Solar Thruster Sailor (STS)

- Đ Frank Ellinghaus -

I N D E X - G E R M A N

POST & AUTHOR & Vision
STS - Solar Thruster Sailor
RSS - Ring Segment System LTH - Launcher Transport Head EFO - Experimental Flying Object RSC - Rotational Slingshot Catapult

main - Outer Ring - Inner Ring - comparison - mass - Stringing Net - advantages - Fig.1 - thruster - Fig.2a-2b - Fig.2c - Fig.2d - Fig.3a-3b - Fig.8b - Fig.4a-4c - Fig.5 - Mini STS - Mission Proposal - Micro STS - new parts in red updated March 26 / 2006

Main Structure of the Solar Thruster Sailor solar sail spacecraft

The main structure of the Solar Thruster Sailor (STS) is a solar sail inside a flat, disk shaped Thruster Ring Spacecraft were the Outer Ring is carrying the sail, the ( electric ) thrusters and a stringing net which serves as means to support the sail and and take on payload like solar cells and also holds a docking and payload carrying station ( the Inner Ring ) in itīs center.

Newest development:

The design showīn on Fig. 1 underneath is the basic design of a highly steerable spacecraft through the combination of solar sail propulsion and thrusters. This design has evolved further through Using roller reefing for fuelless steering, station keeping and attitude control, - additional mobile dockingable thruster units for STS-spacecrafts.
Fig.13 - Fig.14-15 - Fig.16 Mobile Thruster Unit - Fig.17 - Fig.18 Fuelless Steering -

As an alternative to the STS which has to get constructed in space, take a look onto Fig.19-21d - The Solar Sail Launch System, which uses also roller reefing for fuelless steering, station keeping and attitude control, carries a docking and payload station, ample solar cell arrays and additional thruster units.

The main purposes of such an STS would be
  1. to serve as a carrier for fleets of saucer shaped Thruster Ring Space crafts and satellites as daughter units.
  2. to serve as a Container Carrier Ship ( freighter ) for fleets of flying containers as daughter units.
  3. to serve as a Carrier ship for asteroid landers and docking able equipment units.
  4. to serve as a Carrier ship for mobile space processing and manufacturing utilities and equipment.
  5. to serve as a Mobile Solar Power Station ( MOPS ).
  6. to serve as a Space Tug.
  7. to serve as a mobile carrier unit for space based telescopes, radar / broadcast stations, mirrors, lenses.
The design is very versatile and enables many uses for several different space based reusable spacecrafts which through the option to use their thrusters as main propulsion donīt have to carry a sail at all. Since the base design is always the same - (the outer thruster ring with a net of structure and payload carrying threads or belts) converting or retooling for instance a swarm carrier ship into a Space Tug or into a Mobile Power Satellite is very easy and often would only require to exchange daughter units which can dock in and out by remote. Unlike todays deployable solar sail designs which waste the valuable center of the sail craft to carry breakage prone mast and thruster combinations for steerage (were and how will payload get placed?), the STS-design reserves this premium space for docking and payload handling.

This enables for secure handling of the center payload through low thrust electric thrusters which are strong enough to turn the sail through the large lever which their location on the Outer Ring provides. Moreover the pairwise placement of thrusters on the Outer Ring enables acceleration and deceleration of a Thruster Ring Spacecraft without moving parts and without having to turn the craft for deceleration. That gives the STS an unprecedented high level of smooth steerage which no other solar sail design provides. This high level of steer ability is only possible through low thrust steerage propulsion devices.

The Solar Thruster Sailor as a mother carrier ship provides cheap transportation through the possibility to hold a huge free photonic thrust producing sail (and therefor saving fuel mass and costs) with the ability to carry a lot of payload combined with the secure docking ability for also highly steerable Thruster-Ring daughter units.

This is done mainly through combining Low Tech KISS (keep it simple stupid) lightweight and few parts spacecraft design with High Tech materials and High Tech but proven electric thruster technology.

In conjunction with authomatic - remote controlled docking actions for similar shaped daughter units and with authomatic remote controlled payload handling the carrier-/daughter-units design allows to serve all space based transportation needs in the inner Solar system - from Sun to Mars and short beyond Mars without involving humans in space. This is a huge cost and security advantage for transportation, processing and manufacturing in space - not having to take care for humans and their life support and yet getting things done like asteroid mining for instance.

That does not mean, the author prefers not to send humans into space. I would like to get things done for less, getting infrastructure into space first and sending out humans later when we have already the ability to transport and process space materials cheaply and to produce and spread energy were needed.

