Grav Skimmers

[Icosahedron]

For a couple of decades or so, I have handwaved the notion of Skimmers - grav vehicles that are restricted to flying a metre or so from the ground.

I want these things IMTU, and as they are less versatile they ought to be cheaper, but I haven't yet come up with a rationale to explain them.

Why would they be cheaper?
How/why do they have a limited altitude?

The obvious Inverse Square Law won't work - it might stop them achieving stationary orbit, but it won't stop them reaching the clouds.
Simpler avionics is not promising either - I imagine you'd need more and better avionics to fly close to the ground.
I can simply use legal fiat, but it just feels like a handwave, and players just want to buy a cheap Skimmer, then 'remove the governors'.

Any ideas?

 

[aramis]

Invoke a ground resonance and only 0.5-0.6 G's lift.... but double that within 2m of solid surface, 1m above liquids...

or use the real-world "Wing in ground effect" skimmers, which make less than 3m (they stall out above that), which are non-gravitic.

 

[Gadrin]

Why would they be cheaper?
How/why do they have a limited altitude?

Any ideas?

some rambling ideas...

Power usage might be doable. In other words the power reqs to maintain the constant, go-anywhere-including-orbit of an air/raft are rich, versus those of a repulsor-type craft being a fraction. This might allow a much cheaper, much lighter and maintenance-light version of the air/raft. It might be workable with batteries/energy cells/energy banks. This can put significant time restrictions on the players as they have to recharge to move from A to B to C, or carry spares.

You could conceivably fly in one, but the power drain becomes enormous after X altitude (say more than 50 to 100 feet). This means your destination is no longer possible, or returning from it.

The old, yank out the cells and put in a MHD, cause weight problems as well significant frame mods. It can be done, but in the long run, you should really just buy an air/raft. GURPS offers the "NPU in a tiny box" at some Traveller tech levels, but just discourage it by making it too much for the simplicity of the TL8 or TL9 skimmer.

Skimmers offer only modest shielding/cover. So flying one at 10,000 feet means you better have arctic clothing, plus oxygen, etc. At some point trade-off between skimmer and air/raft blurs so much that any sensible person would buy an air/raft. And work involved to transform skimmer into air/raft just isn't worth it. In short, they are two different animals.

 

[Scott Martin]

I'd use a combination of Aramis' suggestions: A grav "Skimmer" is a grav vehicle with just enough lift (thrust) to get the airfoil to engage in WIG mode. This is probably ~ 1/4 of the thrust needed for a "true" grav vehicle, but is offset by requiring the skimmer to have a lifting body airframe.

My WAG (Wild-Ass Guess) is that his would reduce the cost of such vehicles by about 1/3, (less grav needed, less powerplant needed) and would make this type of vehicle unusable in rough or broken terrain, and hazardous to use in strong winds.

Of course this also gives you some additional adventure possibilities (deus et machina style) such as having a storm break out while a transport skimmer is over an ocean, forcing it to run into or in front of the wind, and leaving it without sufficient fuel to return once land is found (or crashlanding it, whatever...) since crosswinds would be death to a WIG transport in heavy seas...

Just in case anyone was wondering why the 3I uses G-Carriers...

Scott Martin

 

[Ishmael]

Because I use mass instead of volume, I figure power requirements for grav vehicles to be equal to changes in the vehicle's potential energy. I also treat it as energy that must be input continuously to keep the thing from falling. This takes mass and altitude and gravity into consideration. Because of high power requirements, trains and wet ships are viable imtu. I always hated that grav vehicles take over all transportation as soon as they are introduced.

