Okay, here’s the scenario. Let’s assume that two things happen in the next few months.
[ol]
[li]I suddenly inherit an embarrassingly large fortune from a relative I didn’t know I had. (Ha!)[/li][li]A working nuclear fusion reactor is developed that puts out very little radiation and scads of electrical power from a smallish package. Perhaps Proton Boron 11 fusion.[/li][/ol]
I decide that I will use my new fortune to build an awesome, kick-a*se space ship. What are my options for getting my ship into orbit without using chemical rockets? Preference is given for the most purely electrical and most reusable engine.
There are serious proposals for laser driven craft, where the laser remains on the ground, and enough energy is imparted via the beam onto the craft that it can reach space.
The problem with mass ejector is that all the acceleration happens right at the start. You would be pureed by the time you reached orbital velocity.
Also there has been serious work on direct nuclear powered rocket engines. However materials issues, amongst others, tend to prevent them being viable for the first stage, limiting their use to upper stages or outer space duty. These were all fission reactors, but a fusion reactor would be beset with the same problems.
Your fictional device actually appears to have solved two problems, not one. Not just a useful fusion reactor, but how to make electricity with it. If we ever do get a viable fusion reactor, no-one really expects anything other than the traditional method of boiling water to drive turbines for the electricity part. Magnetohydrodynamic ideas are even more fanciful than the fusion.
We talked about this in the Aerospace Propulsion class I took. It uses nuclear explosions to push the mass of the craft ahead. From the calculations we were shown, it would work pretty well and most of the challenges could be overcome with 60s-70s tech. Unfortunately, the treaties banning the testing of nukes above ground kinda killed this project. It would have been able to carry massive payloads at incredible speeds, all without killing the passengers.
This isn’t really true. The biggest problem with MHD is that it is inefficient compared to methods of thermal expansion propulsion (in a rocket engine) or torque conversion into thrust (turbine, propeller) owing to the need to convert thermal energy into electricity, requiring a powerful generator that is typically larger and heavier than the propulsive system itself. Aneutronic fusion that produces ionized products, like p-[sup]11[/sup]B or p-[sup]7[/sup]Li, if made practicable by advanced in fusion technology, elimination the conversion step and allow direct recovery of energy via electrostatic or electrodynamic means, i.e. direct conversion to current or an energized plasma. The reason that such fusion avenues are not being more aggressively pursued relative to D-T fusion is because the threshold to obtain stable conditions for sustained fusion are an order of magnitude or two higher for the heavier fusion interactions.
Given the o.p.'s presumption of feasible and lightweight fusion generation, many of the propulsive problems that currently plague spaceplane-type designs are minimized, and performance issues with vertical launch SSTOs are surmountable. Currently the problem of going from subsonic propulsion to supersonic to scramjet to vacuum conditions are restricted on the ability to configure engines that can function well at all regimes and convert from air-breathing to stored oxidizer once they reach the upper atmosphere. Also, the energetic yield, and therefore propulsive efficiency and thrust-weight ratio are restricted by the amount of energy available by the chemical constituents of the propellants; in other words, there is a maximum amount of performance you can obtain from any chemical propellant reaction regardless of the configuration of the vehicle or engine/motor. However, if you are using an external energy source (and for the purposes of this discussion, can ignore thermal or erosive limits of the materials used in constructing your engine) you are now no longer limited by the chemical energy of the propellant and can instead focus on getting maximum effective exhaust velocity from a low molecular mass propellant to achieve both high thrust and high propulsive efficacy. In other words, the trade-offs that we play with chemical propellants between compactness (to reduce dead weight like tankage), low molecular weight, and maximum energetic output are now simplified, as is the necessity for precise control of supersonic flow in a scramjet-type engine. Given the at least couple of order of magnitude improvement on propulsive efficiency and thrust/weight ratio, as well as the potential for reducing the dead weight of propellant needed to achieve exoatmospheric altitude, spaceplane-type craft may become more feasible, although the problems with scramjet design and materials for thermal protection upon re-entry may still drive you to a vertical launch/blunt body re-entry type configuration, like the proposed Phoenix/Chrysler SERV/Delta Clipper vehicle utilizing all stored propellants.
