Saturday 9 December 2023

The Lofstrom Loop: A Bridge to Space

Imagine you could take a train ride to space. Tracks that slope up into the sky, higher and higher, until you reach a plateau above the planet where it’s a straight line up to orbital velocity.

That’s what’s possible with a Lofstrom Loop. But sending you into orbit is just one of the things it can do!

The mechanics of a Lofstrom Launch Loop (presented here) are simple and straightforward but it is the implementation of each of its parts that is difficult. 

Let’s start with what we have today: rocket launch.

A see-through SLS rocket revealing how much of it is just propellant liquids.

Modern rocket engines are very effective at converting the chemical energy stored in their fuel and oxidizer into heat. Large expansion nozzles then do a decent job at turning that heat energy into thrust to accelerate a spacecraft. However, most of a launch vehicle today is propellant, not payload. And nearly all of that propellant is spent accelerating the rest of the propellant. A vehicle like the Falcon 9 FT is 4% payload by mass. In other words, it is wasting 96% of its energy on liftoff.

Ideally you want to spend energy on accelerating only the payload and nothing else. You could do this by putting the payload on a track and pushing it along with magnets, like a train. A design like the Maglev, which can be thought of as an electric motor unrolled into a long line, can reach theoretically unlimited velocities with great efficiency. 

Japan's 603 km/h Maglev train.

However, there’s the problem of drag. A train on the ground has to push air out of its way. That’s the main source of energy losses and the reason why it has a top speed. It’s a problem that cannot be solved with just More Power: at high enough speeds, the air doesn’t get out of the way fast enough and instead compresses in front of the train. Aerodynamic heating is dangerous - at Mach 2.5, it becomes dangerous for aluminium… at orbital velocity (equivalent to Mach 25), it is enough to vaporize every material in existence.

There are ways around that issue, like enclosing the train in a vacuum tube or equipping it with an enormous heatshield. These are difficult and expensive options. 

StarTram envisaged a shuttle accelerated within an enclosed vacuum tube from the ground.

It’s best to get rid of the drag problem altogether. A train raised out of the atmosphere can accelerate to any speed on a long enough track. But how do you lift tracks up to an altitude of 80 km or higher and keep them there? You can’t attach them to balloons. There’s no propeller or wing that can generate lift up there either.

An 80 km tall pillar of steel won’t work either; it would have to be shaped more like a pyramid and weigh several billions of tons. That's due to the specific strength limits of construction materials.

The solution is a dynamic support structure; held up not by the strength of its materials but by the momentum of a high velocity rotor.   

Dynamic Support Structures

The Space Fountain is a simple early concept for a dynamically supported structure.

This is the key component of the Lofstrom Loop. Dynamic support is the only way to build things over 80 km tall without mobilizing mountains of resources. That’s because we do not have materials that are strong enough. Conventional structures like a pillar have a limit: a maximum height beyond which the materials it is made of cannot be stacked any further without collapsing under their own weight.  

That height limit depends on the ratio of their mechanical strength to their density, divided by the acceleration of gravity. A traditional building material like bricks has a compressive strength of 20 MegaPascals and a density of 2000 kg/m^3. A pillar of bricks can only go up to a height of around 1000 meters before the lowest layer breaks. Construction steel is better, with a strength of 350 MPa and a density of 8000 kg/m^3. It can be stacked to an altitude of 4460 m. That’s still very far from 80 kilometers of altitude. 

Wikipedia's table of specific tensile strengths.

To go further, we need to use shapes that get wider at the base, like a pyramid, or use materials that are extremely strong but also very light. Neither is a good option.

A gigantic 80 km tall pyramid, large enough to support a train track at its peak, would need several hundreds of billions of tons of steel. A structure made of a super-material like carbon nanotubes would be much slimmer, but also cost trillions to make because the material is so expensive. 

14 cm long carbon nanotubes are considered 'ultra-long'. We would need several thousands of km of them.

A radically different structure is necessary, and that’s where the dynamic support structure comes in. It’s actually easy to understand. You’ve worked with one before without realizing it. Have you ever played with a water hose?

Water in fountains following parabolic arcs.

If you turn the tap all the way open and point the nozzle up, you can make tall arcs of water. Push your hand into the stream and you will feel a force lifting it up and away. That force is due to the momentum of the moving water resisting your attempt to bend its trajectory. 

Your hand takes the place of this 'deflection plate'.

A dynamic structure relies on that same principle to raise a platform up to the sky. Instead of water, we’ll use a rotor, which is a metal cable going around in loops inside a tube.

A basic dynamic support structure section. This is possible to build in a vacuum. Inside an atmosphere, we have to protect those moving masses from drag. 

The air in the tube is removed, creating a vacuum that reduces drag forces on the rotor to zero. Magnets surrounding the tube can also levitate the metal cable, so it does not have to slide against anything.

Cross-section of the rotor and vacuum sheath.

