What amazes (and saddens) me is that we don't seem to have not even a far fetched theoretical approach to dramatically improve rockets within the next few decades.
Beamed propulsion throws out the rocket equation altogether, as do light sails.
Not an issue for flybys, but an issue if you want to get into an orbit.
False. Robert Forward has a technique for deceleration for interstellar lightsail craft, which is described in Rocheworld and one of his science books. Magsails using beamed particle propulsion can also exploit the interstellar medium to decelerate without onboard fuel.
Not an issue for flybys, but an issue if you want to get into an orbit.
Carrying a small rocket for a circularization burn is vastly cheaper than a 1st stage. This is covered in Jordin Kare's modular laser launch proposal, and this technique was also considered for Bull's light gas space gun. Those can produce launch economics approaching those of space elevators.
Sorry, but you're citing a science fiction author as proposing a credible method, yet don't even state what that method is.
Yes, I know Robert Forward used to work in aerospace, but his fiction writings are... fiction. He's been out of the industry since 2002 and a hand-wavy "he has a technique" doesn't strike me as being particularly credible.
(I was thinking of free-space skyhooks, exchanging momentum between inbound and outbound trips, but the ones for planets are also cool. Just harder when the planet is heavy like Earth and Venus.)
Chemical rockets are also much more efficient devices than is given credit for. A good, very optimized two stage rocket like SpaceX BFR (and the ITS concept before it) can get payloads to orbit using just ~300 Megajoules per kg (250MJ/kg in the more efficient tanker configuration). The specific kinetic energy of orbit is about 25MJ/kg (40MJ/kg if we include gravity and aero losses and changes in altitude), so rockets are total 10% efficient at converting chemical energy directly into orbital energy, and with an optimized, high pressure and variable mixture ratio hydrogen rocket engine architecture with some improvements in materials science, we could improve that to 20% or more. That beats the chemical to mechanical efficiency of the typical car.
That translates to between $1 and $10 per kg to orbit. That's 3 orders of magnitude less cost than today, if we can master reuse. So arguably, chemical rockets are perfectly fine for achieving orbit cheaply.
However, rramatically improving rockets might be like trying to improve the carriage to get a much faster personal transport, while keeping the horse.
It seems to be the case, that the cost of launching commercial payloads are still "low" enough compared to the cost of the payloads since there are few big budget ventures that tries to make lifting much cheaper through alternative methods.
I'm not saying that space elevators or other alternative lift devices are feasible, but we tend to get huge investments even into tech that seems absolutely implausible if there is a big market. The lack of these makes investments makes it plausible that the current launch costs are actually acceptable for all current, and expected near future commercial actors.
Space elevators, skyhooks, and launch loops are all big enough that it's an all-or-nothing deal. You can test individual components, but a whole end-to-end space elevator has to be exactly the distance between ground and geosync or else it just doesn't work. At best, you could build smaller versions of this stuff on the moon first and then scale up to Earth-size later, taking into account such factors as water, atmosphere, and the fact that the Earth is not tidally locked with the sun. But that requires hauling metric fucktons of parts and equipment to the moon, even if you're using lunar resources to build the space elevator itself, so there's no guarantee you end up ahead in the end.
The basic physics behind rockets is simple as all hell, and it still required von Braun to spend his entire adult life building progressively bigger and bigger rockets and refining the design details. The V2 only needed to be able to land nose-first 200 miles from where it was launched, but without that knowledge and experience, the Saturn V would have been impossible. That's the kind of iteration that's fundamentally impossible with these other launch systems.
Nuclear has amazing power ratios but detonating a nuclear warhead to get moving is somewhat of a problem when you're close to earth.
Fission Heat Engines are quite good but not good enough for takeoff. Ion engines suffer the same fate.
Almost no alternative propulsion method can replace takeoff boosters, thusly limiting the mass you can put into space. With limited mass you can only make engines that big. Or bigger if you assemble them in space which opens other cans of worms.
It's not really a 'near earth' issue. Fallout concerns can probably be mitigated (and Freeman Dyson came up with a few ideas in that direction). Research in this direction, however, got killed as part of the various test ban treaties.
From the perspective of trying to reduce any country's desire for having a nuclear bomb program, Project Orion, and other nuclear solutions, is a no go. So even if you were to promise "We'll only start the nuclear component when we hit the moon" the proliferation concerns would still kill NASA's interest in pursuing it.
So yeah; it's not really a technical issue at it's core (though you definitely have technical issues to solve), but rather a political one (which certainly doesn't invalidate it).
There’s a lot of reason why they haven’t been used yet, but I think most the compelling argument against them is that they won’t be used until fuel costs become the competitive factor in launch prices.
https://space.stackexchange.com/questions/3004/why-arent-lin...
Nuclear rockets are quite the concept but have their obvious limitations. This video is worth checking out if you haven’t seen it yet.
But maybe there won't be one dramatic change that makes rocket travel cheaper. Maybe we keep iterating on rocket and rocket engine designs and eventually looking back it seems revolutionary. As composites get better and better more and more of rockets will be made from composites instead of the current material of choice, lithium aluminum alloy. Plus the increased use of oxygen rich combustion[3] with it's improved efficiency. Also it looks like Ch4/LOX might be more common in the future[4][5][6], which is good because Ch4/LOX has a higher ISP then RP-1 and also is significantly more dense then H2 propellant. There's also so called "slush" or "super chilled" propellants[7], Spacex has even been using it for awhile[8]. So maybe the future will be extremely light, reusable, oxygen rich, staged combustion, methane burning rockets with super chilled propellants?
[1]: http://www.astronautix.com/g/gnom.html
[2]: http://www.astronautix.com/p/pr-90.html
[3]: http://www.americaspace.com/2015/05/29/new-oxygen-preburner-...
[4]: https://spaceflightnow.com/2017/10/20/worlds-largest-methane...
[5]: https://en.wikipedia.org/wiki/Raptor_(rocket_engine_family)
[6]: http://www.russianspaceweb.com/soyuz5-lv-ptk.html
[7]: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/197900...
[8]: http://www.businessinsider.com/spacex-deep-cryogenic-cryo-li...
Does Skylon fit the bill?
https://en.wikipedia.org/wiki/Skylon_(spacecraft)
It pushes the limits of materials and design. But I think a reusable 2-stage rocket—as boring as it sounds—will end up working better in the near term. It can be done with safe margins.