Since light pressure is a very weak force it is essential to get extremely large solar sails and yet light construction for appropriate photonic propulsion.

This is possible through the Ring Segment System ( RSS ) which enables simple and fast mounting of very large structures using a lot of ring segments from carbon fibre reinforced light weight plastic pipes. The pipes are just sticked together in space.

The Outer Ring

The Outer Ring provides the stability of the spacecraft structure and holds sail, thrusters and payload stringing or belts. Through the RSS the sail does NOT need a self inflating boom! In fact, it does not need a boom at all. All the hassles with inflating the structure and possible instability donīt have to be.

Think about it, with an outer ring holding several thrusters each sitting on a large lever, because they are located on the structures edge, youīll get optimal steerage. The ring does provide also lots of room for payload with a net of belts or stringing inside.

In contrast if you have a huge solar sail with a boom. How large could the boom get with all the load dragging at the structures center? You will have a stress point on the masts foot. So the solution would be a short mast, but that means weak steerage for such a big sail. The boom sail may be of use for smaller crafts but what if it getīs bigger?

The STS sail through itīs outer ring will have at least two thrusters working together on opposite sides of the ring to turn the craft - each pushing on a large lever, the levers getting bigger with the sails size. So it is possible to use the weak but fuel effective ion thrusters for steerage even for gigantic structures. Result - no stress point, no self employing needed and optimum control.


If I write about large structures, I am thinking of square kilometers!
"... for example, a 2 x 2 km square sail of mass 19200 kg can deliver a 32000 kg payload to Mars orbit in 4.2 years from an initial Earth parking orbit. The solar sail can then return to Earth in 2 years to be loaded with another payload for delivery to Mars." (Source: Solar Sailing, Technology, Dynamics and Mission Applications, Colin R. McInnes, page 22).

Can we compare the 4 square-km structure above with a Thruster Ring Spacecraft of the same sail area?

Colin Mc. Innes does not specify how he came to those 19200 kg for the square sail mass. The most critical point seems to be the making of the sail cloth. If we would use the rather conservative specific weight of 7 g/mē a four square kilometer sail would amount already for 28000 kg for the sail cloth alone. Structural members, bus, other parts and payload not included.
So he must have used more optimistic numbers for much thinner sails. As I donīt know the numbers Colin used, I just canīt compare the masses of the square solar sail with the Solar Thruster Sailor (STS). But what I can do is trying to find out in my eyes realistic mass numbers for a 4 square kilometer STS based on already proven technology for very thin sail cloth making. This thoughts are based on the assumption that the STS structure with the sail supporting net allows to get a much thinner sail cloth onto the sail structure than onto a mast based square sail.


Mass of a round Solar Thruster Sailor with Outer Thruster Ring, supporting net and Inner Docking / and Payload handling Station.
The Outer Ring made of CNT reinforced plastics has a diameter of about 2260 m, the Inner Ring Magazine Dock has a diameter of 30 m with a payload / docking space of at least 21214 cubic meters when the space 15 m above and 15 m under the Inner Ring is used. This sail would produce a constant thrust of 36 N at Earth level, 144 N of constant thrust when about 0.5 AU away from the Sun. Wouldnīt this craft be well suited to transport materials and fuel for Mars exploration to Earth orbit - mined and processed at asteroids in the vicinity of Sun were a lot of power for both (transporting and processing) is available?

The mass for a 4 sqkm sail for a round Solar Thruster Sailor I found in this " comparison " is about 13812 kg

- Payload and solar cell arrays are not included.

  • Outer Ring
  • = 3566 kg
  • Inner Ring
  • = 100 kg
  • Stringing Net
  • = 4850 kg
  • Sailcloth with a thin substrate
  • = 5200 kg
  • Electric Thruster propulsion steering including fuel
  • = 96 kg
    13812 kg

    If assuming only a small payload a sail of about 15000 kg could amount for a solar sail loading of 3.75 g / mē. That is not so far away from the loading of 1.55 g / mē which is necessary to reach the lightness number of "1" which means that a sail of that lightness number would be able to cancel our Sunīs gravitational pull - it could stop itīs rotation around Sun and not fall into it.
    Since the loads onto a sail in a free fall or low gravity environment are rather flimsy we could shave substantial more weight in keeping the thread thinner or increase the field size of the stringing, use a smaller diameter for the rings and/or thinner pipe walls. When it would be possible to skip the sail cloth substrate also, a solar sail lightness number of less than 1 seems to be feasible.

