[Spaceships] [Alternate GURPS] Building a Hovercraft . . .

[vicky_molokh]

Greetings, all!

So, I decided that I want to, instead of using the one from Ultra-Tech, design a hovercraft using Spaceships/Alternate GURPS/Pyramid Vehicles rules. I thought, the AG article is so good with all the additions, surely I'll be fine with just a couple of tweaks. But now, it seems not that easy.

Now, a hovercraft basically has two systems that are interesting, and seem simple from a gaming standpoint . . . at first sight.
These are the turbine/rotors used for lift (and the attached skirt) and the rotor for horizontal trust.
Turns out, there aren't any thrust propellers that can be installed in the rear. There are jets, but those have a nasty exhaust, and eat fuel directly (thus making elctro-motor versions impossible). The closest thing I found were various Ether Screws, and those are definitely not in any way like the real thing.

Now, in desperation, I looked at Helicopter Rotors, but I'm not sure. They provide wonderful lift for just a single system, and, interestingly, a top speed that is more-or-less similar to the one of UT hovercraft. But what should be gained by 'downgrading' those from a full-flight rotor to a ground-effect-skirt-only rotor?

Perhaps there are other considerations for hovercraft?
Thanks in advance!

[vicky_molokh]

One idea: helicopter rotors are much more expensive than, say, wheels?
One of the benefits of a 'mere' hover rotor compared to a full-power heli rotor might be cost. How much should it be compared to wheels/tracks?

[Ashtagon]

At least in the 3e version of vehicles, there's no real difference between a propeller giving thrust to an aircraft and one doing the same for a hovercraft. Is there something new I am missing?

[vicky_molokh]

Originally Posted by Ashtagon

At least in the 3e version of vehicles, there's no real difference between a propeller giving thrust to an aircraft and one doing the same for a hovercraft. Is there something new I am missing?

Isn't there a way to maintain 1G lift for less power when the vehicle is near the surface? Ground effect or something? If hovercraft require as much power for lift as full-fledged vertical-flight rotor aircraft, what's the point of hovers?

[Ashtagon]

Originally Posted by vicky_molokh

Isn't there a way to maintain 1G lift for less power when the vehicle is near the surface? Ground effect or something? If hovercraft require as much power for lift as full-fledged vertical-flight rotor aircraft, what's the point of hovers?

Yes, there is. If they are used for vertical lift (not horizontal thrust; the book uses the two terms distinctly) and the vehicle has a ground-effect curtain or wall, the lift from those rotors has a multiplier while the vehicle is in close proximity to the ground.

Relevant section is page 9, hovercraft sub-assemblies, page 33 (ducted fans, the conventional hovercraft lift engine), and page 136 (hovercraft performance).

[vicky_molokh]

Originally Posted by Ashtagon

Yes, there is. If they are used for vertical lift (not horizontal thrust; the book uses the two terms distinctly) and the vehicle has a ground-effect curtain or wall, the lift from those rotors has a multiplier while the vehicle is in close proximity to the ground.

Relevant section is page 9, hovercraft sub-assemblies, page 33 (ducted fans, the conventional hovercraft lift engine), and page 136 (hovercraft performance).

Okay, so I see that thrust is multiplied by 2 to 5 depending on subassemblies. Let's assume a skirt (×5), but reduce the modifier to ×4 to account to possible losses from converting a helicopter rotor into a ducted hover fan (after all, some mass spent on the duct/skirt is not spent on the drivetrain). That seems to indicate that a hovercraft lift system can be reduced to a one-SM-smaller system, unlike usual helicopter rotors. Seems neat, actually, since presumably those 2/3rds of mass can be dedicated to more forward propulsion. I guess an SS-based streamlined hovercraft could theoretically be faster than a helicopter. Interesting.