# Designing vehicles for different gravities

In my story, humans have colonized both a high-G and a low-G world in the same star system. How would surface vehicles have to be designed differently for practicality and economy on each of those worlds?

World A (Yanacocha) is a large carbon planet with surface gravity 2.12 times Earth. Theoretically, the crust of a carbon planet would be mostly graphite. The atmosphere is relatively thin (~44kPa) and primarily carbon monoxide, with potential for graphite dust storms, so I expect the colony to be mostly contained within sealed domes and tunnels. The primary function of the colony is mining.

World B (Serana) is a smallish Earth analogue with surface gravity 0.7 times Earth. The surface conditions should be broadly similar to Earth aside from the low gravity and associated thinner atmosphere (about 68kPa at sea level).

Edit: Added carbon planet link and details re: surface conditions on both planets.

• We'll need to know more about the atmosphere as you've made it hostile. That means materials and lubricants are significant. Commented Dec 14, 2018 at 15:13
• @Separatrix added more details for Yanacocha's atmosphere. Commented Dec 14, 2018 at 15:44
• @RichDurst I think you should change your hard-science tag to science-based Commented Dec 14, 2018 at 16:50
• @SherwoodBotsford I agree. The hard-science tag isn't needed. The science-base tag is the better choice. Please edit the tags, RichDurst. Commented Dec 15, 2018 at 4:03
• @RichDurst How fiendish your atmosphere is: Carbonmonoxide is a strong poison, and all humans will have to wear really gas-tight suits or sit in gas-tight enclosures or they die. Commented Dec 17, 2018 at 16:46

General

The majority of vehicle design need not change. This is because the mass of the vehicle remains the same, no matter what the gravity is. When dealing with the classic equation:

$$F = mA$$

The effect of gravity is in the "A" (acceleration) term. When the gravitic force is perpendicular to the vehicle (when you're cruising down main trolling for a date Friday night), that force has no affect on acceleration. On the other hand, when you're ascending or descending a hill, a component of gravity is affecting your acceleration. But, in the end, it's just force, so there's little difference from modern design. But, of these differences, I can think of the following:

• Fuel and coolant pressures will change moderately between gravities. However, this is easily overcome with a pressure regulator that consistently feeds the pressure appropriate for the engine despite changes in torque. This problem is already solved today (since torque and fuel feed already change when you climb a hill) using the engine's vacuum system. Basically, as vacuum pressure increases, so does fuel pressure or volume (depending on how the engine is actually designed).

• Braking will change between the gravities, but again, only somewhat. The greatest problem will be the need for increased braking descending a hill on a high-G planet. Once again, modern tech already exists (via the ABS systems) to adjust both braking pressure and oscillation frequency to manage braking heat vs. braking friction. In the end, you'll use the same pads on both planets — they'll just last longer on the low-G world.

Friction Traction

Braking isn't simply pads on a rotor, it depends on the friction between the driving surface and the wheel or tread. This is affected by gravity. We could get into the process of calculating the coefficient of friction, parallel motive force, blah, blah. The problem is that it depends on much more than just gravity. It depends on the material of your tires, the material of the street, the angle of ascent at any given moment, the square acreage of material against the road, etc., etc.. Suffice it to say, if you have good tire rubber, you need more of it on a low-G world than you do on a high-G world to compensate for the change in gravity. Unless you're planning to lift-and-lower wheels, there's not much you can do here but change tire widths between planets. But, one-size-don't-fit-all here on Earth, so you should expect that.

Suspension, on the other hand, is something you can do on the fly. If you were to design your suspension for the high-G world and try to use it on the low-G world, you'll find yourself bouncing all over the place. You could use a pneumatic suspension that adjusts the compressive force used for smooth driving by analyzing the weight (not mass) of a known mass (yeah, there's the mass). In other words, if your 1Kg mass weighs 4 pounds on your high-G world, the compressive force is increased by adding more air to the penumatic shock absorbers. That same mass may weigh only 1.8 pounds on your low-G world, thus air is removed.