My version of contra-grav produces no thrust and needs
mass(kg) * altitude(m) * g's * 10 watts of power while in operation. ( G accelleration rounded to nearest whole number; its easier that way )

A 426 hemi's ( 700hp =~.5MW ) output could
lift ( not very high; 10-20 meters/ aerodynaimcs can augment lift ) about a
2000kg speeder ( loaded) and drive ducted fans for thrust. Power plants can
be overdriven to give that extra oomph to get over obstacles in emergencies
and higher altitudes could mean tossing stuff ( people? ) overboard to reduce
mass....works better in lower grav enviroments..
efficiency would be 60% at tech 10 and improve by 5% for each higher tech
level. ( divide power needed by efficiency for power from power plant; waste power is heat/noise/etc )

I also use this for starship take-off; power comes from jump hi-perf power plant to get ship to an altitude where other thrust agencies can take over....
Landing, it takes over when the ship reaches a low enough altitude.

ymmv

 

[Gadrin]

one of the factors you'll definitely want to use is make sure the skimmer can't be converted to an air/raft with a small amount of money and some technology.

by heading the players off at the pass on this one, it'll help obviate the guy who knows how to tweak something costing 10Kcr into a 1+ Mcr item, with a fraction of the cost and minimum effort.

 

[Icosahedron]

Thanks for the ideas, guys.

I like the ground resonance, Aramis; that's a new one, I'll give it more thought.

I'd considered WIG airframes, I have these as alternative transports anyway, but ordinarily you would still need 1G to get airborne before the WIG kicked in.

Maybe, as Scott suggested, a combination of the two? This would make G-WIGs efficient and therefore popular.

Power usage could be an option, but why would a skimmer use less power than a high-flyer, when they are all using the same grav tech?

Yes, the lack of a protective environment would keep a vehicle to within a few thousand metres, but not a few metres. It is an option for restricting some high-flyers.

The 'change in potential energy' is good, but I couldn't go with it needing a different continuous supply for different altitudes. No matter how it is manipulated by handwave technology, gravity just doesn't work that way. I, too, like to retain other modes of transport, and I will still keep ground cars alongside skimmers, as an even cheaper option.

My grav vehicles incorporate lateral thrust in the manipulation technology.

I didn't say I'd LET the players make a conversion, Gadrin, just that they WANT to.


The more I think about it, the more I like ground resonance. How does it work, Aramis?

Musing aloud: what properties of the ground could induce a resonance in the gravitic field, or in the handwave manipulation of the field? Mass is out, because it influences things by gravity. Hmm, didn't Tesla or someone figure out the Earth had a powerful static electricity field near the surface? Is that common for all worlds? Perhaps that would do it. Maybe Skimmers use static repulsion as gravity compensation INSTEAD of grav manipulation, making them an entirely different (and probably older) beast? This could bring in different power requirements. It might also incorporate some potential energy factor.

Any more ideas along the ground resonance lines, guys?

 

[aramis]

Actually, the WIGE designs don't use lifters to get up; they use pontoons or free-wheeling ground wheels, and forward thrust. Since the stall speeds in ground effect are about 30KTS, they take some roll to get off the surface. A grav assist of 1/4-1/2 surface G's would reduce ground pressure, and quite likely, stall speed, quite a bit, and could then be used for viffing once fully transitioned to flight.

Note also: almost every pilot experiences ground effect. In a schweitzer S32, the stall speed is 30kts airborn (25 with flaps), but when in GE, it stalls at about 12-15 kts.

 

[Gadrin]

I didn't say I'd LET the players make a conversion, Gadrin, just that they WANT to.

:rofl:

oh, yeah, dems do stuff like dat.

 

[Kharum1]

How about the the power plant is smaller, the grav plates are smaller and to replace all this would also require the strengthen of the undercarriage and basic framework of the craft, as well as, the upgrade in electronics. By the time your players are done with all that, they would be better off just buying an airaft.

 

[Icosahedron]

I know conventional WIG(E) craft taxi, Aramis, I just assumed that a Grav vehicle using this technology for additional lift would use grav-based VTOL, otherwise the G drive becomes just another thrust agency for an effectively conventional aircraft.

IMTU I use the notion of airframes and controls from the Striker rules. I have house-ruled that Skimmers use a special airframe, (in response to a former rules-lawyer episode) but I just haven't specified (or decided) why. I routinely use grav vehicles with lifting bodies that can vector their thrust horizontally once above stall speed (adding 1G to the Speed).
Of course, any ground resonance effect would aid such craft, equating to a sort of grav-taxiing, and thereby allowing lifting-body grav vehicles to use smaller (and cheaper) drives.