For interplanetary transit, the use of fusion to generate charged plasma or electricity for electrostatic ion propulsion is within the realm of feasibility (although the technology is obviously nascent or conceptual at this point. I doubt even efficient fusion would provide adequate performance for interstellar transit on human lifetime timescales unless an effective means to extract propellant from the interstellar medium can be developed, as the mass ratio for even high I[sub]sp[/sub] propulsion is still prohibitive at the distances for even the nearest star systems. But ground-to-orbit, and orbit-to-interplanetary? It certainly eliminates a lot of the existing difficulties inherent in chemical propulsion.
Which is actually sort of the problem, from a practical standpoint. Unlike chemical rockets, which reduce in efficiency as you scale up for structural reasons (larger vehicles require more structure, and thus, more dead weight per unit payload), ORION-type craft are problematic as you scale down due to the minimum yield of the propulsive bomblets and the need for mass to damp the impulse to keep experienced acceleration and higher-order momentum differentials within a range that is allowable for crew. This means that you would have to be able to either launch the craft from the ground, boost it to orbit using chemical propulsion by some prohibitively massive launcher, or transport it to orbit in components and assemble it.
The first is neither politically nor practically realistic. Although the amount of fallout that would be produced by an individual launch would be smaller than that of many of the early above-ground nuclear tests conducted by the United States in the 'Fifiteis in Nevada, it would not be acceptable to most of the population (especially those living downwind) to allow deliberate radioactive contamination, regardless of how well minimized or the statistically-demonstrable marginal impact it would have on morbidity and mortality of affected populations. Such a massive vehicle would also not have any realistic abort modes that would be acceptable to any range safety authority. While you could place crew in an ejectable capsule, the large mass of the vehicle and the potential hazard of the propulsive payload represent risks that are just not mitigable by any realistic standard. (Even given the one-bomblet-failure recovery capability demonstrated on the conventional explosive prototype, the reliability required by the system is not achievable by a mechanical feed system for the bomb lets.) The potential for enormous ground hazard in the case of failure, particularly at high altitude where the trajectory of the unpowered craft may bring it across many populated areas and the amount of material that would would survive to ground make it impossible to achieve the E[sub]c[/sub]
You’re only option is to invest in some non-chemical, electric technology and hope that something useful is developed.
Aside from the space elevator, non-chemical is a big hurdle, unless you just mean non-combustion. You either have to get hurled off the earth with enough energy imparted at takeoff to make it into space, or you have to throw something toward the ground to make you go up. That something would usually be called chemicals, unless your just throwing particles, and then people would just argue about whether that constituted a chemical rocket or not.
What’s the big deal about combustion anyway? Hydrogen/Oxygen rockets are non-polluting, and the fuel will be cheap with all that cheap electricity available.
Cryogenic propellants, and particularly liquid diatomic hydrogen (LH2) is difficult to handle, cannot be stored for long periods of time, is a significant personnel and material hazard, and rockets based upon chemical propellants ultimately suffer from scaling problems; the amount of tankage and supporting structure that has to be carried with payload increases disproportionately with the mass of the payload. The ideal propellant would be something like liquid nitrogen (LN2) or even liquid water, which are chemically non-reactive, relatively easy to store, and have known stable thermodynamic properties. However, these require an external energy source to heat them to useful temperature differentials for propulsion. An even better propellant in vacuum is something very lightweight such as ionized monotomic hydrogen which allows for very high exhaust speeds (albeit at lower thrust per volume of propellant), but aren’t practical in atmosphere and at thrust levels from the Earth’s surface.
Even with a sufficient ground-based cannon/catapult/mass driver, you still need rockets on the craft itself. Any non-escape orbit which intersects the Earth once (i.e., at the launch) will intersect it again. You need to first launch up to orbital height, and then fire thrusters to circularize your orbit so you don’t just come right back down.
Note that I said “non-escape orbit”. But if you’re trying to end up in orbit around another planet, you’ll usually end up with the opposite problem: Any orbit which comes from a long distance away will end up going back out to a long distance away. You can largely get around that by using chaotic interactions with other bodies, if you’re trying to orbit something that already has a bunch of other things orbiting it (like, say, going into orbit around Jupiter). But then, you’ve got yet another related problem: Any orbit you can get into via chaotic processes, you can also get kicked out of via chaotic processes. In practice, dealing with that problem is much more efficient than dealing with the other two, since you can use fairly small thrusters to keep from getting into a pattern that would end up ejecting you. But you still need at least some thrust on the vessel itself.