Turning, is a form of braking — we just don't think of it that way, expecially when Hollywood has been advocating that a constant acceleration through a turn will increase your final velocity. That's actually true (of course, it's true without the turn, too. Hollywood tends to ignore that part.)— so long as your tires actually remain firmly fixed on the road. Exceed the force of friction and you slide off the cliff, usually in a brilliant (if unexplained) fireball. The flaming tire bouncing into the distance is mandatory. However, while your vehicle's onboard computer is calculating the amount of air to force into your pneumatic shocks (and your air brakes, for that matter...), it can also be calculating now much to lean the tires into the turn, ensuring greater friction on the lower-G planet.

OK! Hovercraft don't touch the road, but (today) use skirts to hold a cushion of air beneath the vehicle. You have a propeller for motive force, but otherwise the same rules apply to the engine (although the operator is manually increasing the throttle rather than the engine vacuum system doing it. No friction on the ground, no increase in torque.). In fact, other than having to increase the air pump system to hold the unit up on the higher-G world and a bit more throttle on the prop to climb a hill, there's not much difference in how hovercraft would be handled.

But what about real hovercraft? You know, the stuff we haven't invented yet?

OK, you're not holding that hovercraft above the ground on a cusion of air, your riding the magnetics (hard) or using anti-gravity (harder). In reality (if that word can be applied to something we can't actually do in reality), it's all the same problems. More force toward the center of the planet to hold the vehicle above the ground and more force behind the vehicle to move it up a hill.

So, one size really doesn't fit all

No, it doesn't. The design of a dump truck is very, very different compared to the design of a scooter. The fundamental physics are the same, but the needs and purpose of each vehicle vary so dramatically that it's impossible to claim that one size could or should fit all applications. And since you shouldn't do it in real life, you shouldn't do it in your story, either.

But, for the sake of argument, lets assume that your world is building a one-size-fits-all colonizing space ship. The Atlas Corp doesn't want to customize for every planet, they want to send the same thing to every planet (to manage costs). In that regard you might legitimately have a use for limited-design vehicles, in which case my suggestions apply.

But there would be so much customization anyway that I don't believe it's practical to think about it.

But, then again, I can believe that the tried-and-true bicycle wouldn't be different due to gravity. Two wheels, 1.5 inch wide tires, gears and brakes. The bicycle probably wouldn't change at all.

• Hard Science tag was changed to science-based, so you're good :) Commented Dec 14, 2018 at 18:50
• @Basher, oh thank goodness! I'll edit my question to remove my complaint.
– JBH
Commented Dec 14, 2018 at 18:52
• No need to analyze the weight of a known mass. Just pump the suspension pistons until you have the correct average clearance. The car's own weight directly serves as a reference mass here. Apart from that, I'd like to stress that the road contact is really the crucial point here: Cars on the high G planet will feel like Formula-1 cars, while driving without special instructions on the low G planet will be next to suicide: You'll never manage to start braking early enough with your earth-adapted reflexes... Commented Dec 16, 2018 at 14:29
• @cmaster, you make a fantastic point about human reflexes. While not intrinsically inside the scope of the OP's question, I hope he takes note of your comment. The vehicle's design might not be the primary problem.
– JBH
Commented Dec 16, 2018 at 16:09

The question asks how would surface vehicles be designed on two exotic alien worlds; and the question asks for hard science. Unfortunately, industrial and engineering design are not engineering disciplines in themselves, but rather semi-artistic crafts. So I don't believe that a hard-science answer is truly possible, or else we wouldn't have seen, for example, the numerous weird and wonderful designs practiced during the ages here on Earth for bridges, from cantilever to suspension, or for surface vessels, from rafts and monoxylon boats to caravels and clippers.

Nevertheless, a general answer can be provided.

• Step 1: Feasibility studies.

First of all, we can safely assume that different organizations would be in competition to develop and provide surface transportation services for those worlds; and it is very likely that each of these organizations will attempt to develop several concurrent transportation modes, such as individual transportation for persons, mass people transit, transportation for small quantities of goods for deliver or supply, bulk transportation of goods, containerized transportation and so on. For each of the envisioned modes of transport, the organization will establish an exploratory group, tasked with elaborating a feasibility study.

In order to elaborate the feasibility studies, each exploratory group will employ the services of a project management team and a small army of subject-matter experts with expertise ranging from alien geology, to chemistry, metallurgy and the myriad subdisciplines of materials science, including without limit, lubricants, plastics, glass and carbon fiber and so on.

