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Of the general options that exist in terms of hypothetical space launch systems, what is the most plausible option for a relatively near future SF setting? The focus here is on Earth to LEO, as the level of space development this cheaper access would imply also lowers costs by giving refueling infrastructure in orbit.

The few obvious ones that come to mind are:

Space elevators would be the best option overall in terms of lowering costs, but there is the question about whether we could actually pull this off in a practical sense. Besides the question of finding a strong enough material, a failure would be catastrophic in scope. It would also likely require literally perfect forecasting of lightning strikes, as the most likely material is carbon nanotubes that would be very conductive to electricity.

Laser launch to orbit works similarly to a space elevator in terms of offloading the mass of the fuel, but it trades the problems. While it would not require borderline impossible physics, it would require utterly massive energy storage or production at the launch site. It would also be an extremely potent weapon system that would make other nations that the owner nervous and possibly blind anyone looking at the launch.

Improving the fuel via metallic hydrogen is also a possibility, as probably the most high energy and high thrust rocket fuel that doesn't involve setting off nuclear weapons. It isn't the same level of extreme savings as something like a space elevator, but it could potentially lead to savings as it would allow proper mature SSTO designs with large payloads to develop. If fusion rockets could produce enough thrust to get off the ground, they would be also be an excellent option in roughly the same category, but that is not likely.

Reusable designs and their descendants. This is obviously what is happening already in terms of design, but even the most optimistic(and probably more than a little unrealistic) projections would not approach even the conservative projections of the more advanced concepts. One other possible related approach is that of the SABRE(or RAPIER for KSP players) engine in which it uses a ramjet/rocket hybrid, if that can ever be made to work.

Of these or any others I'm not familiar with, what is the most plausible general solution to give significantly cheaper access to orbit within the next century?

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  • $\begingroup$ If you are asking us to predict the future of real Earth, that seems off-topic. If you are asking about a fictional Earth-like planet, then 'plausible' depends upon the storyteller, not the technology. Futurama spacecraft are powered by poop, but it's internally consistent enough and sold quite well, so the audience does not mind. $\endgroup$
    – user535733
    Aug 6 at 18:34
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    $\begingroup$ everyone probably has his own preferences and will praise his choice. Launch loop arguably is a good thing, its biggest flaw is reliability - it has to be bulletproof in a technical sense, no malfunctions for decades and more. otherwise is quite a sound concept. otherwise, reusability by far, as other things may need assumptions of all kinds and it is a question if your setting/vision has those capacities. There are lesser-known ideas, but it is for a reason in most cases. $\endgroup$
    – MolbOrg
    Aug 6 at 20:33
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    $\begingroup$ IMO, plausibility can be assessed based on the current state of each technology's development including engineering and building parts. It should be assumed, though, that no significant and sudden changes will happen in related fields of science and technology. This cannot be guaranteed, of course. However, we cannot guarantee that some breakthrough technology/research will appear in the next 100 years and make all our current knowledge and understanding completely irrelevant. Thus, with the assumption of no big changes, the question can have non-speculative answers. $\endgroup$
    – Otkin
    Aug 6 at 22:17
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    $\begingroup$ Space elevators are even worse than you think. I've read through a number of proposals and the average payload capacity is around 15 tons per week. I suspect that space elevators are such a popular concept, because they are easy to understand. They make stuff go up high. There is no need to even understand orbital mechanis. Active support, laser launch or skyhook are much more complicated to explain. $\endgroup$ Aug 7 at 10:10
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    $\begingroup$ VTO on the basis of its usefullness and reusability for other authors. $\endgroup$
    – MolbOrg
    Aug 9 at 0:06
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Pick one or more to satisfy your story telling needs.

As mentioned, the Non-rocket spacelaunch Wikipedia article gives a nice overview of proposed cheaper space launch systems.

To that I will also add, Big Dumb Boosters. You've perhaps heard of the SpaceX BFR - this is an example of the fact of the square-cube law effect on rocket launch systems (resulting in a higher fuel / weight ratio) as rockets are made larger. There are diminishing returns on bigger rockets, so you won't be launching small mountains this way. The Sea Dragon was the largest booster ever considered at the time and although dated, still a valid example design.

