Robots as Spaceships: Flyers

Not all robots are bound to the ground or to the water. Some of them can fly! Here we look at the various options Gurps: Spaceships provides for atmospheric locomotion.  Some options we might want are missing: propeller systems are the most notable. Others we want to adjust the stats on a little, and still others we just want to understand properly. Most of the time we will be adjusting speeds down in the name of modeling real or fictional vehicles: the new lower speed we call "Downshifted".

While this article is part of robots as spaceships, its probably just as useful when building vehicles, and I suspect I'll come back to it more in that context than for robots.

Jet Engines

Spaceships 1 provides a simple "Jet Engine" System. It is based on a scramjet or a ramjet engine, which makes a lot of sense for a spaceship, but aren't what we normally think of when we hear "Jet Engine". Scramjet and Ramjets exceptionally powerful for their mass, have exceptional performance, and only work at exceptionally high speeds. That's great for a spaceship coming in from orbit, but unsuitable for commercial jet liners or even fighter jets.

Classic Turbofan Jet Engines are found in spaceships 7, and these are much closer to what we need. We do want to tweak those stats a little,  but Turbofans from spaceships 7 provide most of what we need, including a base price.

When using turbofan engines, we should take note of the section on afterburners. This is a large performance boost to the craft, and a really cool bit of engineering and physics that adds immersion to the game.

Downshifting

The thrust for Turbofan Jet Engines are twice as high as they should be: individual turbofan jet engines have thrust to weight ratio's of about 5, not 10 (to be honest, even that is a high number, 4 is more typical). The thrust for down shifted turbofan jets are half of their listed values.

Helicopter Rotors

Rotors are turning out to be one of the more popular ways to move robots around. Twenty years ago, who would have guessed? Spaceships 7 gives us our rules for helicopter rotors, but those rules are a little sparse, and we've got some fine-tuning we'd like to do. Its also worth looking at the differences between quadcopters and traditional helicopters

Quadcopters

Many modern drones use quadcopter technology, which has some differences when compared with helicopters. I wrote a whole post about it here. Most robots reliant on rotors should consider if they are helicopters, controlled via the swash plate, or the more simple quadcopters.

Acceleration and Maneuverability

Spaceships gives us top speeds for helicopter rotors, but not base moves. While Helicopters are extremely maneuverable vehicles, they aren't able to reach their top speeds in a mere 1 second, especially those with extremely high top speeds. As a general rule, we'll say helicopters have a base move of 1/4 their top speed. I'm not actually sure if this is correct, but it feels in the correct ball park (helicopter maneuverability and acceleration is extremely complex topic). Even with that limitation, they are exceptionally nimble.

Downshifting

The numbers given in spaceships 7 for helicopter rotors are a bit generous. Operators CAN push the limits of aircraft, but speed records are the result of skilled pilots and exceptional runs, and should not be considered a simple top speed. The Fastest Modern helicopters operate at 170 mph, not 250 mph. Slower ones hang around 120 mph. Of course, PC's are likely to be among those skilled operators who can get extra speed out of the craft, and we recommend that a GM be extra generous about this with helicopters.

Systems	Streamlined Move	Unstreamlined Move
1 Rotor	15/60	7/30
2 Rotors	21/85	10/40

The power requirements for helicopters in spaceships are a little generous, but increasing the number of power points required by a whole number is overkill.

Gas Bags

Spaceships 7 has rules for Light-Than-Air Craft, in the form of gasbags. It gives a wonky set of prices that scale differently than the rest of the series, and gives no guidance about air-speed or volume, other than to say 4 systems are usually sufficient to lift a craft off the ground.  We will address most of these, At least for earth atmosphere and pressure.

Volume

If not using some other form of lift, it takes 4 gasbag systems to lift a "spaceship" off of earth, with a built in margin of error of 20%. If we really want to push our limits, we can use three systems and a miniature system instead of the full four, but that leaves a minuscule margin for error, something a GM should remember in times of trouble. If working with lower technology, even more gas bags may be appropriate. Heated air is also a possibility, but these systems only have 1/3rd the lifting power, and require 12 systems for the 20% margin, or 10 systems for no excess lift. The volume remains the same.

Lighter than air craft have an SM four times larger than their counterparts (If we want to be super precise, just slightly more) . This is almost all gasbag, of course, and if the SM of the rest of the ship matters, or its weight, the lower SM should be used.

Shape and Speed

Lighter than air craft are limited in speed, regardless of how much power is available to them. They can be made in streamlined shapes, but their extreme surface area to weight ratios make it difficult overcome air resistance.

When designing lighter than air craft it is important to note if they are rigid or if they are inflatable. Rigid craft can have more streamlined shapes and thus travel faster, but inflatable gas bags can be emptied and collapsed for convenient travel or when undergoing activities that would destroy the bags. It should also be noted that heat can turn

There are many inventive configurations to arrange the gas bags, including having high above the rest of the craft, or surrounding the craft, with only the envelope visible. Its generally a good idea to have the majority of the weight towards the bottom of the craft for the sake of stability.

Gas bags don't have a listed speed in spaceships, because they don't produce forward thrust, but we don't have a propeller system listed either. Historically, airships have not driven themselves to go faster and faster, because other vehicles do "Fast" better. Speed is mostly related to rigidity, though size has a fairly noticeable effect. Bigger is better: the square cube law likes balloons. Engines and propellers of historical lighter-than-air craft rarely reach 5% of the craft's mass, which is a single system. Non-rigid craft are usually confined to 20 mph or less, while the largest rigid ships have reached 80 mph. rigid airships have a minimum size to be effective about about SM+8 (by volume, SM+4 by mass).