The results of these efforts will be a set of broad design guidelines, describing possible approaches for each of the transportation modes under consideration, or, in some cases, an indication that the specific transportation mode is not feasible. It is generally expected that the feasibility studies will also include broad estimations of the cost and effort for development, and an appreciation of the costs and revenue of operating the vehicles.

Undoubtedly, some of the interested organizations, either very rich or very foolish, will attempt to perform this work in-house, while others will keep in-house only a core project-steering group and will farm out the work to various consulting, research and engineering organizations, such as universities, research institutes or specialized firms.

Expect the development of each feasibility study to take one to five years, and cost on the order of millions or tens of millions of euros.

• Step 2: Prototypes.

The decision factors of each of the organization will then prioritize the results of the feasibility studies, will select one or two or four of them and assign each of those selected to a prototype design group led by a master designer. The prototype design groups will be given very broad requirements, flexible budgets and somewhat flexible deadlines; their task will be mostly to prove that it is indeed possible to design and build a vehicle along the lines of the vision articulated in the feasibility study, to identify engineering problems, and to find and explore solutions to them.

Prototypes will be subjected to a sequence of practical tests and demonstrations, with the goal of dermining the limits and potential efficiency of each approach, so that those approaches which prove to complicated, too limited or too inefficient can be pruned out.

Expect the development of each prototype to take two to ten years and cost on the order of tens or hundreds of millions of euros.

• Step 3. Engineering and industrial design.

Armed with the knowledge obtained from designing, building and testing the prototypes, the leadership of each interested organization will pick one or two or four of the prototypes as the basis for engineering and industrial design, with a view of moving them to production.

At this point, the organization will begin talking to potential customers, in order to find out what their expected needs are, so that the engineering and industrial design teams will be given concrete requirements and goals; and this is also the point, if not earlier, where financing will definitely be sought, from investment banks, from investment funds, venture capitalists and so on.

Generally, the industrial and engineering design work will be given to those enterprises which will actually manufacture the vehicles; it is not common, but definitely not unheard of, for railways or maritime transport organization to build their own locomotives and ships.

During the engineering and industrial design phase, a number of production prototypes or concept vehicles will be developed and shown to potential customers and to the public; many practical tests will be performed, exploring the various engineering solutions identified during the development of conceptual prototypes; processes and tools will be developed and implemented, suppliers sought, contracts made and so on.

Expect this phase to take two to ten years and cost hundreds of millions to possibly even billions of euros.

• Step 4: Initial production.

One or two or four of the engineering and industrial designs will the be put in limited production, with the goal of testing the market and performing final design adjustments. This is the point, ten to twenty-five years and billions of euros after the start of the overall project that the organization will find out whether their vision is successful or not.

While not as hard-sciencey as some other answers, I thought I would talk a bit about the considerations that may go into designing such a vehicle.

I will say that designing a single vehicle to operate on both worlds is a bad idea. While a heavy-gravity vehicle will operate on a low gravity world, it will be hugely fuel inefficient compared to something purpose made, and shuttling vehicles between planets is unlikely to be practical anyway. So I suggest your vehicles are made on their planet of origin, for their planet of origin. I also suggest building refinaries on your carbon world - no point shipping ore offworld.

Before I start, I'd suggest having a browse of unusuallocomotion. They have a great many weird and wacky vehicles that have been build by humans. Many of the images I link to here come from that site.

High Gravity Considerations

1. The ground you are driving over has to hold the vehicle up, and prevent it sinking into the ground. On Wikipedia, you can find a list of ground pressures. On a 2.12 gravity world, those figures will about double. As a result, An M1 Abrams tank will exert the ground pressure of a typical car would on Earth. And a typical car would exert a ground pressure somewhere between that of a mountain and racing bike. This says big things about getting stuck. Relativly normal things such as mud become a much larger obstacle. Yes, M1 Abrams tanks do get stuck in Mud on earth. They would get stuck twice as easily on World A. While Mud may not exist on World A due to it's composition, you would likely get sand, and you can (and will) sink into that.