Many of the systems are, or can be made complimentary. e.g., combine a rail-gun system like Star Tram to improve the initial stage efficiency and something else to help pull your craft into orbit such as a skyhook system.

In real life, if I had many billions to spare for such, I would not be looking at a space elevator as my solution because I would want something that I was certain could be achieved - and we still don't know whether it is possible to actually build a space elevator on Earth, though I am certain other system could be built - no new science or technology breakthroughs required. Room temperature superconductors could make your rail-gun more desirable, but you don't have to have them for the plan to work.

You can also combine rockets with most of these systems., e.g., rail gun for a good head start with rockets to complete orbital insertion, or perhaps rail gun, then rockets to get it the rest of the to your sky hook.

Plausibility does not require certainty though. It's all about what makes your story work.

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Skyhook!

Ok, I don't know if it's the most plausible, but until one of these technologies is realized nobody will know which one was most plausible. And I like skyhooks, so I'm going to roll with it.

The basic idea of a skyhook is to put a tether into orbit with a hook on one end and a weight on the other, then set it spinning. The hook end dips down into the very uppermost reaches of the atmosphere periodically, where ships can attach to it and get flung up into orbit. The physics of it is actually really simple, but of course there are a few problems (all surmountable).

First, materials. Unlike a space elevator, we wouldn't actually need any breakthrough advances in materials science to make a skyhook work. Most skyhook designs would work fine with existing materials, although production of these high-strength materials would need to be scaled up and made economical. This is actually one of the easier challenges with the skyhook design.

Another easy problem is conservation of momentum. Essentially, every time you grab and launch a craft into orbit, you're leeching a bit of energy / momentum from the hook. There are two ways to solve this. The first is to equip the skyhook with small chemical or ion rockets that would periodically boost and adjust its orbit, keeping it from falling into the atmosphere. These would need periodic refueling, and in general are inferior to the second solution: send stuff back down. A skyhook works in both directions, and if you catch things out of orbit and lower them back to Earth with the skyhook you'll be recovering energy / momentum from them, keeping the whole skyhook up in the sky for "free". Some smallish amount of stationkeeping would probably be required, but overall this is a much better option than just reboosting after every launch.

The hardest problem to solve with a skyhook is the problem of spacecraft needing to actually catch the hook. At its lowest point it will still be moving quite fast relative to the ground, and that lowest point is still in the upper atmosphere. So you'll need specialized ships designed to accelerate up to the speed of the hook, and these craft need to be able to find it and latch on. This is probably solvable with some combination of computer guidance, careful scheduling, and assistance from the hook itself (the Kurzgesagt video on the subject suggests having drones on the hook end of the skyhook to assist in catching spacecraft; the linked video is a great resource, by the way, but do note that it's focused on using skyhooks to facilitate interplanetary travel rather than travel to LEO).

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  • $\begingroup$ Speed relative to ground at its lowest point can be adjusted with trade-offs, it could be set to zero m/s for a few seconds. But at the cost of much lower frequency of availability, like once every 48 hours(or more). Thus allowing a dirigible/balloon platform. A set of space hooks set to dip sequentially every 15-30 min would make this more viable. $\endgroup$ Aug 6 at 20:53
  • $\begingroup$ What do you actually save compared to normal rockets? You still have to boost the skyhook. This means you have transport fuel to it, then accelerate the whole thing. The only advantage is that you can boost more slowly, so ion thrusters might be feasible. $\endgroup$
    – Michael
    Aug 7 at 9:34
  • $\begingroup$ Thinking about engines … if the skyhook is equipped with nuclear engines (maybe even using fusion instead of fission) it might actually be nice. Refueling would require less mass and a lot of the mass and cost is in the engines which can stay in orbit. $\endgroup$
    – Michael
    Aug 7 at 9:41
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    $\begingroup$ @Michael As I understand it, you can theoretically capture the potential and kinetic energy from returns, rather than burning it off in the atmosphere with aerobraking. Effectively, the skyhook can act as a "battery" storing the orbital energy from one returning orbiter and using it to launch another later, reducing the total cost for frequent round-trip travel. It's particularly attractive if you have a net downward flow, e.g. from asteroid mining. $\endgroup$
    – R.M.
    Aug 7 at 13:33
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Rockoons!

