A side shot from my series on a computer-less future; see here, here, and here.

Project Rho defines a torch-drive as an engine with both high acceleration and high exhaust velocity. A torch-ship is a ship with a torch-drive and a specific power of 1 MW per kilogram or larger. The linked page has discussion of various types of torch-drives and others are available on Wikipedia.

Some torch-drive technologies (there are many more that meet the criteria):

Which torch-drive technology could be implemented in the near-future with the least complex (here defined as cost of design and production) control systems?

  • $\begingroup$ Just a side note, I wouldn't want to be anywhere near a fission reactor that wasn't being controlled or monitored by a computer. In my opinion, Nuclear pulse propulsion will probably be the winner. Takes only knowledge of the blast energy, structural capacity of your vehicle, and some orbital mechanics to work out how long to set the timers on the things before dropping them. $\endgroup$
    – user44399
    Nov 13, 2018 at 13:00
  • $\begingroup$ @B.fox Just pretend you are in interplanetary space, with 500 meters and a radiation shield between you and the reactor. $\endgroup$
    – kingledion
    Nov 13, 2018 at 13:02
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    $\begingroup$ @Separatrix I think the question is clear-cut once you analyze the listed propulsion devices. Each method requires certain parameters which themselves require certain delicacies of control. $\endgroup$
    – user44399
    Nov 13, 2018 at 15:10
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    $\begingroup$ @Separatrix It's more like a secretly disguised hard-science question. There are plenty of questions that people on the site deem too broad or opinion based, but that I know has a simple answer because it lays within my area of expertise. I don't know much about control systems; for someone who does this question shouldn't be that hard. I hope. $\endgroup$
    – kingledion
    Nov 13, 2018 at 15:15
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    $\begingroup$ @kingledion, I see that a lot as well, answerable questions that people seem to be closing on "I don't know". As in this case I tend to withhold from voting on things I don't know, and hope that the people voting to close know enough to make that judgement. $\endgroup$
    – Separatrix
    Nov 13, 2018 at 15:23

4 Answers 4


Nuclear Pulse Propulsion and Nuclear Salt-Water Rockets

Of the other propulsion methods, these two seem to be the least complex. The other methods require control systems that dive into the operation of the reactor itself, in some cases to an almost intimate and impossible level (for the constraints).

For a fission-fragment rocket, one would want to monitor the quantity of neutrons being released by the fissionable materials consistently and without much error, else they risk an overreaction of neutron bombardment—a meltdown. These kinds of torches also require some electromagnetic channeling, which itself warrants a subsystem of monitors and controls. Should this fail, you may risk damaging the exhaust nozzle of the rocket and perhaps other things. There are also other constraints of the device's operations one would want to consider monitoring and controlling, such as the temperature of the ionized particles (you're working in the range of potentially tens of thousands of degrees Kelvin, a few thousand, minimum), and the various integrities of the components of the device; if its a rotating reactor, then you have moving parts and specific alignments to worry about.

Gas core rockets have the advantage that they can gain much higher temperatures, thus higher specific impulse. With great power comes great responsibility. A control system for this torch would need to monitor the temperatures of the core's nozzle and containment, which are the structural crutches of the design (for their temperature limitations). One would want to monitor the structural integrity of the containment and control a neutron moderator to increase or decrease the frequency of reactions. This design also requires an effective cooling mechanism, either through external radiators or gaseous/liquid coolant passing through the nozzle. For the sake of maintaining structural integrity or maximizing/minimizing specific-impulse, one would want to control the rate at which this mechanism dissipates heat. In addition to that, it is thought that there needs to be an internal absorption of thermal radiation as well, to control the rate of reactions. Tungsten particles would seem to be the popular choice, and the rate at which these particles are pushed through the reactor would need to be controlled for similar reasons as the cooling mechanism, to delay the degeneration of the device's components.

A fusion rocket has the benefit that it cannot meltdown. Much of the concept of a fusion torch is theoretical. There seems to be two predominant types: inertial and magnetic containment.

  • For the first, one would need to inject pellets of fuel into the reactor and ignite them with high-energy beams of photons or electrons, take your pick. You would need a reliable method for injecting the pellet fuel and a reliable method for activating the beams at precisely the right time and at the right intensity. Typically, you'd do these things inside of a vacuum chamber, so the state of vacuum would need to be controlled. For a successful fusion reaction, one would need to control many beam-to-beam energy ratios on very short timescales (perhaps microseconds). This would seem impossible for an analog system to handle.
  • For the second, you would require a myriad of control mechanisms and monitors for maintaining a magnetic containment field within small error margins. What you are essentially doing is balancing two powerful forces: a magnetic pressure and a plasma pressure. A fusion reactor is constantly working to overcome turbulence created by high-velocity, flowing ions as they are carried and conducted by magnetic field lines. You have to concern yourself about quantities of the fuel escaping the magnetic containment and interacting with the reaction chamber's walls. All in real-time, and without electronic computers. As for the control surfaces required, in the operation of a fusion reactor, you would need to be a helicopter parent to it, controlling, monitoring, and real-time analyzing just about every aspect of its operation.

Nuclear salt-water rockets look promising. The amount of fuel you feed one of these and the degree of temperature it gets to are almost of no concern. One manner of control would be the fuel-feeding system. The particles constituting the nuclear fuel are said not to diffuse all at the same, or even at a regular, velocity. They occupy a broad range of many orders of magnitude, and so it is possible that free-moving particles of the fuel may backtrack into the fuel-feeding system, potentially damaging or destroying the system. Of course, you'd want to have control of the fuel injection rate anyway, to adjust acceleration and whatnot, but this is another reason to have that control. If a detector detects a large enough quantity of the fuel being accelerated back into the system by its own diffusion rate, then you may want to close the system for a short time and let the disturbance pass. This seems to be the only control surface for this device.