    How did I get those numbers?

    The pipe Rings:

    Compressed Volume of a pipe ring is d1 x pi x wall thickness x length of the wall which is d2 x pi were d1 is tube diameter (8 cm) and d2 is ring diameter (2260 m) for the Outer Ring.

    The compressed mass volume of the Outer Ring is about 0.08 m x 3.14 x 2260 m x 3.14 x 0.001 m = 1.783 mģ.
    Those 1.783 mģ for the Outer Ring have a weight of about 3566 kg when using 2 g/cmģ for CNT reinforced plastics.

    The compressed mass volume of the Inner Ring plus four 30-m support pipes as a magazine docking station is
    (0.08 m x 3.14 x 30 m x 3.14 x 0.001 m) + (0.08 m x 3.14 x 30 m x 4 x 0.001 m) = about 0.054 mģ.
    The Inner Ring plus support pipes would just account for a mass of 108 kg.

    Stringing Net

    Now we have to add something what holds the inner Magazine Docking Station tight in the outer ring and supports the sail at the same time. I choosed a stringing net (like at a Badminton racket) of SpectraĐ2000 fibers which are 10 times stronger than steel (for instance thought also for space tethers) with a terrific low specific weight of 0.97 g/ccm.

    The net supports the sail cloth keeping it pretty flat and therefore avoiding the application of tension forces to the edges of the sail and can carry also payload like solar cells which are placed around the Inner Ring. The stringing got square fields of 50 cm size what amounts to about 10 Mio. meters of fiber threads with a thickness area of 0.5 mmē. Because of the immense strength of the fibers the net can take on a lot of load. It would have a weight of about 4850 kg. Enough for mission designers to look for some more weight to save here.

    Than we have to add the sail itself.
    Through the net the sail cloth can get a lot thinner than those used for self deploying sails. The most mass adding component is the substrate which getīs a coating with a suitable metallic reflector. Since the plastic substrate is only used in favor to support a pure much thinner ( mostly aluminium ) coating, it could even be possible to skip the substrate or let it vaporise when the sail is already set. But in this example I choosed a substrate of KaptonĐ film which can get etched down as thin to provide a area density of 1 g/mē (Mc. Innes, Solar Sailing, page 62) along with an aluminium coating of about 0.27 g/mē and a much thinner coating on the backside a high-emissity chromium coating for thermal control through re-radiation.
    When assuming 1.3 g/mē the whole sail cloth of the solar sail would have a mass of about 5200 kg.

    Those 4850 kg invested into the supporting net help to use a much thinner sail of 5200 kg instead of 26000 kg when using conservative 7/g per mē sail cloth. They also help to avoid other structural components like masts, booms and struts. And they help to hold a docking station in the sails center which enables properties like docking and payload handling - just what is needed for the main purpose of a huge solar sail - to serve as a carrier- or mother ship.

    For the Thruster steering unit I choosed 24 double thruster units, each carrying 2 Russian SPT-60 electric thrusters which have a weight of about 1.3 kg each, 1200 g of fuel for each double unit and 200 g structural materials. So we get (1.400 kg x 24) + (1.3 kg x 48) = 96 kg for the thruster propulsion.

    Which advantages does the Solar Thruster Sailor have?

    Fig. 1 - Solar Thruster Sailor

    Fig. 1 shows the main design of the solar thruster sailor using belts and a payload platform in the center. I have now changed my mind and prefer a stringing net instead of the belts for weight reasons and a docking and payload station in form of an Inner Ring construction.

    Main elements of fig 1 are thruster-ring "1.", solar sail and solar cells with carbon belts stretched within "2", carrying a (carbon) payload-platform in the middle "3". Thruster units 1.5 are for moving sideway or for right - left- rotation around the pole. Thruster units 1.6 are for up - downwards propulsion or for rotation around the diameter.

    Instead of the payload platform 3 an Inner Ring as a docking and payload handling station can be used, to carry swarms of spacecrafts and satellites and to handle payload and even big objects using winch devices, so that the solar sail becomes a huge space tug.

    Fig. 1, STS with thruster-ring and solar sail

    Ion thruster steering

    How could little ion thrusters with their weak force steer a space craft of square kilometer size?
    There are at least two thrusters working together at the same time, each mounted on the opposite side of the thruster ring and each pushing a large lever. This lever getīs bigger with increasing size of the craft.