rockoon https://www.semanticscholar.org/paper/Development-of-a-Rockoon-Launch-Platform-and-a-Fuel-Johnson-Roberson/aa576b73f3b9ebe1f1201b8d7119e4707e9d595c

  1. Float up above sticky grabby draggy gas with balloon and attached rocket.

  2. Launch rocket!

  3. Pump floaty stuff into cylinder so balloon descends.

  4. Get new rocket.

  5. GOTO 10.

https://en.wikipedia.org/wiki/Rockoon

A rockoon (from rocket and balloon) is a solid fuel sounding rocket that, rather than being immediately lit while on the ground, is first carried into the upper atmosphere by a gas-filled balloon, then separated from the balloon and ignited. This allows the rocket to achieve a higher altitude, as the rocket does not have to move under power through the lower and thicker layers of the atmosphere.

https://www.livescience.com/47692-high-altitude-balloons-floating-satellites.html

High-altitude balloons, some of which can reach altitudes of up to 100,000 feet (30,500 meters) or 19 miles (30.6 kilometers), have been used to observe weather patterns since at least the early 20th century, and NASA has been using these types of balloons to conduct scientific experiments for the past 70 years, Crites said...

"We think the key is to build a platform to get people exploring up there," Crites said. "What we want to see is students and entrepreneurs using new applications on this platform and getting really creative."

My personal favorite though is the railgun rockeloonannon in which a railgun on a high altitude floating platform fires the rocket up to orbital altitude and the rocket then attains the lateral velocity to stay in orbit. Rock Rock, Rockeloonannon!

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    $\begingroup$ +1 for subtle homage to BASIC via its line numbering convention $\endgroup$
    – Bohemian
    Aug 7 at 2:09
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    $\begingroup$ Rockoons are neat, but in terms of delta-v, it's basically not with the effort because they're much more difficult to launch and generally very complicated. The majority of the energy a rocket expends goes into horizontal velocity anyways, and there's a reason air-launch systems never really took off $\endgroup$
    – Dragongeek
    Aug 7 at 4:03
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    $\begingroup$ The greatest advantage of airplane launched rockets is that a plane can easily fly hundreds of kilometers, therefore change “launch site” quite easily. The delta-v savings for the actual launch to orbit are small. A balloon wouldn’t be able to move, therefore would lose this big advantage. $\endgroup$
    – Michael
    Aug 7 at 9:38
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Space Fountains and Launch Loops

A space fountain is essentially a space elevator that could work with current materials. Instead of relying on material strength, the space fountain is supported by the active motion of iron slugs cycling through it, similar to a stream of water arcing up from a hose.

A launch loop is a possibly more practical version of space fountain that doesn't try to go straight up. Instead it extends horizontally for hundreds of miles, providing a runway 50 miles up from which to launch ships, and acting like an enormous aircraft carrier catapult. A big advantage is that the iron slugs can have more room to turn around when they touch earth, so you can use many miles of electromagnets to gradually turn them. A disadvantage is that the launch loop is more vulnerable to ground attacks, because it covers more territory and enough damage at any point could destroy it.

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    $\begingroup$ I was going to go with an orbital ring with active support towers. Since the towers are the part that directly answer this question, and they are pretty much just space fountains by another name, +1. $\endgroup$ Aug 7 at 5:53
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Question as posed: What is the most plausible way to lower costs to orbit?

Just wait.

No stipulations have been given as to how much the costs must lower, only what's most plausible. However, it's plausible (even likely) that we'll see a substantial reduction in launch costs in our lifetime, through three mechanisms...

  1. Private sector companies replacing inefficient government monopolies.
  2. Adoption of greener energy lowering the price of crude oil, and thus, kerosene.
  3. Economy of scale, in technology and production.

As noted in the question, private companies have succeeded in developing reuseable rockets, with very considerable cost savings. They're also driven much more to bring down costs, as decreasing costs result in increasing profits, not decreasing budgets. What have we seen so far? Private companies have brought the cost of launch into orbit from 18,500 USD / kg down to 2,720 USD / kg, an 85% reduction in cost. (https://theconversation.com/how-spacex-lowered-costs-and-reduced-barriers-to-space-112586) As these companies are driven by profit and the future of society (in Musk's case), they have reason to continue to lower these costs as far as they can.