With no drag and no friction, we can accelerate the metal cable to incredible velocities and then not have to worry about it experiencing resistance or slowing down afterwards. Those incredible velocities can give a lightweight rotor a lot of momentum, which in turn means it takes an enormous amount of force to bend it. A dynamic support structure makes use of that enormous force to carry a load. 

Sketch of a bridge supported by moving masses.

To do that, we point the moving mass of iron inside its vacuum tube upwards. It will try to form an arc. Place a load at the top of that arc and it will flatten down until the load’s weight matches the forces resisting the bending of the rotor. That flat section is perfect for a train ride all the way to space. We call this sort of arrangement a Lofstrom Loop after its inventor (first presented in 1985). 

Increasing the load a rotor can support or the altitude it can reach is simply a case of making the rotor loop go faster. It can far surpass the performance of any solid material without becoming much heavier or more expensive.

There are disadvantages of course.

Dynamic structures require power. This may be acceptable for a space launch system when the electricity is replacing thousands of tons of propellant. It’s not suitable for a building or tower that normally stands for a century without maintenance. And even if costs were ignored, it is not reasonable to expect no loss-of-power or blackout events over the course of an entire century! 

They are not intrinsically self-correcting either. A regular structure made of solid materials withstands deformation from all directions. A loop will resist being bent in one direction but is fine with flopping over to the side. This exacerbates another problem: dynamic structures need active control.

Tacoma Narrows bridge is an infamous example of a structure lost to uncontrolled vibrations.

An iron rotor spinning inside a tube of magnets is not passively stable and will hit the walls unless its drift is corrected. The most dangerous event to look out for are certain vibrations that will self-amplify, creating waves in the rotor that grow until they strike the tube walls. 

And while dynamic structures are scalable, so they can be built and demonstrated using small loops first, they only have advantages over regular steel and concrete when reaching for extreme heights. Not 1 km, but several tens of kilometers above the ground. We rarely need to build that tall, so dynamic structures won’t often be practical to use. 

Lofstrom Loop

As stated above, the basic concept for how the launch loop operates is simple to understand. The implementation is the hard part.

Note that there are few technical details on how a Launch Loop would actually work outside of Lofstrom's publications, which include an earlier version and a later paper. In this post are included illustrations from both papers, but only numbers from the latter.

The Orions Arm depiction of a Launch Loop.

Lofstrom’s ‘Launch Loop’ uses an iron rotor travelling at 14,000 m/s. It’s a hollow tube only 5 cm in diameter with walls just 2.5mm thick. A meter length section of it would mass just under 3 kg. A vacuum sheath protects the rotor, with the vacuum maintained by pumps spaced every 10 km.

Another cross-section of the loop's sheath. 

Ferromagnetic levitation is achieved using a combination of permanent magnets and electromagnets. 40 megawatts of electricity are required to maintain their field strength. The currents running through these magnets generate heat, which needs to be dissipated using radiator fins along the sides. 

In total, the iron rotor weighs 15,600 tons. It runs a complete loop in around 5 minutes.

The iron rotor could be a ribbon, as presented in the earlier work by Lofstrom.

Additional equipment includes pumps to maintain vacuum conditions along the track, position sensors and high frequency electronic controls that can adjust the magnetic field strength to compensate for vibrations, magnetic instability, buckling and so on. 

Track weight. A lot of optimization is possible.

On average, the track mass is 7.1 kg/m. The weight of the track is less than the vertical force from the dynamic structure, so it can hold itself at any altitude without external support. Still, it is attached to the ground with stabilization cables that compensate for the push and pull of winds or other perturbations. A little bit of surplus lifting force pulls the stabilization cables taut and allows the track to carry the additional weight of a train on top without bending. 

Lofstrom adds three more components to complete the launch loop: an elevator, deflection stations and a rotor motor.

Pulley elevator at 80 km altitude.

The elevator carries a payload (a train) vertically up to the top of the track. It is a simple set of pulleys connected to the flat part at the top of the loop. Lightweight, yet sized to lift several tons at Mach 1 so that the 80 km climb takes less than 4 minutes. An electric motor on the ground moves the elevator. 600 tons per hour costs 130 MW of electricity from the motor. 

Deflection stations turn the loop around at either end.

One version of Lofstrom's deflection stations.

They have to take the momentum of the hypervelocity rotor and turn it around 180 degrees… a difficult task! To manage it, they use a 28 km wide semicircle of magnets. 100 megawatts are consumed here to produce strong enough magnetic fields to bend the iron rotor. Each station has 5000 tons of mass and has to be well anchored into the ground. They form the most significant pieces of surface infrastructure.

The final piece is a large electric linear motor, 10 km long. It constantly pulls on the iron rotor to increase its velocity, which is necessary to compensate for the small drag losses the rotor incurs as it runs around the loop.

A linear motor as an 'unwrapped' rotary motor.

This comes from both magnetic field discontinuities slowing down the rotor and aerodynamic drag from residual gases inside the vacuum sheath. 60 MW of motor power are needed to overcome it all. As we will see in the next section, launching off the loop also takes away from the rotor’s momentum. The main task of the motor is actually to make up for the momentum lost with each launch. Lofstrom proposes an additional 300 MW of motor power to maintain a high launch rate of 48 payloads to Low Earth Orbit per day, with each payload massing 5 tons.

