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I am writing a short story about a fighter pilot on Venus, I was advised on my other post to break up the post into multiple others.
What are viable methods to power a fixed-wing aircraft in the upper Venusian atmosphere as it exists right now (i.e. not terraformed)?
Given that these aircraft would have to engage in combat maneuvers, the engines would have to be sufficiently powerful to allow for that.
Given there's no free oxygen in that atmosphere (that we know of), you're pretty well limited to non-combustion power sources (or rockets, but they have very poor duration due to having to haul both fuel and oxidizer).
First, battery electric, either via propeller (strictly subsonic) or ducted fan (potentially at least transsonic). Energy storage is the bottleneck, but if you don't mind handwaving a major (physics breaking) advance in battery or supercapacitor energy density it might be plot-viable.
Second is nuclear (fission or fusion), either nuclear powered propellers (see nuclear B-36 project history) or nuclear jet (itself divided into turbojet/turbofan and ramjet). Nuclear ramjet was demonstrate in southern Idaho in the 1960s, as I recall, and the engine worked -- and maybe Venus is the right place for this sort of thing (or maybe fusion as a heat source will be less prone to leave particles from the engine and core in its wake).
The fact you want to drive fighter jets basically rules out all sensible electrical power options. Without a ready source of atmospheric oxidiser, fuel cells have no particular benefit over batteries and batteries just don't have a high enough energy density to let you do any fancy high-speed manoevering.
Maybe if you wanted WWI air combat i spaaaaace on Venus then batteries would work fine, but if you wanted anything more aggressive than that (even WW2-style) you need nuclear power.
Nuclear jet engines are certainly possible. None have actually flown an aircraft, but a working nuclear reactor has certainly been run on a (large) aircraft by the US, and the USSR seems to have run similar tests.
This is the HTRE-3 test nuclear turbojet, minus a support structure. It did run, and could power up on reactor heat alone. It is pretty hefty though, and would have fitted in a bomber-sized aircraft, not a fighter.
It would be very difficult to miniaturize such an engine. Fission reactors are tricky to shrink, and there's a minimum mass of uranium required and a minimum mass of shielding so the rest of the aircraft (and the crew!) don't get cooked.
Supersonic nuclear ramjets were also developed as part of Project Pluto, with the Tory II-c model being actually run for several minutes.
(image source: Nuclear Powered Jet Engines: A Bad Idea that has Not Gone Away)
The problem with ramjets is that you need to get them up to speed somehow, so that the flow of air into the intake is sufficient to provide enough thrust to keep it flying. For going fast they'd certainly outperform a nuclear jet, but they won't run at lower speeds which probably harms their usefulness as in a fighter craft (as opposed to a missile).
In either case the biggest problem is finding some fissiles. Venus probably has similar reserves of Uranium to Earth, but good luck digging that stuff up. You'd have to import it, but then you'd have to import almost everything so it isn't the end of the world.
On the bright side, any catastrophic failures aren't going to be causing any environmental concerns, because you won't get much fallout above the clouds and no-one will be visiting the crash site in person.
If your tech level can swing to compact fusion plants, that might be better. Fusion might scale down better than fission, and there are fusion reactions that reduce the amount of seriously problematic radiation such as p+11B. Though often billed as "aneutronic", you'll still a small number of neutrons and some very nasty high-energy x-rays so you can't skip the radiation shielding.
You can use a fusion-to-electricity setup to drive jets or propellers, or a simpler and probably much more efficient heat-exchange system that uses fusion energy to heat incoming air in the same way that the HTRE-3 design did. There are even more exotic possibilities such as using direct energy conversion of high-velocity fusion products to drive a megavolt-level electron beam which could be used to heat up incoming air like a "normal" nuclear jet, or use any other solid propellant for a quick high-thrust boost (up to and potentially including operating as an SSTO rocket for space operations).
Burn metal!
That is also the title of the book. Insert power chord here.
Venus is loaded with oxidizer if you are burning the right stuff. There are a number of scholarly papers looking at reduced metal as fuel and CO2 as the oxidizer. Metal is an awesome energy storage medium and fuel - stable, energy dense, easy to transport and abundant in the universe. Reduced metal does not burn easily outside on a cool spring day, but if you get metal hot it will rip oxygen away from lots of other elements including water, CO2 and other oxides. Consider the thermite reaction where normally staid and unflappable aluminum metal gets hot and bothered and grabs oxygen wherever it can get it, including from iron oxide.
In your aircraft, the burning of metal fuel in CO2 will provide the heat and ambient CO2 the working fluid to drive a piston engine and a propeller. Your aircraft exhaust will be incandescent particles of metal oxide and glowing carbon soot.
Background reading:
Combustion of aluminum particles in carbon dioxide
...Because Martian atmosphere consists largely of CO2, a propulsion system using metals as fuel and CO2 as oxidizer would enable one to utilize Mars's resources very efficiently.
Ignition and combustion of metals in a carbon dioxide stream
In the prospect of using metals as fuel of breathing combustion engines in carbon dioxide rich planet atmospheres without oxygen such as those of Mars and Venus, a fundamental study was performed experimentally on the ignition and combustion of metals in an impinging pure CO2 gas stream. Metals selected were lithium, magnesium, boron and aluminum, because of their high heats of reaction with CO2.
The lack of oxygen combined with the very dense atmosphere of Venus may provide a unique kind of approach to battles in the skies of you Venusian future. NASA has said solar-powered aircraft would be uniquely suited to the skies of Venus. But this doesn’t get you the high-speed fighters you’re imagining.
