Cable-based space launch system

A rail gun is a simple electromagnetic motor that very rapidly accelerates a metal slug by using it as part of an electrical circuit. A strong magnetic field arises as the current passes through the slug, propelling it forwards.

An ‘inverse rail gun’ uses the same principle, but rather than a short slug, the force acts on a small section of a long cable, which continues to pass through the system. As that section passes through, another takes its place, passing on the force and acceleration to the remainder of the cable. That also means that each small section only has a short and tolerable time of extreme heating resulting from high current.

This can be used either to accelerate a cable, optionally with a payload on the end, or via Newtonian reaction, to drag a motor along a cable, the motor acting as a sled, accelerating all along the cable. If the cable is very long, high speeds could result in the vacuum of space. Since the motor is little more than a pair of conductive plates, it can easily be built into a simple spacecraft.

A suitable spacecraft could thus use a long length of this cable to accelerate to high speed for a long distance trip. Graphene being an excellent conductor as well as super-strong, it should be able to carry the high electric currents needed in the motor, and solar panels/capacitors along the way could provide it.

With such a simple structure, made from advanced materials, and with only linear electromagnetic forces involved, extreme speeds could be achieved.

A system could be made for trips to Mars for example. 10,000 tons of sufficiently strong graphene cable to accelerate a 2 ton craft at 5g could stretch 6.7M km through space, and at 5g acceleration (just about tolerable for trained astronauts), would get them to 800km/s at launch, in 4.6 hours. That’s fast enough to get to Mars in 5-12 days, depending where it is, plus a day each end to accelerate and decelerate, 7-14 days total.

10,000 tons is a lot of graphene by today’s standards, but we routinely use 10,000 tons of steel in shipbuilding, and future technology may well be capable of producing bulk carbon materials at acceptable cost (and there would be a healthy budget for a reusable Mars launch system). It’s less than a space elevator.

6.7M km is a huge distance, but space is pretty empty, and even with gravitation forces distorting the cable, the launch phase can be designed to straighten it. A shorter length of cable on the opposite side of an anchor (attached to a Moon tower, or a large mass at a Lagrange point) would be used to accelerate the spacecraft towards the launch end of the cable, at relatively low speed, say 100km/s, a 20 hour journey, and the deceleration phase of that trip applies significant force to the cable, helping to straighten and tension it for the launch immediately following. The craft would then accelerate along the cable, travel to Mars at high speed, and there would need to be an intercept system there to slow it. That could be a mirror of the launch system, or use alternative intercept equipment such as a folded graphene catcher (another blog).

Power requirements would peak at the very last moments, at a very high 80GW. Then again, this is not something we could build next year, so it should be considered in the context of a mature and still fast-developing space industry, and 800km/s is pretty fast, 0.27% of light speed, and that would make it perfect for asteroid defense systems too, so it has other ways to help cost in. Slower systems would have lower power requirements or longer cable could be used.

Some tricky maths is involved at every stage of the logistics, but no more than any other complex space trip. Overall, this would be a system that would be very long but relatively low in mass and well within scales of other human engineering.

So, I think it would be hard, but not too hard, and a system that could get people to Mars in literally a week or two would presumably be much favored over one that takes several months, albeit it comes with some serious physical stress at each end. So of course it needs work and I’ve only hinted superficially at solutions to some of the issues, but I think it offers potential.

On the down-side, the spaceship would have kinetic energy of 640TJ, comparable to a small nuke, and that was mainly limited by the 5g acceleration astronauts can cope with. Scaling up acceleration to 1000s of gs military levels could make weapons comparable to our largest nukes.

Mars trips won’t have to take months

It is exciting seeing the resurgence in interest in space travel, especially the prospect that Mars trips are looking increasingly feasible. Every year, far-future projects come a year closer. Mars has been on the agenda for decades, but now the tech needed is coming over the horizon.

You’ve probably already read about Elon Musk’s SpaceX plans, so I won’t bother repeating them here. The first trips will be dangerous but the passengers on the first successful trip will get to go down in history as the first human Mars visitors. That prospect of lasting fame and a place in history plus the actual experience and excitement of doing the trip will add up to more than enough reward to tempt lots of people to join the queue to be considered. A lucky and elite few will eventually land there. Some might stay as the first colonists. It won’t be long after that before the first babies are born on Mars, and their names will certainly be remembered, the first true Martians.