Lofstrom’s design slopes off the ground at 9 to 20 degrees to reach an altitude of 80 km. It creates a 2000 km long ‘launch track’ outside of most of the atmosphere. That length of track is used to accelerate payloads into space. 

Deploying the structure requires the motor and deflection stations to be online. The iron rotor is slowly brought up to speed - a process which may take weeks. As it rises off the surface, stabilizing cables are added until the whole track sits at 80 km altitude. 

Deployment can be sped up by providing more power to the linear motors. 

To summarize: the iron rotor travels at nearly twice orbital velocity, pulling itself and all attached structures off the ground. Magnets turn it around at each end of the loop. The rotor is held, levitating, inside a vacuum tube while a linear motor keeps it up to speed. It’s more accurate to imagine this structure as a ‘flying rope’ than something solid you can walk on. 

Altogether, the launch loop consumes 500 MW of power and delivers 240 tons to LEO per day. It has the capacity to send off payloads much more frequently, but it would require proportionally more power. For example, using 17,000 MW, the launch loop can send off 9600 tons per day to the Moon! 

If no launches are scheduled, it continues to consume 200 MW of power. Theoretically, rotor has enough momentum that it’ll continue gliding along for a long time, but an actual prolonged loss of power would end the active control that suppresses wobbles, vibrations and other shakes to keep it safe. 

The Launch

So how does a launch actually get done?

The key element here is the magnetic eddy currents that the fast-moving iron rotor can induce in a nearby magnet. If we place a row of magnets next to the launch track, it will be both repelled and dragged along by these eddy currents.

This is similar in principle to the electrodynamic suspension felt by a Maglev train passing over a track of magnets.

A 10 meter long ‘magnet rack’ passing over the Lofstrom Loop will feel 50 kN of lift force and 150 kN of drag force. This is enough to hold a 5 ton payload up against gravity while pulling it along at up to 3g of acceleration. 

As the payload accelerates, three things happen: the iron rotor slows down (a momentum transfer), the relative velocity between the payload and rotor decreases and the rotor gets deflected downwards (equal and opposite reaction to payload weight).

If left unattended, the payload will gain speed while losing lift, until it crashes into the rotor, destroying the Launch Loop. 

Instead, the Launch Loop must compensate for each effect. The linear motor on the ground can gradually bring the iron rotor back up to its initial velocity in between launches. The magnet rack has to continually get closer to the iron rotor to keep increasing the lift force as the relative velocity decreases. Magnets closer to each other feel stronger forces! 

The payload’s passage pushes down on the loop. The acceleration track falls by 1.2 m/s when a 5 ton weight passes over it.

The passage of payloads over the track make it look like they're surfing a giant wave.

Lofstrom proposes to release counterweights, timed exactly, to match that push and prevent the track from actually moving. 5% of the track mass would be dedicated to counterweights fired by solenoid coils. Simultaneously, the entire launch loop must release tension at the deflection stations to allow the track to stretch appropriately. 

A 5 ton payload accelerated at 3g down the whole 2000 km length of the acceleration track removes 735 GJ of kinetic energy from the iron rotor, slowing it down by 3.6 m/s at the start of acceleration and 14.3 m/s by the end.

Now, the payload design.

We have 5 tons to play with. Lofstrom places a pressurized passenger capsule on top of the 10 meter long magnet rack. Let’s call this the Space Train. In case of a launch abort or other failure, the Space Train simply falls off the acceleration track. It may be travelling at extreme velocity, so it needs a heatshield. Also, wings and a parachute to land on the ground (or more likely, splash down). In the back, a rocket engine is necessary to circularize the Space Train’s orbit with an apogee kick. The deltaV required to enter a circular Low Earth Orbit at 300 km altitude, after leaving the Launch Loop, is a mere 65 m/s. So the apogee kick engine can be very small. 

The Space Train can also have a ‘cargo’ configuration. This may hold 2-3 tons of useful cargo if it needs to be recovered, or nearly the whole 5 tons if it’s just given an unpressurized aerodynamic shell on top of the magnet rack. 

Launch performance is adjustable. Use the full or partial length of the track, use the full or partial strength of the magnet rack, to reach the desired final velocity.

At full 3g acceleration, the Space Train can reach 7450 m/s after accelerating for 943 km. That same velocity can be reached by using the full 2000 km of the track at a milder 1.41g. It’s enough to send the payload to a near-Low Earth Orbit trajectory with 300 km altitude. 

At 2.49g and using the full 2000 km, the payload can be accelerated to 9875 m/s. That is enough to reach a Geostationary orbit. The apogee kick deltaV necessary to stay in GEO increases to 1490 m/s. 

The maximum performance is 10,547 m/s. Passengers would have to experience an uncomfortable 3g for six minutes and a half. But this puts the Space Train on a trajectory to the Moon! DeltaV to circularize is 833 m/s.  