2. Similarly, any pneumatic tire pressures will have to double to keep the same contact patch. This will require thicker sidewalls, stronger beads and so on. I'd say pneumatic tires would almost certainly be a poor way to go, because the same sharp rock will be twice as bad with twice the gravity.

3. You require more energy and torque to go up a hill. Think about how much more gas you require when going up a hill than on the flat. This will be worse on a high-gravity world. As a result, you'll have to carry more fuel, which weighs more, which makes your uphill performance worse. I couldn't think of a way to quantify this, but expect your vehicles to have many many low gears, and go uphill very very slow.

4. Much mass-mining equipment has great big crane arms. If a worlds primary industry is mining, I would expect to see giant equipment similar to, say, the Bagger 288. Unfortunately, these designs wouldn't work too well on a planet with double gravity. The strength of a beam goes up with the square, but the mass goes up with the cube. This applies to everything from cables on cranes (a free-hanging rope will break under it's own mass in half the distance) to the girders that hold things up. Everything will have to be smaller given known materials. You could almost assume that the largest equipment would be half the size as it is on Earth. I'd expect this makes profitable mining a bit harder.

Where do we end up with this? I'd expect most of your vehicles to have tracks or fairly large tires. Airless tires would be a large contender. They'd move slowly, and the maximum size of your vehicles would be small.

• For personal transport inside the city, I'd expect to see power-assisted bicycles. You'd have to pave the surface really smooth, but there's no reason they wouldn't work. A bicycle is much more practical than a car for an indoor city, and the light-weight of the vehicle with respect to the payload helps with the fuel usage. If possible, use airless tires, otherwise a modern racing bike tire can take really high pressures (a 200kg guy on a racing bike?). Bikes outside a paved surface would be virtually impossible.

• For personal transport outside the city, I'd expect to see lightweight vehicles with large low-pressure-like tires. This could range from large-tire dirt bikes which rely on low vehicle weight, to rolligon-equipped cars relying on large surface area.

• For anything industrial, I'd expect to see tracked vehicles dominate. Tracked vehicles aren't common on Earth because they have lots of friction losses which necessitate low speeds. Because of the higher traction and lower ground pressure, I expect most equipment to be equipped with them.

I think this is a photo-mash-up rather than a real vehicle (because this is the only photo like this anywhere), but you may well see vehicles like this:
It's relatively small (use in the background on the left), but has a huge amount of engine power and a huge amount of ground contact. You can imagine something like this dragging trailers out of a mining pit

You may even see vehicles like the tactical tree stomper for the ultimate low-ground-pressure.

Low Gravity Considerations Low gravity is much easier than high gravity. You aren't worried about your materials breaking and ground crumbling, instead you're worried about remaining in contact with the ground. 0.7g isn't much difference, so you'd see many similar vehicles as you do today. That said there are some considerations:

1. Staying on the ground is harder. Here's the video of the moon buggy driving. You can see it bouncing around all over the place. But it's driving, and that's at 0.2g. But your family car will likely require functional aerodynamics (ie spoilers) to push it onto the ground.
2. Similarly, the friction with the ground changes. Friction is dependant on material and normal forces. On a lower gravity world, you have lower normal forces. If a car takes 10m to stop at maximum braking force limited by ground friction, it's take 14.7m to stop on a planet with 0.7g. As a result, I'd expect to see lower speed limites for safety reasons.

One thing we can compare with on earth are underwater vehicles. While it doesn't provide 0.7g, you can see some of the issues with riding bikes underwater here, and here's a video of an RC truck underwater (and another) that looks like it's having issues similar to the moon buggy. I was hoping to find some footage of larger vehicles (perhaps for servicing oil rigs?) but it appears there are no underwater negatively-buoyant driving vehicles (for fairly good reasons). Underwater has a lot more drag than in an atmosphere, so you'd probably see much less tire slip than these vehicles anyway.

So, Because 0.7g isn't too low, I'd expect to see vehicles similar to what we have here on earth. Cars, bikes, trucks. You could get bigger payloads with the same engine, but safety would be hard. Some vehicles that are impractical here on Earth may become practical. I'd expect to see more legged vehicles, hovercraft, hydrofoils, small aircraft and so on.