Nuclear pulse propulsion has been experimented with in the past (see Project Orion) and is fairly well-understood. What you are essentially doing is dropping small (or large, if you're looking for a good time) nuclear explosives behind your ship and detonating them at a distance where it is both "safe" and the explosion will impart a good fraction of its momentum onto your craft. You'd need a control to drop the bombs and a control to detonate the bombs. The intermittent stuff is up to your analog computers to figure out. Dropping should be simple. While your craft is experiencing imparted momentum from the previous blast, release another bomb so that your vessel accelerates away from it. My personal recommendation is that you don't put the actual timer (detonation control) on the bomb itself. Let the bomb be activated by some external force, like a focused laser beam vaporizing some part of its surface, a "fuse," or a hypervelocity projectile smacking into it, fired from the ship, a gun perhaps. Why? Safety reasons, of course. Not because it'd be cool to shoot at nuclear bombs all day.

  • $\begingroup$ Shooting lasers at nuclear bombs? No. Can’t see what would be fun about that at all. Nosiree. No fun to be had there... $\endgroup$
    – Joe Bloggs
    Nov 15, 2018 at 8:27
  • $\begingroup$ I particularly like the nuclear salt-water because of the nuclear capture cross-section of Sodium as in Sodium Chloride (and I am remembering my high-school era explorations of Van Nostrand's so I could be off here) had an anomalously high cross-section suggesting its use as a moderator or absorber, which means it converts more neutrons to hot-water. $\endgroup$ May 19, 2022 at 3:43

The Orion Nuclear Pulse Drive is the closest candidate. With the exception of the timing mechanism to fire the nuclear pulse units, you could actually have everything else done by straight mechanical drive. The mechanism to load the pulse units was based on a vending machine, and the cannon to fire the pulse units behind the ship was essentially a giant air cannon. Mechanical timers can run most of the devices, and timing of the shots could be moderated by a cam system in the shock absorbers to trigger the timing cycle.

It is perhaps the ultimate dieselpunk sort of technology imaginable, and if the General Atomic crew had been given their funding, they could have gone to Mars by the end of the 1960's, and Saturn by 1975 according to their timetble


Least complex: all by hand.

steelworks source

If you want retro, go full retro. Chains, pulleys and main strength. Your model will be a mid 1800s iron foundry. Instead of that (alarming-looking) crucible you have a self-sustaining fusion reaction lifted from H.G. Wells The World Set FreeYour crew points their various rockets in the direction called for by the captain, quenching or firing up the reactions as warranted. It will take several straining shirtless men to turn off one of the rocket tubes - which is how you get the thing to turn.

  • 4
    $\begingroup$ If you thought Victorian foundry workers had bad working conditions, wait until you see the acute-radiation-poisoning and vacuum-of-interplanetary-space related death rates for the Atomic Rocket-Tube Men. Cool name for a metal band, though. $\endgroup$
    – kingledion
    Nov 13, 2018 at 16:23
  • $\begingroup$ "All by hand" isn't a type of engine: it's a proposal for how to replace certain control systems with human power. How, and if, that is feasible will depend on the type of engine used. For fusion power, how are you planning on containing the plasma "by hand"? Generally, that requires a complex and fragile set of electromagnetic fields/lasers/etc. to accomplish. $\endgroup$
    – ckersch
    Nov 13, 2018 at 17:51
  • $\begingroup$ @ckersch - Ships could use a diamond crucible full of /Carolinum, which belonged to the beta group of Hyslop’s so-called ‘suspended degenerator’ elements, once its degenerative process had been induced, continued a furious radiation of energy and nothing could arrest it. Of all Hyslop’s artificial elements, Carolinum was the most heavily stored with energy and the most dangerous to make and handle. To this day it remains the most potent degenerator known. What the earlier twentieth-century chemists called its half period was seventeen days.../ See section 4 of linked Wells story for more. $\endgroup$
    – Willk
    Nov 13, 2018 at 18:30
  • $\begingroup$ I might be wrong here but your answer seems to hinge on an entirely fictional process using an entirely fictional element? $\endgroup$
    – Joe Bloggs
    Nov 15, 2018 at 8:31
  • $\begingroup$ @JoeBloggs - well, the guys and chains and pulleys were real. $\endgroup$
    – Willk
    Nov 15, 2018 at 11:59

I don't think it gets much simpler than the nuclear saltwater rocket; you should, depending on design specifics, be able to steer and control a nuclear saltwater rocket using mechanical valves, gimbals, and gyros. This is because nuclear saltwater rockets use the same design, in principle, as liquid fueled chemical rockets just with more delta-v and higher fuel energy density. All you need to control is the rate of fuel flow to the reaction chamber and the direction of the exhaust from that reaction.

Plotting orbital maneuvers is a different kettle of fish and will require a lot more computational power unless it's being done "by guess and by God" using marque one eyeball, maybe a sextant, and hands on manual steering, which it could be. Even a fully digital system needn't be much more complex than that used by the Saturn Vs and your smartphone has orders of magnitude more computing prowess.

  • $\begingroup$ Orbital maneuvers aren't really the focus of the question. Do you have any sourced information on what the controls might be for a NSWR? $\endgroup$
    – kingledion
    Nov 13, 2018 at 13:29
  • $\begingroup$ @kingledion No. They're the same as for a chemical rocket, you only need to control the rate of fuel entering the combustion chamber and the direction of the motor exhaust. $\endgroup$
    – Ash
    Nov 13, 2018 at 13:31

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