Here is a performance table:

M288 is a hypothetical space station in a High Earth Orbit. Note the option to shoot off payloads at 10g. This is not for passengers, but for insensitive stuff like food or rocket fuel. 

The main advantage of the Launch Loop is its ability to send useful payloads into any orbit around the Earth or Moon using only electricity. Higher launch rates make the process more efficient. At the 500 MW scale, 40% of the electricity is spent maintaining the rotor instead of accelerating payloads. At 17 GW, that load falls to 1.2%.

A launch to Low Earth Orbit (7451 m/s) adds 27.8 MJ of kinetic energy to each kilogram of the Space Train. At the 500 MW scale, 180 MJ of electricity is consumed on average for each kg launched. So the efficiency of the launch loop is roughly 15%.

A 10.5 km/s launch to the Moon represents 55.1 MJ/kg of kinetic energy. At the 17 GW scale, the launch loop spends 153 MJ for each kg sent to the Moon. Its efficiency is a much better 36%. 

As of February 2023, the average residential electricity rate in the U.S. is about 23 cents per kilowatt-hour. That translates to 6.39 cents per MJ. That means a Launch Loop can expect an energy cost of $11.5/kg at 500 MW, to $3.5/kg at 17 GW. That's much lower than current rocket launch prices exceeding $2350/kg for a Falcon 9 Heavy, or even the optimistic $100/kg goal for the SpaceX Starship.

The differences become even more stark when comparing between a Launch loop and a rocket headed beyond LEO. 

Of course, these figures do not include the cost of delivering electricity to some remote structure out over the ocean, other running costs like maintenance, or the overheads from paying back the construction of the whole thing. Still, they are a useful reference for how cheap a launch loop could make space travel.

For the environmentally conscious, the fact that all this electricity can be generated by solar panels or nuclear reactors on the ground instead of burning rocket fuels in the upper atmosphere is a major benefit. 

Getting Loopy

Lofstrom’s design for a Launch Loop is a massive structure of over 2000 km in length that needs to be placed near the equator. It has two slopes rising to 80 km altitude that need a large flight exclusion zone around them to prevent run-ins with aircraft. They also make it vulnerable to strong weather, like a hurricane. Preferably, it would be installed somewhere that doesn’t have to face such winds frequently.  

Here is a map of suitable locations:

The yellow bars show how large the Loop looks on a map. The squiggly tracks are the paths taken by violent storms.

All the locations are intentionally far from inhabited areas. That’s because if a Launch Loop fails, it can fail spectacularly. 

The structure can be severed by a collision. A payload crashing into the acceleration track, an airplane crashing into its slopes, a meteor striking the wrong spot, a ship running into the deflection stations… these scenarios can be made less likely but never completely ruled out. What happens afterwards depends on where the loop is cut.

A cut in the ascending slope would disgorge the iron rotor at 14 km/s.

It is travelling much faster than orbital velocity, in both forward and return directions. The iron would therefore sail out into space, escaping Earth’s gravity entirely, only catching the unlucky satellites passing overhead. The hollow vacuum tube that held the iron rotor would start to fall downstream of the cut. Since it would only mass around 5 kg per meter, it drifts to the surface harmlessly.

A cut in the high-altitude acceleration track would have a similar outcome.

The structure might recover from such an event, by rebuilding the cut section, inserting a new iron rotor and raising it back into place.

A cut in the descending slope is more dangerous.

It would point the 14 km/s iron rotor down at the ground. This turns it into a gigantic hypervelocity shotgun that continues firing for around five minutes. A zone extending for hundreds of kilometers east of the cut could be riddled with shrapnel. The deflection station at the foot of the slope might also get hit. That would make recovery much more difficult. 

A very unlikely but still possible event is a cut in the short section between the foot of the slopes and the deflection station would also result in a hypervelocity shotgun, but the damage would be contained to a relatively small area near the surface.

1500 TJ of kinetic energy would be released at that spot, deposited at a rate of 5 TW, so it would resemble a miniature nuke. 

Not all damage causes a catastrophic failure. 

Despite its impressive scale, the acceleration track is very thin and so narrow it would not be visible to the naked eye most of the time. That makes hitting it with anything rather unlikely. And if it does get hit, chances are the strike will create a hole in the vacuum tube rather than a full cut. The hole will fill with air. If it is a small hole or an opening at high altitude, the vacuum pumps will compensate for the inflow of air and maintain a good level of vacuum until repairs arrive. A large hole, or an opening created in the thicker air at low altitude, will fill with air quickly.

A 14 km/s iron rotor encountering air will start to experience drag, which doesn’t slow it down by much, but does create heat. If the rotor heats up to 1000 K, the Curie temperature of iron, it will lose its magnetic properties and crash into the tube walls. Thankfully, this takes a very long time to happen and a sea-level air breach would not be ignored for long. 

This brings us to thermal limits. The biggest cause of iron rotor heating is the launch of a payload. A full 2000 km launch with 3g acceleration and 5 ton payload raises the rotor temperature by 84 Kelvin.