Because of the challenges of surface mining, your planes will likely consist mostly of very light carbon materials (readily obtained from the atmosphere). Similarly, rocket fuel can be made from atmospheric components. And since the atmosphere is so dense, even breathable air is a lifting gas on Venus.
I propose solar-powered cruising and battery recharging for your planes. Prop engines can provide faster maneuvering for sustained skirmishes, probably the same engines used for sustained slow flight. Rocket engines are carried to provide quick responses and sudden maneuvers, and inflatable solar panels allow your aircraft to stop mid-flight as lighter-than-air craft and sit, recharging batteries.
If a rocket fuel (or potentially even a compressed gas) can be produced from electricity and the atmosphere mid-flight, the pace of your wars will look very different.
- I'm trying to find if there is any way to somehow get enough reactivity from the sulfuric acid to use it in place of an oxidizer. IF a way to make this work could be found, then sulfuric acid in the clouds MIGHT be able to substitute for oxygen. Unfortunately, I can't find a better reference than this. Just thought I'd throw it out as a hypothetical.
Some technical options have been mentioned by the others. Chemical, electric, nuclear and fusion thermal and even more exotic options. I would add antimatter and beamed power to that list.
Venus is rich in energy (solar and wind) and carbon but poor in most other resources. The rest has to be imported. Fissiles form Earth or Mars (most concentrated ore), volatiles and fusion fel from the outer solar system, antimatter from Mercury or the outer planets, metals from the inner planets, ... You get the picture.
You could use local energy by using beamed energy, solar or even detachable windpower turbines which can get power from the lower atmosphere. These options can be combined with your best energy storage systems to power you in a fight. During the approach you hang on the cable. You might want to consider dynamic soaring as an option, if it works on Venus. You could use local deuterium for fusion, but it is a lackluster and technically difficult fuel. Mining fissiles on the ground sounds like a pain. Importing them is easier. Antimatter would probably be your best bet for great fighter performance.
Concerning the environment it might be worth considering the tactical interplay of the space (you need rockets), high atmosphere (like Earth's atmosphere), low atmosphere (boyancy is trivial to use, lasers are less effective, missiles can cruse but are slow, as are fighters) (high visibility submarine combat could be a good analogy) and ground (which is, well the ground). The more varied the environment the fighter can operate in the better.
You might want to check out the Exacting Class Starfigter from atomic rockets. It could probably operate in space as well as in an atmosphere.
First, with such a dense atmosphere a plane would not need wings, a lifting body design should be enough. Then, not only the composition of the air does not provide anything useful for a chemical reaction, but the high temperature would render any type of jet engine inefficient. So, the options would be a rocket propulsion or a propeller. But the dense atmosphere will make a lot of resistance and the rocket propulsion would have a limited range. The propeller driven plane will will be slow, but probably it will also lack manoeuvrability, an alternative that would fit in the dense atmosphere would be fins or paddles that will let it move more like a fish than a plane. Actually the most elegant solution would be this one.
For the power source the problem is the same, a nuclear generator will provide a lot of energy, but it would be inefficient. The same would be true for any combustion engine. The best way would be to have advanced batteries or a new type of super capacitor. The range might be more than what you might think. On Earth planes have to use a lot of energy to keep their height on Venus it is more akin to stay afloat, that would take less energy leaving it available for the forward motion, provided it is not too fast and it does not generate too much friction.
I believe that planes would use turbofan engines powered by some kind of non-cryogenic rocket fuel/oxidizer mix. The problem with rockets is that the fuel is also the reaction mass, which subjects you to the tyranny of the rocket equation. With a turbofan engine, the fuel just provides energy, and the ambient air is (most of) the reaction mass. Having to carry oxidizer would cut your range by as much as 3/4, which is a lot. But I don't think anything else in our current toolbox has close to the same power density. The design would have to be different than a jet engine since you don't want the ambient air to interfere with the combusting gases. Perhaps a gas turbine engine attached to a ducted fan by a gearbox.
If you're using a modern fighter jet, you'd need to carry your own oxygen, and have a device that injected that oxygen into the air stream as it entered the engines. You'd need an oxygen supply for the pilot, too, of course. Beyond that, I think you'd be just fine as long as you stayed above the clouds, in the appropriately pressurized part of the environment.
In response to the comment that "this would be just a rocket":
There are numerous difference between a jet and a rocket. For instance, jets have flight surfaces to provide lift, and they use the atmosphere as reaction mass, pushing it out behind it to increase the amount of thrust that can be achieved from a gallon of fuel. Jets at less than the speed of sound can also use incoming air to cool off the engines, whereas rockets have to cryogenically freeze their fuel and pump it through the bell at the bottom of the engine to keep the bell from melting off of it.
I could also point out that most rockets are designed to turn on once, burn through all of their fuel, then detach and ditch. Jets are designed for a wide degree of throttling, and can even restart the engines while in flight (usually).
The stoichiometric ratio of kerosene to oxygen is around 1:2.6, so you have to expect that a jet that carried its own oxygen would have about a quarter of the flight time of an air-breather.
If we convert an F16 to this technology, it would be able to stay in the air for about an hour. With afterburners going, a NORMAL F-16 has a flight time in single digit minutes.
If you're talking fighter planes, it's entirely a question of energy density. Normal aircraft can be lighter than air and solar powered, and run everything off of batteries. For fighter craft, you can't get even close to the energy density of chemical combustibles, even adding in oxidizers, without going nuclear, and then you've moved into the realm of hand-wavium, and there's no point in even doing math.
