Okay, the time machine itself is science fiction. I've got some handwavium about transformation of Einsteinian equations and unconventional polarization of electromagnetic frequencies and so forth. Here I'm trying to conceptualize the support equipment for it and I want to keep that strictly true to known science and engineering as it existed in 1962 (as a tip o' the hat to BTTF my protagonist had his flash of insight in November 1955 and it took him seven years to develop a workable prototype). Here are the rules I'm following:

  • The time field generator requires an "activation energy" proportional to the mass involved. The activation energy works out to 260 ft-lbf of energy per pound-mass, or 778 J/kg in SI units (Australian character in 1962; uses SI and EES units interchangeably but his consulting engineers use mostly EES). This must all be delivered to the device in approximately two seconds or a 5th order decay reaction overcomes it and the time field "fizzles". (Another hat-tip to BTTF: This is exactly the energy needed to accelerate a mass from a standing stop to 88 miles per hour...)
  • The time field generator also requires a sustaining energy input proportional to the volume of the field. This is much less but needs to be delivered continuously. The "slope" encountered within the time continuum is proportional to the frequency of the energy input. During my hero's first time jump he is using a 400 Hz aircraft alternator and inadvertently jumps back in time some four centuries earlier than he planned! (A little difficult to get petrol in Western Australia in 1519...)
  • Upon loss of the sustaining energy the object exits the time continuum, randomly displaced in position by up to 500 or so feet, and with an imparted kinetic energy from the decay of the activation energy. This manifests as a velocity vector (summed with the object's initial velocity, if any), in an entirely random direction with a normal distribution centered around 58 feet per second and extremes of 36 and 80 fps.
  • If an object emerges from the time continuum superimposed on a solid object, its structure merges with that of the solid object. (With, need I say, fatal results for the occupant!) However, the process of emerging takes a few tenths of a second and gases, and to a lesser extent liquids, can 'get out of the way'.
  • Aside from the displacements in both time and speed due to the decay of the activation energy, the object emerges exactly where it originated with its initial velocity vector regardless of how much time has elapsed in either direction. The time continuum is affected by gravitational fields and planetary rotation and objects within it follow them in a stable manner. Of course, I'm not touching the question of a planetary impactor...
  • Simple molecules, especially gases, have a multidimensional presence and are present within the time continuum. There is breathable air at normal pressure within.
  • The preceding rules, save for the frequency relation, were "discovered" by my character (a professor of Physics at a fictional Perth college) during lab bench tests between 1955 and 1960. He also served as an RAAF pilot during the Pacific War, and he hits upon the idea of building his contraption into a DC-3.
  • To supply the activation energy he uses a pair of counterrotating 830 lbm (each) 60" diameter flywheels designed by the college's engineering department. Rotor windings are built into the flywheel rims and stator windings surround them. Except for the windings, the rim of the flywheel is built of martensitic steel and the hub, bearings, and counterrotation transmission are lightweight titanium. The flywheel spins at 5000 rpm driven by titanium alloy turbine rotors (direct drive) which are in turn driven by bleed air from an AiResearch GTCP85 APU unit.
  • To activate the contraption, while in the air at a safe altitude he shifts the APU from max bleed air to max electrical output to energize the sustaining field, then energizes the rotors through slip rings to supply the activation energy. With a flash of blue light, the DC-3 disappears from our skies...

...and emerges over Western Australia in the year 1519 (or so). Which leaves me with a couple of questions: First, how is he going to find out when he is and navigate back? Full disclosure: I posted this on astronomical SE and they directed me here. If you want to answer that specific question you can go there or, if comments suggest, I'll make it a separate question here.

The more important specific question I have is that, as I said, he takes off using a 400 cycle alternator and overshoots his destination...badly. He was misled by his lab tests which seemed to indicate that the time displacement decreased with increased frequency; what was actually happening is that he was supplying power from outside the field as he had not yet assembled a suitable self-contained apparatus and it took longer for the lower frequencies to be affected by loss of input power.

Once he realizes that, he "detunes" the alternator as much as possible. So, not being well versed in electrical engineering, I'd like to know how much a 400 Hz alternator frequency could be lowered without overheating and blowing the windings. Let's say, for purposes of the exercise, it's a 40KW maximum capacity alternator being operated at 20KW output and that he has the tools with him to bypass/alter the settings of the (mechanical, 1962) governor. Presuming that the contraption is able to handle voltage variations, could the ability of the alternator to handle frequency changes be increased by making voltage adjustments?

Although I want him to have to make some stops along the way, I'd like for him to finally blow the 400 Hz alternator in 1943...just in "time" to get a fill-up of petrol and paraffin (kerosene) and to find a 25 Hz alternator which can be adapted to his APU, courtesy of a wartime buddy who wonders why he suddenly came back to the base he was transferred from six months ago but who welcomes him with open arms. My, those six months really aged you....

P.S.: I should also ask those knowledgeable whether a flywheel such as I described (60" diameter, martensitic steel rim, spinning at 5000 rpm) is compatible with 1960-era technology. I've worked out the energies and needed moment of inertia, but I'm not up to computing stresses at the rim.

  • $\begingroup$ Welcome ehbowen. Please take our tour and refer to the help center for guidance. Nicely explained ideas. We deal with just one single question per post, but you can write other questions in their own threads, hyperlinking to the main one - we like question-series. Please use the edit function to give us just the one for starters. $\endgroup$ Sep 10, 2022 at 17:12

1 Answer 1


You might find this document interesting it has the schematics and training information for the Douglas DC-3. I couldn't tell is your APU was ever used on DC-3s, but seems to have been used on DC-9's. I don't know much about planes, though so just a comment.

If it is a synchronous alternator I think you could lower the frequency quite a bit. The output frequency unloaded is just related to speed that you are turning it. I am assuming it is not having to synchronize with another bus that has to be at a certain frequency. For your case, it seems you would have options.

If for some reason, there was another bus at a higher frequency that you were trying to connect to, then you could have a problem since your would have a lagging phase and you could have a large current. So it would depend on your story details.

Since it is 1962, you might have some options for some other kind of problem. For example solid state inverters to convert DC to 400 Hz or 400Hz to DC - since silicon power electronics were pretty new perhaps they were somewhat unreliable, or easy to blow up with a transient. Before the silicon rectifiers and switches were used for inverters there would have been other methods available in the 1940's

In your alternator if it was to charge the batteries for example one of the diode could go bad and there could be a some why to have to rig some kind of mechanical relay or other rectification method.

  • $\begingroup$ Actually, I found that ground training manual already while doing research, but thanks for the suggestion. The installation will be a 'one-off' as DC-3s didn't use APUs (except, possibly, for other unique one-offs). I know that reducing the rotational speed of a synchronous alternator changes the frequency, but it also has effects on current and voltage as well as heating of the armature. I'm looking to see how low I can get it (the frequency) believably before those side effects would ruin the hardware. $\endgroup$
    – ehbowen
    Sep 11, 2022 at 1:25
  • $\begingroup$ @ehbowen the Engineering stack might be a better place to ask, (but without the world building context). I may misunderstand your set up, but I think it is more about how the armature is loaded (which drops the frequency), vs running an alternator/generator at a low frequency which is o.k. (through a power factor of 1 or a resistive load) until you either try to synch it to a bus at a different frequency. 20% change in frequency or more?. A 50 Hz motor can run on 60 Hz and vice versa especially if you take care to adjust the voltage properly. $\endgroup$
    – UVphoton
    Sep 11, 2022 at 14:37

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