At 900 K, the rotor radiates away enough heat through blackbody radiation (the only way to lose heat in a vacuum tube) to accommodate 80 successive launches of 5 ton payloads per hour. The permanent magnets and other structures surrounding the rotor would have to be shielded from this heat to enable such a high launch rate. 

Smaller payloads cause a smaller temperature rise.

The thermal limits create two additional failure modes: magnetic failure in the iron rotor from a heat spike above 1000 K, or overheating if the launch rate exceeds the sheath’s cooling capabilities, disabling the magnets and breaking the loop. 

Beyond Lofstrom

A depiction of a deflection station, by Katie Byrne for this video.

Some aspects of Lofstrom’s design proposal can be modified. Larger Space Trains launched less frequently seem more practical than a handful of passengers accelerated every few minutes. The use of loaded counter-weights seems unnecessary when the stabilization cables could be tensioned for the same effect. High temperature superconductors to replace some or all of the permanent magnets would be an expensive option today, but would be a plausible option by the time we actually consider building a Launch Loop.

The vertical elevator that places a Space Train atop the acceleration track seems dangerous and impractical when the Space Train could climb the West slope instead. As the incline is about 310 km long, and the majority of it is at stratospheric height or higher, so a Space Train could climb it at supersonic speed in less than 10 minutes.

It would also be reasonable to install an actual set of rails to support the Space Train when it is not being dragged along on its magnet rack.
A launch loop with a more substantial track structure.

They can be relied upon at low speeds, or the Space Train can slow down to rest on them if the launch is aborted - Lofstrom’s design has ditching into the ocean as the only other option. 

We can further expand on Lofstrom’s design. 

A shorter track can be envisaged.

Maybe because LEO is the only destination you want, an acceleration track of around 950 km long is suitable. Perhaps because the Launch Loop is something new and not yet rated for human spaceflight, meaning it is reserved for cargo launch until it has proven its safety. That cargo could survive higher accelerations on the shorter track. At 10g, a track length of only 283 km is needed to reach LEO, 567 km to reach the Moon. 

Perhaps your destination is not somewhere beyond Earth. A short dynamic structure could serve as a launcher for intercontinental flights.

London to JFK in 1 hour at Mach 5.

It doesn’t even have to reach 80 km altitude. Merely pushing a passenger plane to 10-20 km altitude is enough to eliminate sonic boom issues entirely. It could obtain almost all the speed it needs from the launch loop, eliminating the need for complex and heavy hypersonic engines, instead covering most of the flight distance by gliding at high average velocity. The only propulsion needed is landing or divert engines, which can be small, subsonic and loaded with a minimum amount of fuel.

If a hypersonic glider is too much to consider, then a launch loop placing a merely supersonic plane directly into its most efficient cruise speed and altitude is already a huge benefit.

Imagine a Concorde, but affordable and without sonic booms. 

The low speed takeoff, the climb to altitude and the high thrust acceleration to supersonic flight all cost a significant load of fuel, which has knock-on effects on the size of the wings needed to lift that fuel, the unnecessary drag incurred during the rest of the flight and so on. All that could be skipped.

Furthermore, passenger flights are much more frequent than space launches, which works to the launch loop’s favour as it only becomes better with higher launch rates. The location of a plane-launching loop is flexible as the plane can maneuver to its destination and lands on its own. The small low-altitude dynamic structure wouldn’t need a 14 km/s iron rotor inside, but a much slower rotor that doesn’t have a ‘hypervelocity shotgun’ failure mode. It may be safe enough to place near cities…

Or, you could imagine a longer track. One which lets a payload reach velocities that directly place it on a trajectory to Mars, Venus or beyond. It might need to deliver 15.5 km/s to its payloads, so it’d have to be 4081 km long if limited to 3g acceleration.

A 4000 km launch loop anchored between two Atlantic islands. 

A multi-interplanetary launch loop would need at least one end to be mobile so that it may align itself with the target planet’s inclination; a job perhaps for a floating platform that sinks itself into place before launch. 

Size of a 4000 km launch loop stationed at Cape Canaveral.

The track could be extended even further to shorten the trips to other planets. The only limit would be slowing down at the destination. Aerobraking can do wonders in this regard. 

Alternatively, we could exploit the launch loop’s ability to shoot off many payloads in a short span of time before it reaches its thermal limits. Lofstrom’s design could manage 15 x 5 tons in 6 minutes, followed by another 15 x 5 tons after half an hour of cooling. In two hours, that’s a total of 300 tons. A thicker rotor with greater heat capacity could manage even more. These many small payloads would all be travelling in roughly the same direction with small differences in speed and a separation between them of a few hundred kilometers.

Spacecraft launched in multiple pieces, connecting in space, is a often-proposed concept. But here, it would be done while already underway to Mars. 

They would have weeks or months to group up, connect and form a single large spaceship. That large spaceship would then be used to brake into the destination. 

For example, a futuristic 5500 km Mars Launch Loop could send off a hundred 5 ton payloads in a short burst. They leave the loop at 18 km/s, putting them on a 45 day trajectory to Mars.

