combination with such
cheap rockets for very cheap payloads. The combination of launching
cheap rockets for easily replaced payloads with highly reliable rockets
for expensive payloads dovetails with the capability to assemble in
orbit in the "garage" of the base. There is then a cost-tradeoff
between assembly of spacecraft in space versus launching them fully
integrated from the launchpad on Earth.
So the orbital tug
can only been seen in conjuction with the cheap rocket that can send up
cans of water for refuelling the tug.
Longhaul tug
This tug may be adapted into an interplanetary longhaul tug (to
asteroids and back)
with minimal adaptations:
One end may hold a large tank of water for fuel, while the other end
holds the payload (hopper, crewed habitat on a spinner, etc... ).
Manipulative capabilities
Robonautics are expected to become very important. This will help in
coordinated movements of arms during grappling procedures etc. It is
also conceivable that small robonauts are attached to the ends of arms,
which would bring them into position to perform small detailed
manipulative work - similar to what we have at present when an
astronaut stands on the end of the CanadArm of ISS.
|
Bootstrapping the system
For bootstrapping such a system the following seems to suggest itself:
Phase 1: Proof of concept
1. Launch a fully fuelled miniature tug with enough manipulative
dexterity
into orbit.
2. Launch three fuel cartridges on one cheap rocket and have them
captured
by mini-tug.
3. Mini tug boosts the three full cartridges to orbit of future "base".
4. Mini tug refuels with one cartridge, leaving one empty cartridge
along
with the remaining two full ones, and is thereby fully refuelled for
next
operations.
Phase 2: Accumulation
Accumulate more and more fuel at base. Build a "cartridge rack" as
preliminary fuel depot for mini tug. Possibly launch further mini tugs.
Phase 3: Large tug
Launch a large tug in bits and pieces that can then be assembled by the
mini tugs at the base (through tele robonautics).
Phase 4: Begin regular operations
Begin to earn money by providing transfer services to satellites and
collecting space junk. Mini tugs may begin to concentrate on collecting
small space junk in the important orbital regions.
Begin to build base with "garage".
PV panels?
The configuration of panels around a tug are still very preliminary. It
could be that for long boosts it will be advisable to have large arrays
stuck out on booms and tracking the sun.
METs
Due to an interesting relationship between available electricity, total
mass and size of METs, there is a tradeoff between large METs and small
METs. Large METs provide strong thrusts with fast response of total
mass, something worth having during final approaches and rendezvous.
But for long burns small METs on longer burns may be advisable, since
the mass of the large METs can be saved. Analogue to tugs shoving
around large ocean ships, orbital tugs will
have large METs with little fuel. But longhaul tugs will have large
fuel tanks with relatively small METs.
More work needs to be done on METs to achieve space-worthiness on first
tugs. |
The
NEAmines group welcomes feedback on these preliminary designs.
We also
welcome people who may want to join the effort.
Please
surf to
http://www.asteroidmines.net
Contact sitemanager
|