Summary
For much of the time on paved surfaces, normal vehicles would suffice on both worlds. You get 1000kg cars, 2000kg cars, and 500kg cars. You get 10 tonne trucks, 20 tonne trucks, and 5 tonne trucks. All can drive on paved roads here on Earth, and all could probably drive on paved roads on both of the worlds described. Only when you go offroad or want better efficiency do you need custom designs.

• Mashup or not, I love that picture. That is one fearsome-looking machine.
– JBH
Commented Dec 17, 2018 at 8:41

WORLD A: Yanacocha

PURPOSE: The most common vehicle that will be used in this world would be Haulers. As a mining planet, precious ores and minerals needs to be processed and shipped to another planet which will use the processed materials for manufacturing.

REQUIREMENTS: Due to the hostile environment that the planet has for human life, all types of vehicles must have these requirements when outside the dome city.

• Air-sealed, self contained system - Since there is no breathable air in the world, all vehicles must simulate what planes or spaceships have. The vehicles need to be able to provide oxygen, remove C02 and keep the air pressure at levels where a human can live properly.
• Structurally solid, and armor - As the inside of the vehicle has a higher pressure than outside, it should be structurally solid to withstand the difference in pressure. Not only that, it needs to be able to withstand the deadly dust storms (assuming that these graphite dust storms are indeed deadly) that happens within the planet.

POWER SOURCE: Typical combustion engines are out of the question due to the composition of the atmosphere. Batteries can be used to power small vehicles, but for the larger ones like Haulers, a nuclear reactor is recommended.

PROPULSION: With a hostile environment, wheels can still be used but can cause wear and tear more than usual. Instead, to be able to haul large amounts of ores, as well as lift all the requirements and power source, heavy tracks are the way to go. Large tracks similar to NASA's crawler transporter but wider could be used. It needs to be wider due to the higher gravity, so that it exerts less pressure on the ground. So you can imagine how big the Hauler would be.

World B: Serana There's not really much of a problem when using any typical vehicles that we have for a lower gravity planet. The lower air pressure maybe a bigger hindrance for helicopters and planes. The atmospheric pressure of World B is similar to Earth at 3500m above sea level. Typical helicopters would be able to fly up at least 1km above sea level with such a low air pressure, instead of 3-4km at Earth. Planes would fly lower, and possibly are more inefficient than what we have on Earth.

With a low gravity and air pressure than Earth, it maybe more economical to have rocket-propelled planes instead (SpaceX?).

The one big factor that changes, is the mass to forces ratio: Your car can only exert forces on the road which are proportional to its weight.

### High-G regime, Yanacocha

Your car designers will love this. Your tires can transmit 2.12 times the force as they could on earth. No danger of skidding your tires before you are decelerating with a whopping $$20\frac{m}{s^2}$$. Your braking distance will half, and you can do turns unimaginable on earth.

That said, you need sturdier tires and suspensions than on earth. The forces have to be transmitted, after all. Efficiency will go down, but since when has efficiency been a concern of car manufacturers?

There is one more point that needs considerations: Energy dissipation when going down-hill.

Twice the weight means that your car's brakes need to absorb twice the amount of energy for the same hight difference. Running brakes hot on long descents is a problem on earth, on Yanacocha this would be much worse.

As such, I guess your car designers will invest into electric braking. Either to recuperate the energy for the next ascent, or to dispose it off with a large, specialized radiator under the car's floor: Using electric braking allows you to move the energy disposal anywhere you can place that radiator, and you have no constraints sizing this radiator.

### Low-G regime, Serana

What can be said for Yanacocha can be said for Serana in reverse. Braking will be a headache, even though not downhill. Your tires simply won't allow for more than $$7\frac{m}{s^2}$$ deceleration. Even though any car designed for earth will have no trouble driving, using such cars would be much more dangerous than on earth.

Because of this, and the gruesome accidents that happened shortly after the colonization of Serana, individual driving has been restricted to human powered vehicles, only. It's already perfectly possible to kill yourself with a bike, cars are just too dangerous for daily traffic.

Instead, Serana has built a big, well developed rail-road system. While acceleration is even worse for rail-roads, trains can easily be controlled by computers which don't do the same mistakes as humans do. This public transportation system allows anyone to get anywhere.