Loop-launched craft aimed at Mars could slow down with intense aerobraking, or if going really fast, with powerful rocket engines. 

The majority of those payloads are blocks of propellant, the rest are 5 ton nuclear engines, 5 ton crew compartments and so on. Assembled, they form a 500 ton nuclear spaceship. The rocket engines could have a specific impulse of 1200s. So, with a mass ratio of 5 (100 tons dry, 400 tons of propellant), the onboard deltaV capacity is 18.9 km/s. This is enough to slow down into a Mars orbit without aerobraking.  

By the way, those propellant blocks could also be fusion fuel pellets, making the Launch Loop an excellent way to set up a Fusion Highway

The Launch Loop would replace the solar sails setting up the pellet 'highways'.

And the Launch Loop is not restricted to Earth. It could be installed on the Moon, Mars or anywhere else.

A launch loop anchored to a distant planet. Illustration from a page of JayRock's Runaway to the Stars.

An extraplanetary launch loop wouldn’t even have to rise to a higher altitude if it is installed on an airless body. A Lunar Launch Loop could, for example, sit flat on the ground. This makes the launch loop the equivalent of a gentle, easily scalable mass driver with less extreme pulsed power requirements than a railgun or coilgun. 

A launch loop delivering a payload to a skyhook. Also a JayRock piece

It may even be paired up with an orbital tether, so it would not need circularization engines. The tether could handle inclination changes, so the launch loop facility would not need to move itself to reach different destinations.

In the far future, if Launch Loop technology is accepted despite its faults and developed into a mature, reliable service, then we could have one loop launch a Space Train perfectly onto the track of another launch loop. The Space Train would grip onto the returning iron rotor to slow down at the destination. The entire journey could be completed without using a drop of rocket fuel, at incredible speed. It may or may not be more difficult than lining up a rendezvous with the end of a rotating tether.


  1. Just a thought about the launch loop/skyhook combination: they would most likely limit launch loops to be near or on the equator, because away from the equator the skyhook would have to be in an inclined orbit exactly matching the latitude of the launch loop in order to deliver payload with zero velocity perpendicular to the loop. But with the skyhook in an non-zero inclination orbit, you only get one point where the launch loop and the skyhook line up, meaning you would need very good timing to release payloads (and also be limited to 1-2 payloads every 24 hours due to the rotation of the Earth moving the insertion point away from the launch loop 2500 km every 90 minutes)

    1. You make good points - but it means the use of 'relay' tethers would be extremely useful.

      One tether grabs the payload from the launch loop, another changes its inclination.

  2. I'm concerned about maintenance of the cable. Its undergoing huge amounts of thermal and mechanical stress and I can't really see a way you could repair it without lowering the entire loop. Hard to fix something moving at 14 km/s inside a tube. I wonder if a liquid or plasma based system would be possible, you wouldn't have to worry about the cable wearing in that case, but stability and drag would be harder.

    1. I doubt these kinds of actively-supported structures will ever have just one rotor. You'd probably have a bundle of three or more rotors running in parallel, so you can stop one for maintenance without lowering the loop.

    2. Agreed, but I should note that having multiple rotors in parallel makes them all vulnerable to a single strike from a large object like a plane or a crashing Space Train.

      Using a liquid or plasma instead of a levitated frictionless rotor is not possible: the drag effects would be unsurmountable! Try and use the drag equation when you have 14000^2 as one factor.

    3. Well, a lateral offset of a kilometre or so would prevent single-strike incidents from taking out the entire structure. I'm envisioning multiple independent tracks that are tied together by long tethers, with separate rotor bundles supporting each track. Launch loops in practice might be way more complex structures than Lofstrom's initial proposal. The expensive part is constructing the deflection stations as you stated; once those are built I don't think there's anything really restricting one from throwing up multiple tracks. One could even envision a hub-and-spoke type model where there are multiple loops radiating in different directions from a single deflection station.

      Regarding the rotor itself, would solid pellets work as an alternative to a single-piece rotor?

    4. Drat, I originally tried to reply on my phone and posted it as a new chain. I can't seem to delete it. I've repeated it below with some minor edits.

      I'm not sure if drag would be completely insurmountable. The liquid or plasma would also be in a vacuum tube of course, and like with the solid rotor if the stream was dense and contiguous the drag from the trace gas left in the tube would mainly just be grazing the surface, not slamming into it head on. And at those speeds the structural strength of the iron is pretty irrelevant, the gas molecules are penetratingly both pretty deeply when they do hit. The main difference I see is the liquid or plasma is more vulnerable to turbulence disturbing their surfaces and cause droplets or wisps to break off, but if the magnets are strong enough and the vacuum pure enough it might be possible to prevent splattering. And in the case of the plasma I imagine the trace gas wouldn't last long before also heating up enough to become ionized and joining the plasma stream.

      Though then the question becomes how do you get the plasma stream up to speed in the first place. The iron rotor can be placed in the tube beforehand and then slowly spun up. The plasma or liquid wouldn't be well constrained if they aren't passing by the magnets rapidly.