Even at dozens of kilometers in length, such an accelerator would have to have to accelerate the payload at tens of gees in order to attain enough speed to achieve orbital altitudes and overcome atmospheric drag. A conservative estimate for a terrestrial accelerator capable of launching a payload with enough length to achieve Low Earth Orbit speeds (including atmospheric drag, requiring an exit speed of about 9.5 km/s) is on the order of 450 km in length at 100 m/s[sup[2[/sup] constant acceleration (10 g). this estimate scales roughly linearly, so you could get to 90 km length for a 500 m/s acceleration, although how you would build a useful payload or launch apparatus to survive such inertial forces and aerodynamic heating is nearly unprecedented. I say “nearly” because the interceptor Sprint of the short-lived Safeguard anti-ballistic missile system accelerated to ~100 g, and the HiBEx testbed accelerated to more than 300 g. However, both only had to survive only a short altitude (~30 for Sprint, ~8 for HiBEx) before intentional destruction, not survive to orbit, so they could utilize ablative cooling measures and accept high thermal loads. The payload, (a compact nuclear weapon for the briefly operational Sprint, an instrumentation package for HiBEx) could be purpose-designed to accept such high loads for a brief period of time without having to perform the delicate deployment and maneuvering operations that most spacecraft payloads do on orbit.
In short, using a linear accelerator to launch payloads from the surface of the Earth to orbit is as much or more science fiction than beanstalks or antimatter rockets, and as a practical means is scarcely more credible than anti-gravity or teleportation (insofar as we can at least assert that there may be unknown principles that make they latter feasible). Launching payloads using a very long solar-orbiting accelerator may be just possible some day (albeit beyond conventional materials and propulsive technology) but launching payloads from a planetary surface would require beyond conception material strength for both the accelerator and the payload.
Yeah, I got interrupted in that post and was going to finish with exploding spacecraft. I think we’ll be using combustion for a long time, but if ground based lasers or hypothetical fusion reactors are used in spacecraft, heating a container of water or liquid nitrogen are likely propellants. I’m considering mainly reaching earth orbit. Combustion rockets combined with air breathing engines like ramjets or scramjets, or or combined with electric rail gun or cannon type technology seem likely to be the next step on the horizon. Also, manned versus unmanned missions will also make a difference in the technology used. Unmanned rockets can afford a lower safety factor, and more strenuous launch conditions. I couldn’t find the cite, but someone posited that we are nearing the point where Single-Stage-To-Orbit craft could be built if they could be launched at 10,000 feet above sea level at the equator. We could build spaceports in the Andes or Himalyas, or use balloons or craft dropped from air-breathing planes to achieve that using combustion rockets.
Although there is a definite advantage in terms of orbital momentum for launching from the equator, launching at altitude offers only modest advantages (albeit for a single-stage-to-orbit vehicle, it does mean less compensation for changes in ambient pressure). Although it may seem that being 10,000 feet higher is as many feet less to travel, the true measure is the gain in speed required to achieve orbital energy, which is marginal from such a small increase in altitude.
Actually, the SSTO is at least marginally possible with existing propulsion technology. The first stage of the the LGM-25 ‘Titan II’ is effectively an SSTO (albeit one with a minimal payload). Very modest improvements in existing engines combined with altitude-compensating nozzles (like the plug nozzle or aerospike) would allow for SSTO capability with very useful payloads; indeed, one radical proposal for the American Space Transportation System proposed by Chrysler Aerospace had a large craft with an Apollo Command Module-like outer mold line using both aerospike engines (for ascent) and air-breathing engines (to provide some modest cross-range and soft-landing capability after a blunt-body energy wasting re-entry maneuver) capable of carrying heavy-class payloads and a personnel shuttle to orbit, and returning as a unitary vehicle. The DC-X ‘Delta Clipper’, being originally developed under the aegis of the Strategic Defense Initiative Office and latter transferred to NASA, showed good promise at developing the necessary technology for an SSTO vehicle, but was poorly funded and ultimately cancelled despite significant successes in suborbital flight.