I am optimistic that the costs and travel times involved in getting to Mars can be reduced enormously. Today’s space travel relies on rockets, but my own invention, the Pythagoras Sling, could reduce the costs of getting materials and people to orbit by a factor of 50 or 100 compared the SpaceX rockets, which already are far cheaper than NASA’s. A system introduction paper can be downloaded from:

Click to access pythagoras-sling-article.pdf

Sadly, in spite of obviously being far more feasible and shorter term than a space elevator, we have not yet been able to get our paper published in a space journal so that is the only source so far.

This picture shows one implementation for non-human payloads, but tape length and scale could be increased to allow low-g human launches some day, or more likely, early systems would allow space-based anchors to be built with different launch architecture for human payloads.

The Sling needs graphene tape, a couple of parachutes or a floating drag platform and a magnetic drive to pull the tape, using standard linear motor principles as used in linear induction motors and rail guns. The tape is simply attached to the rocket and pulled through two high altitude anchors attached to the platforms or parachutes. Here is a pic of the tape drive designed for another use, but the principle is the same. Rail gun technology works well today, and could easily be adapted into this inverse form to drive a suitably engineered tape at incredible speed.

All the components are reusable, but shouldn’t cost much compared to heavy rockets anyway. The required parachutes exist today, but we don’t have graphene tape or the motor to pull it yet. As space industry continues to develop, these will come. The Space Elevator will need millions of tons of graphene, the Sling only needs around 100 kilograms so will certainly be possible decades before a space elevator. The sling configuration can achieve full orbital speeds for payloads using only electrical energy at the ground, so is also much less environmentally damaging than rocketry.

Using tech such as the Sling, material can be put into orbit to make space stations and development factories for all sorts of space activity. One project that I would put high on the priority list would be another tape-pulling launch system, early architecture suggestion here:.

Since it will be in space, laying tape out in a long line would be no real problem, even millions of kms, and with motors arranged periodically along the length, a long tape pointed in the right direction could launch a payload towards a Mars interception system at extreme speeds. We need to think big, since the distances traveled will be big. A launch system weighing 40,000 tons would be large scale engineering but not exceptional, and although graphene today is very expensive as with any novel material, it will become much cheaper as manufacturing technology catches up (if the graphene filament print heads I suggest work as I hope, graphene filament could be made at 200m/s and woven into yarn by a spinneret as it emerges from multiple heads). In the following pics, carbon atoms are fed through nanotubes with the right timing, speed and charges to combine into graphene as they emerge. The second pic shows why the nanotubes need to be tilted towards each other since otherwise the molecular geometry doesn’t work, and this requirement limits the heads to make thin filaments with just two or three carbon rings wide. The second pic mentions carbon foam, which would be perfect to make stratospheric floating platforms as an alternative to using parachutes in the Sling system.

Graphene filament head, ejects graphene filament at 200m/s.

A large ship is of that magnitude, as are some building or bridges. Such a launch system would allow people to get to Mars in 5-12 days, and payloads of g-force tolerant supplies such as water could be sent to arrive in a day. The intercept system at the Mars end would need to be of similar size to catch and decelerate the payload into Mars orbit. The systems at both ends can be designed to be used for launch or intercept as needed.

I’ve been a systems engineer for 36 years and a futurologist for 27 of those. The system solutions I propose should work if there is no better solution available, but since we’re talking about the far future, it is far more likely that better systems will be invented by smarter engineers or AIs by the time we’re ready to use them. Rocketry will probably get us through to the 2040s but after that, I believe these solutions can be made real and Mars trips after that could become quite routine. I present these solutions as proof that the problems can be solved, by showing that potential solutions already exist. As a futurologist, all I really care about is that someone will be able to do it somehow.

 

So, there really is no need to think in terms of months of travel each way, we should think of rapid supply chains and human travel times around a week or two – not so different from the first US immigrants from Europe.