    5. There are other problems.
      How do you get a plasma dense enough to provide the momentum you need? At typical ionization temperatures of 10,000K+, it is hundreds of times less dense than air! If you ramp up the pressure, you'll need substantial containment all along the 2000 km+ length of the launch loop.

      Liquids are relatively dense, but they suffer from internal turbulence and shear forces. You're not going to get perfectly laminar flow at 14 km/s! And even small velocity differences are enough to create enough friction to boil the liquid instantly...

    6. Ah that's true. A pellet stream might be the best of both worlds, dense and not as prone to turbulence but you can still gradually add fresh pellets and remove worn ones if necessary.

  3. The liquid or plasma would also be in a vacuum tube of course, and if the stream was dense enough the drag from the trace gas left in the tube would mainly just cause turbulence on the surface, and if the magnets are strong enough and the vacuum pure enough it might be possible to prevent splattering. And in the case of the plasma I imagine the trace gas wouldn't last long before also heating up enough to become ionized and join the plasma stream. Though then the question becomes how do you get the plasma stream up to speed in the first place. The iron rotor can be placed in the tube beforehand and then slowly spun up. The plasma or liquid wouldn't be well constrained if they aren't passing by the magnets rapidly.

  4. I’m wondering about the impact of Earthquakes at one or both deflection station sites. Sure, you could try to find a site that doesn’t get very many Earthquakes, but it’s almost impossible to find a place on earth that doesn’t get a very rare mild-to-moderate earthquake. While traditional earthquakes are limited to areas where continental plates meet, intraplate earthquakes can happen almost anywhere and aren’t completely understood. With the deflection stations being so far apart, you could have one station experiencing a mild to moderate earthquake while the other station doesn’t. Could the active control systems manage to stabilize the rotor under those conditions?

    1. I guess it will depend on the type and location of the earthquake. A small/distant earthquake will shake the loop less than a typical SpaceTrain launch, meaning the vibrations can be dampened out without a hitch. A massive/close earthquake would move the deflection stations themselves, which might cause displacement so severe it breaks the loop sheath.

      The only saving grace is that the structure is so large that only a small part of it would be affected by an earthquake, and 99% of the time it will be some random region along the acceleration track that can afford to lose a stabilization cable or two.

  5. Ah, another interesting post.

    Two thoughts:

    1: A trope in science fiction is submerging persons in oxygenated breathable liquids, so that they can bear much higher G-forces (at the cost of great discomfort). And there already exist high G-suits in reality.
    This could lower the needed length of a Lofstrom Loop, by increasing acceleration with at least a few Gs, but neither has been mentioned in your post. Are there problems I am not aware of which make those insufficiently hard sci-fi to be included (even the high G flight-suits)?

    2: At the moment I have just begun writing a Stargate SG-1 fanfic; I thought to have Earth use some hard sci-fi technologies in order to minimize the usage of space magic, err, naquadah and trinium. So, I thought to use a few of the ideas I had encountered here, like the usage of solar sails to resupply asteroid mining operations; would you want I mention where I got some of those ideas from in my author's notes or is that unnecessary?

    However, this whole Lofstrom Loop things seems a rather impractical and unfeasible megastructure to me, even after taking into account that this Earth already was more advanced before the start of the Stargate program*. So I wonder whether I should instead have the SGC 'find' somewhere a carbon nanotube space-elevator cable and take it to Earth through the Stargate. Though, maybe I could mention a world much more advanced than Earth having Lofstrom Loops.

    *In the unlikely event somebody wonders why Earth already was more advanced before the start of the Stargate program, there exist a certain tech group whose CEO is secretly the Goa'uld (Ancient Egyptian space alien) Isis.

    Oh, and I also thought to somewhere in my story include an offhand mention of Goa'uld Ha'taks being designed with the option to be equipped with anti-submarine weaponry. A result of the Goa'uld Isis once having exploited the tendency of energy shields to be much weaker in atmosphere by ambushing spaceships who had entered atmosphere in order to invade one of her worlds with Fusion Powered Anti-Orbit Laser Submarines...

    1. 1) You could imagine liquid immersion seats for specially trained astronauts that are launched only a few times a year. But you will not find enough of that sort of passenger to fill up a Lofstrom launch loop's capacity for thousands of launches a year. You would instead have to deal with the general public, and they will not put up with the difficulty and constraint of a liquid immersion. You know, if something goes wrong, the passenger could drown or die from an aneurysm...

      2) That is probably unnecessary. You can make a nice note in the dedications though!

      Regarding the SG-1 setting: you could 'find' a gigantic strange of carbon nanomaterial to make a space elevator... but you would not know how to maintain it, nor can you repair it or replace damaged sections of it.

    2. 1) So, if thanks to limited resources you can only afford a small launch loop, you are limited to only sending specially trained astronauts into space? Still better than nothing I suppose.