Wind speed is of course very important for craft with gas bags effective speed, and many hot-air balloons travel simply by going up or down and finding a breeze going the right direction.

Normalized Costs

The costs of Gasbags in spaceships 7 doesn't scale the same way as other systems. The SM+9 system  costs the same as a fuel tank, while the SM+6 system costs as much as a habitat. The habitat is about 3 times as expensive as the fuel tank.  I don't know why the scaling decision was made. I suspect that nothing goes too wrong if we give it a base cost of 5.  Put simply, it costs the same as a system of light alloy armor.

Contragravity and Repulsors

Robots floating around in the air are found from Star Wars to WALL-E to XCOM. Despite this, most of the time the behavior is based on what looks cool rather than on any actual physics. There are a few things to consider when building these types of robots.

Fictional floating robots tend to have one mechanism that lets them float and another that moves them forward. It takes a lot of power to levitate off the ground, especially if you're not using the air to do the lifting. Only the most powerful jet aircraft are capable of "standing" in the air, using their engines to keep them up. Most floating robots don't have that kind of power, relying instead on contra-gravity of some sort to keep them in the air. And most of them don't seem to need the powerful (and expensive) 10 G version found in Spaceships 1. A single reactionless drive is usually sufficient for their forward movement.

I would encourage GM's to play around with these numbers if they are interested in floating robots. This isn't real technology, and while spaceships provides a starting point, that's all it is: a starting point. If you want to emulate star wars and you want large but cheap contra-grav systems that care how far off the ground they are, just build them. Say each system is 1/20th the cost, but only cancels .5 G of gravity. And you're done. Just have a feel for the costs of the various systems. Relative costs help here (SM+10 cost divided by 1 million). Tracks cost 5, helicopter rotors cost 20, and Jet and rocket engines cost 100. Internal combustion engines cost 3, and Fuel Cells cost 5. (Wheels cost 2, unless you use normalized costs to bring them up to 5)

Its also worth asking if the particular floating vehicle you are using isn't just a floating version of a different vehicle. Many levitating fictional vehicles are very much just floating cars, bikes or tanks, performing only slightly better.

Air Speed Formula

The base rules for speed in an atmosphere are on spaceships 1 page  35. The formula for calculating air speed is obviously wrong for fractions of a single G. Actually, I don't know if it is correct at all, but we're interested in smaller and slower craft than spaceships normally works with, and I can promise you nothing is going to hit 250 miles per hour with .01 G of force, no matter how you streamline it. Except maybe Ant-man using comic-book physics.

Low Acceleration Speeds

At under 1 G of acceleration for winged aircraft, we will use the following formula:

base speed * acceleration in G's

Even these down shifted numbers are generous, but they'll give reasonable results. In the standard spaceships airspeed rules, the base speed number is 2,500.

Downshifting

The 2,500 mph number given as the base speed in spaceships is reasonable for the very fastest and most spaceship-like of craft: narrow needles speeding through the sky. Streamlining is extremely important for craft moving at high speed in an atmosphere. At high speeds, drag is much more important than weight. While spaceships treats craft as either streamlined or not, there is actually a lot of variance in how streamlined an chassis can be. We will assign different levels of streamlining different base speeds.

SR-71 Blackbird
Sleek and Slim

The most streamlined craft of all do so at the expense of maneuverability. Orbital rockets, the spaceshuttle, the SR-71 blackbird, and the X-15 all have a very similar long needle-like shape, designed for extreme speeds. They lack the maneuverability of a properly winged craft, and while they have control surfaces, they should not be considered winged. They are also fairly large craft, at 50 feet or longer (though because of their shape 50 foot versions are probably still SM+5). The 2,500 mph base speed is appropriate for such craft.

F-35 Lightning
Look at Those Control Surfaces

Airplanes designed for maneuverability or efficient flight are still streamlined, but exchange that needle shape for large control surfaces providing maneuverability and lift. Examples include Fighter Jets and Passenger Jets. These craft should use 1,500 mph as their base speed rather than 2,500 mph. Wings provide a lot of maneuverability, and they provide meaningful lift at lower speeds, but they also increase drag, and the very fastest craft minimize them. Fighter Jets find it more useful to be maneuverable that to operate at the very highest speeds, and are the basis for out 1,500 mph number. These vehicles also tend to be SM+5 or larger: the square-cube law favors larger designs for comparatively less drag. Extremely streamlined craft lacking control surfaces under SM+5 should also use the 1,500 mph base speed: the square cube law catches up to them.

Mazda MX6
Still Streamlined

Vehicles can still be streamlined while under SM+5 or intended for lower speeds. The classic example is the automobile, but many early airplanes also fit these sorts of designs. The shape is streamlined, but functional construction comes first. While all the corners are smoothed, the underlying useful shape of the craft remains. Such craft should use 500 mph for 1 G as their base speed. They are still much faster than non-streamlined craft, which honestly shouldn't be going much faster than 100 mph, if even that.

Happy Flying

This article is listed under my "Robots as Spaceships" Series, but I don't think I'll be using it for robots as much as I'll be using it for building vehicles. These rules are useful for mad science, steampunk, and custom aircraft. I hope you enjoy your adventures in the wild blue yonder!