    Fig. 2.a and Fig. 2.b - Thruster Units up - down and right - left

    Fig. 2.a, Fig. 2.b

    Thruster units, parts 1.9 are at the same time brackets and tank units for xenon-gas. Parts 1.10 are jet tubes consisting of two thrusters with outlet to opposed directions.

    Fig. 2.c - Thruster Unit - Installation

    Fig. 2.c

    The two splints of the bracket 1.9 run completely through the segment-pipes and are bolted onto them with counter nuts. The splints are also conductors for the electric power yielded from the solar-cells near the thruster-unit. Half-pipes (could be from metal) 1.9.2 enfold the segment pipes to give this joints some more strength since they have to resist heavy rotational forces.

    Fig. 2.d - Double Thruster Unit with two typical ion thrusters.

    Fig. 2.d, double thruster unit with typical ion thrusters

    The bracket for the unit is at the same time tank for xenon gas

    Fig. 3.a - 3.b - (carbon) belts bolted to the pipe segments

    Fig. 3.a - Fig. 3.b - (carbon) belts bolted to the pipe segments

    As simple as it can get. If you have one plate with inbuilt screws and the other plate with holes there is not much work to do to mount the belt.

    The Magazine Docking station - held in the center of the craft enabling docking and payload handling.

    Fig. 4.a, Fig. 4.b, Fig. 4.c - Launcher Transport Head (LTH) Getting the pipe segments into space

    Fig. 4.c

    Launcher Transport Head (LTH)
    pipes transported outside of the launcher

    Fig. 4.a

    one layer LTH from above

    Fig. 4.b

    multiple layer LTH from above

    Fig4c Fig4a

    If the upper stage of the launcher has a diameter of 4 m and the pipe segments a diameter of about 6 cm, it is possible to transport up to 200 pipes. Given a length of about 15 m per segment youīll get a perimeter of 3000 m from one layer. The compressed mass volume of this layer is about 0,6 cubic meter of lightweight material. Not a weight to worry about for a big solar sailor-structure.


    A multilayer LTH could transport pipes for very large ring structures.

    to the next Fig.5 -

    Mini STS

    The Mini STS is a little solar sail based on the Solar Thruster Sailor (STS) design steered with the help of ion thrusters. It getīs transported to itīs near sun orbit using a mothership or with the Rotational Slingshot Catapult (RSC). This craft could even land on asteroids with low gravitation using the ion thrusters in this case as main propulsion and the thruster brackets as landing feet.

    How could a small solar sail work at all carrying relative heavy ion thrusters too? Shouldnīt it have a pretty big sail area because of the low force of the light pressure?
    This is correct - in principle, but as you get nearer to the sun as stronger getīs the photonic force also. At 0,25 AU this force should be 16 fold stronger than at near earth position!

    A solar sail of about 10 m diameter (about 78,5 square metre area) could gain the force of a 1256 square metre solar sail on near earth position. This would be considerable more than the area (and the force available) of the 30 m diameter Cosmos 1 solar sail of the Planetary Society, hopefully to be launched soon (written on Dec. 2002). Building and launching such a lightweight STS to LEO would be a special offer compared with the costs (if possible at all) of missions to and around the sun with other propulsion technics. Planning and installing the Slingshot Catapult at LEO would be more demanding but this could be a one time investment for a multitude of cheap missions enabling a cluster installation of near sun satellites, with many fly by observations of venus and mercury.

    In contrast to conventional satellites this Mini STS would be able to fully navigate for a long time, spiraling towards the sun through positive declination and changing direction spiraling away from the sun through negative declination. The ion thrusters would have to do just this one turn to steer the craft when changing direction hence staying functional for a long time of operation since they need only sparse fuel.

    Mission Proposal for a Mini STS- Asteroid Sample Return

    using Mini-STS Solar Thruster Sailors (solar sails which are compounding solar sail with ion thruster propulsion) to retrieve samples of asteroid regolith to land on one or several asteroids, drill asteroidal sample cores and bringing them back to earth or to the ISS for analysation. One craft would do all this tasks, no other equipment would be needed.

    The Mini-STS ion thrusters will enable the craft to navigate like a lander. Better yet itīs special structure (and the micro gravity environment on asteroids) allows to hover for finding sampling places or to pitch up and down without turning the craft. While even a very heavy Prospector equipment would get in trouble not to throw itself off the asteroid when pressing to remove samples off the ground, the Mini-STS can do that without problems. That is because itīs thrusters can press the craft down while drilling or removing samples. If you take a look on Fig. 2.a youīll see why the craft can change movement into the opposite direction or press itself down to the ground. Each thruster is located in a jet tube together with a twin thruster firing into the opposite direction when needed. Each thruster pair has an identical twin pair on the other side of the thruster ring for symmetric reasons. In fig 1 the up- down-pressing thruster pair is designated with the part number 1.6.