      2) Hmm, I did not have any dedications in the two fanfics I had written previously; ideas I had because they were suggested by readers were instead put in authors notes below/above the chapter they appear in. Well, I could mention your blog in only one of them and note that a few things in other chapters were also partially inspired by things I had read there.

      The knowledge how to maintain and repair a space-elevator should hopefully not be a large problem.
      The CEO/Goa'uld Isis had earlier experience working with space elevators, the result of being a multimillenarian alien with an interest in science.
      And according to my current plans she would in the year 1993 reveal herself to the Stargate Program, introduce herself as 'Aset, mispronounced Isis by the Ancient Greeks', and offer her services, coming up with a whole story about how 'She had always been a just ruler who unlike the other Goa'uld had a willing human host; however, she ended up imprisoned in an artificial hibernation, and her husband Osiris death, because they had disagreed with Ra's policy of keeping humanity primitive and thus plotted to overthrow him, but were betrayed by Seth; and now, some decades after accidentally being released, wants to serve Earth'*. The SGC has some doubts about her trustworthiness; however, her knowledge is simply too important to not work with her.
      (Hmm, that turned out longer than expected.)

      *Well, actually she was neither a real defector nor the real Isis. As the real Isis and Osiris were located in the White House and had created her to serve as a Trojan Horse; 'What could go wrong? Their daughter having a real change of heart as a result from closely working with humans? That is completely ridiculous.'. Naturally, she has no plans to tell the protagonists about that.

    3. Might I ask for a link to said fanfic?

    4. @Mr. Tucker

      I haven't yet begun publishing it, sorry.
      My current plan is to post the first chapter somewhere in February. I still want to go over it one last time to find things which could still be improved but am busy with something else at the moment.

      I am also still unsure where to post it. My first*, and up till now only, Stargate fanfic had been posted on both the fandom subforum of and as you need an account to visit the former. However, I am planning to include a few images in my new fanfic (both from image generators and from Wikimedia Commons) and does not allow images; so now I am unsure whether to post it on spacebattles forum or DeviantArt for people who do not have an account.

      Anyway, I’ll write down that I will post a link here, after posting the first chapter.

      *It is completely unrelated to my current idea, but instead based on the Quantum Mirror. Also you won't find much hard sci-fi in it.
      Here is a link encase you decide you now want a link to it also; however, I do have to warn you that English is not my first language and I had not found a proof-reader:

    5. Ty, @Tus 3! I eargerly await to see your work! :)

    6. Hello, @Mr. Tucker

      I had finally posted the first chapter on the SpaceBattles forum:
      I hope you think it is not too bad. However, I must warn you I fear that, as a result of real life, I will be capable of at most making one new chapter every one or two months.

      Now, I am thinking about it, we actually went far off topic. However, as I have encountered in the comments on other posts here such things as monologues about the Reagan Tax Cuts, I suppose that is nothing out of the ordinary.

    7. Much obliged, @Tus 3. I will give it a look! Thank you for taking the time! :)

  6. Extreme scale and capability of loop seems to be at odds with organic gradual development of space. It might take many decades before demand catches up. Better suited for mature space civilization than one making its first steps.

    1. The loop can start small. It can serve many markets, from air travel to spaceflight, so it can be larger than strictly necessary for a single market.

  7. Hi Matter Beam, before I begin, I'm very sorry for posting this here but the question I'm going to ask doesn't really fit completely with any of the blogs you posted before so I decided to ask it at the most recent post.
    So, for a current project I'm postulating a world where unobtainium is liberally applied so long as it doesn't really wreck the plot but handwavium is still a no-go, amongst the unobtainiums are quark nuggets which allowed (somewhat) economical antimatter torch ships and an Alcubierre drive (still limited with Novikov self-consistency principle). As the Novikov self-consistency principle seems to roughly mean "sometimes the drive works other times it does not", I was wondering if the two previously mentioned factor could justify a warship that is not completely dedicated to a particular set of weapons?
    The rationale is that, as warships may arrive in a somewhat unpredictable quantity to another system, and the quark nuggets have allowed ships with a generous delta-v budget, it may make sense to have warships carrying a main weapon (likely a macron accelerator, a laser or a particle beam), its own point defenses and some missiles. This scheme seems attractive to me, as from a in-universe perspective it allows a warship to defend itself and carry out the original mission even if the self-consistency principle made it the only vessel to be able to arrive on scene, while from an author's perspective I noticed that the more popular space warships (such as a star destroyer) seem to be always carrying a large range of weapons, and by having a warship designed in a similar way it may be more likable to a reader?
    Again, sorry for posting a largely irrelevant question.

  8. Any launch loop that is on the equator should be safe from hurricanes/typhoons, since they rely on Coriolis forces, which don't occur on the equator. You can see the lack of storm tracks on the equator on your map.

  9. The rationale is that, as warships may arrive in a somewhat unpredictable quantity to another system, and the quark nuggets have allowed ships with a generous delta-v budget, it may make sense to have warships carrying a main weapon (likely a macron accelerator, a laser or a particle beam), its own point defenses and some missiles.