Yet, even as these companies burn mind-boggling amounts of fuel to get into orbit, we can expect the cost of that fuel to go down in the next century, due to crude oil's replacement with electricity. Even if only cars go to (renewably generated) electric, it's reasonable to expect the cost of crude oil to drop dramatically, and with it, fuels like RP-1 that help with launch. Rocket launches may be the one place that hydrocarbons make the most sense, for their energy density (both in MJ/kg and MJ/L).

Finally, as space exploration becomes more common, economy of scale will start to kick in. When private companies find designs that work very well, they can move from essentially experimental launches to mass production, with huge associated cost savings. The bigger private companies are already playing with this idea, but have a long ways to go in the next few decades.

How long will all this take? At the current rate, and seeing as two of the richest people in history are vying to accomplish these very goals, we're looking at a scale of decades, if even that.

This may not be a particularly out-there answer, but that also makes it the most plausible, in my humble opinion. And there's no reason to expect that private companies, mass-producing highly reusable rockets to launch with likely ever-cheapening fuel, will have any trouble reducing the costs of launch to orbit.

All this takes is time. So, in short... just wait.

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  • $\begingroup$ This definetely most plausible answer, lol, so as it adresses factors and chooses technologies, solid answer. $\endgroup$
    – MolbOrg
    Aug 7 at 10:02
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    $\begingroup$ The cost is fundamental energy cost, not anything that can be sidestepped by economies of scale or any of your other capitalist magical thinking. $\endgroup$ Aug 7 at 17:33
  • $\begingroup$ @R..GitHubSTOPHELPINGICE -- the fundamental energy cost is only one factor in several that currently make space launch costly -- we've already seen significant reductions in cost-to-orbit from SpaceX's work on not throwing rockets in the trash all the time :P and there's undoubtedly more that can be done on that front $\endgroup$
    – Shalvenay
    Aug 7 at 18:40
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    $\begingroup$ @R..GitHubSTOPHELPINGICE according to Elon Musk, energy is is only 5% of the cost of a launch. Everything else is (flight and ground) hardware, software, control, etc. $\endgroup$
    – RonJohn
    Aug 7 at 20:50
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    $\begingroup$ @R..GitHubSTOPHELPINGICE but he knows how much his rockets cost, goes in to making them, how much fuel+oxidizer is required and how much they cost. $\endgroup$
    – RonJohn
    Aug 7 at 23:19
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Ecuadorian Railgun

Pick a tall mountain near the Equator. Build a railgun up the side of the mountain. Put a nuclear power station and a big bank of capacitors nearby to produce the phenomenal surge required to energize all those magnets at once. And shoot your cargo into orbit.

Of course, the cargo must be hardy enough to withstand a few dozen Gs. Squishy humans must continue to ride oversized firecrackers into orbit.

You will need a space-tugs at apogee to prevent the cargo from re-entering, of course. Your railgun capacity will be limited by the number of tugs, by the number of launches that your power plant can support, and by the number of UN-employed inspectors ensuring that a "mistake" doesn't blast a city. So the cafeteria at the railgun base needs to be impressively large, too.

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    $\begingroup$ Further reading, but be warned that it's not especially encouraging reading. This is a problem where you have to solve massive engineering issues just to get to the point where you can grapple with the other massive engineering issues. $\endgroup$
    – Cadence
    Aug 7 at 0:19
  • $\begingroup$ I've read "limited by. . .the number of UN-employed inspectors" and it raised my hackles immediately. Yes, this will limit the capacity---but in the opposite sense than the author thinks, namely, the more bureaucracy you add to the project, the lower its efficiency will be. Instead of reaching orbit, the launches will slowly and disgustingly sink into a morass of bureaucratic poop. $\endgroup$
    – Ralf B
    Aug 7 at 11:11
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Linear Induction Launcher

A much more feasible alternative to a railgun, the Linear Induction Launcher not only has no fundamental speed limit, it also levitates the armature for a relatively frictionless (aside from air resistance) journey down the barrel.

The operating principle is to axially stack many coils of wire to creat a barrel. Each subsequent coil will have fewer and fewer turns. When pulsed in an accelerating sequence, a traveling (and accelerating) magnetic wave will be created that, by the principle of induction, will create a force in the same direction of travel on any conductive object within the barrel.

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