    The favoured asteroids would be located very close up to 0,25 AU to the sun, since solar flux for photonic propulsion and solar electric energy is about 16 times as strong there as near earth. So the Mini-STS sample prospectors could be made pretty small. To get the same propulsion force as a 30 m near earth sail the prospectors would only need 7.5 m diameter (or about 44 square metres sail area) even so they could be faster than the 30 m sail at earth level because they would be smaller and lighter having the same force available.

    On fig.5 you see a mothership with 6 smaller sails thought for sample prospecting. The mother ship is for space usage only, it has to transport the prospector sails to asteroids letting all of them or only one ore two off at the first place of action. Letting a smaller sail off would happen through tourning the mother sails backside (were the payload would be located) to sun. The sun pressure would push the sails away. Since the mother sail with itīs lower specific weight and bigger sail area would accelerate faster than the smaller prospectors it could be possible to separate without using the thrusters. The mothershipīs size is 135 m diameter, - about 14320 square metres sail area. The four smallest of the daughter sails in the example would be 30 m (100 ft) diameter - about 707 square metres sail area. I did chose this size because it is about the same as the size of Cosmos 1 the first solar sail hopefully getting into space and showing that solar sailing is feasible.

    If Cosmos 1 would be able to prove the feasibility of solar sailing, perhaps even sailing out of earthīs gravity well, than my prospector crafts which feature also ion thrusters besides sail propulsion would be able to navigate near earth as well, even with some more payload when getting back from their asteroid missions. When the prospectors have drilled their samples they could get off the asteroid with the help of their ion thrusters and accelerate with suns light pressure pushing the sail with high force. So they would not need to accelerate that long because at the end of the journey a longer deceleration phase is needed because of the weaker sailing force near earth. But perhaps some gravity assists from moon or earth could help with the deceleration task also.

    Near earth the prospectors could deliver their samples to the ISS or some other sort of space station or landing on earth using the sail as a parachute. How could the rather "flimsy" sail serve as a parachute? Since those prospectors would need only to return of a location with very high light pressure it would be possible to use strong enough sail cloth for parachuting. The added weight should be no problem in this case.

    So, is this a feasible task? What would YOU think.
    I do appreciate any commentaries on this subject. Thank you. Frank And here are some links for robotic sample returns from the moon which has be done the first time in 1970.

    Taking samples, the first step to space mining operations

    Luna 16 first robotic space probe landing on moon and returning samples of moon regolith to earth in 1970

    Also pictures and vrml-files of Lunar 16, 20 and 24 sample return capsules.

    "The drill was deployed and penetrated to a depth of 35 cm before encountering hard rock or large fragments of rock. The column of regolith in the drill tube was then transferred to the soil sample container. After 26 hours and 25 minutes on the lunar surface, the ascent stage, with the hermetically sealed soil sample container, lifted off from the Moon carrying 101 grams of collected material...".

    among others pictures and vrml-files of Lunar 16 and lunar soil samples

    Mission proposal for sample return from asteroids

    using a Mini-STS solar thruster sailor (solar sail which is able to compound solar sail with ion thruster propulsion).

    Micro STS

    The Micro STS is a little solar sail based on the Solar Thruster Sailor (STS) design steered with the help of micro- or digital thrusters. Since the lower weight of the thrusters would be substantial this solar sails could be even smaller than the Mini STS or they could move faster. Such small propulsion devices could be used in this case because they are only needed to turn the (ultra light) sail. Through weight placement (all heavier parts should be placed as near to the center as possible) and force lever (the steering propulsion devices move a large lever from the outer ring to the center) they are able to steer the craft.

    main - Outer Ring - Inner Ring - comparison - mass - Stringing Net - advantages - Fig.1 - thruster - Fig.2a-2b - Fig.2c - Fig.2d - Fig.3a-3b - Fig.8b - Fig.4a-4c - Fig.5 - Mini STS - Mission Proposal - Micro STS - new parts in red updated June 26 / 2005

    I N D E X - G E R M A N

    POST & AUTHOR & Vision
    STS - Solar Thruster Sailor
    RSS - Ring Segment System LTH - Launcher Transport Head EFO - Experimental Flying Object RSC - Rotational Slingshot Catapult