I restrict vehicles without an airframe (eg grav belts, broomsticks) to NOE (4m) and 30kph, invoking a handwave 'vertigo effect', whereby they become unstable/chaotic beyond these limits. The idea of ground resonance would provide a rationale for this, too. I just need to figure out how proximity to solids and liquids (as opposed to suspension in gaseous matter) would serve to aid gravitational lift. Physics doesn't really support the idea (free pun for you). The proximity of a mountain doesn't noticeably alter the gravitational field, so presumably any such effect must alter the handwave manipulation instead, by handwave manipulating it. Can't say I'm altogether happy with a double handwave, but the ground resonance does at least make my former handwaves self-consistent.

I'm rambling - time to exit.

 

[atpollard]

RE: Ground Resonance

I vaguely recall a discussion from an old Physics Course that there is no specific reason that Gravity and Inertia are related properties, they just coincidentally are. That the resistance to movement need not be related to gravitational attraction. Perhaps this hypothetical “ground resonance” is based on a property of matter separate from gravitational attraction. That it requires concentrated matter to “push” against and atmospheric density is too low to “push”.

This might be similar to the way one force holds multiple Protons together in the nucleus of an atom, while the like charges would push them apart if they were to separate beyond the range of the molecular glue force.

This "Ground Resonance" is stronger than "Gravity" but quickly drops off based on the distance from the surface of the matter. Gravity (in contrast) is based on the distance between the center of mass and operates at much greater distances.

Just “what if-ing” out loud.

 

[BlackBat242]

Sounds like a repulsor field... do you have tractor/repulsor tech in your world?

That would solve it nicely... the small repulsor unit & power plant, the limit on distance above ground, the non-convertability to normal grav-vehicle drives.

 

[aramis]

Once you get sufficient lift to get off the ground (without a ground effect), you have sufficient to get airborne.

So either gravitics have a "surface effect" and sub-local thrust...
or
they have sub-local thrust, used to reduce wing lift needed (and thus takeoff roll)
or
They have local G's plus, and fly.

There are about 5 other scenarios, but of lower likelyhoods.

 

[Icosahedron]

I vaguely recall a discussion from an old Physics Course that there is no specific reason that Gravity and Inertia are related properties, they just coincidentally are. That the resistance to movement need not be related to gravitational attraction. Perhaps this hypothetical “ground resonance” is based on a property of matter separate from gravitational attraction. That it requires concentrated matter to “push” against and atmospheric density is too low to “push”.

This might be similar to the way one force holds multiple Protons together in the nucleus of an atom, while the like charges would push them apart if they were to separate beyond the range of the molecular glue force.

This "Ground Resonance" is stronger than "Gravity" but quickly drops off based on the distance from the surface of the matter. Gravity (in contrast) is based on the distance between the center of mass and operates at much greater distances.

Just “what if-ing” out loud.

Unless you subscribe to Mach's Principle.
I'm not sure I want another handwave force/interaction, I'd prefer to stick with just gravity, but I have looked at the possibility of using the gravitational inverse square law relating in some handwave way to the position of the nearest bulk matter. It's the best I have so far. This raises questions of what constitutes 'bulk' and which nearby objects will influence a skimmer (Can I skim above the deck of a flying aircraft carrier/cloud city, for example).

Just "yes but-ing" out loud. All ideas are helpful.

 

[Icosahedron]

Sounds like a repulsor field... do you have tractor/repulsor tech in your world?

That would solve it nicely... the small repulsor unit & power plant, the limit on distance above ground, the non-convertability to normal grav-vehicle drives.

Yes, I use the canon repulsors, polarity (handwave) reversed to create tractors, but I always figured they were a form of grav projector in much the same way as grav plates in ship decks...
Hang on a minute - reverse polarity deck plates ARE skimmer tech!
All I need to do now is figure out how deck plates/repulsors are supposed to work - and that is a CANON problem, maybe of interest to more people.

This has intrigued me for years too. How DO those plates work? Why don't the plates in the deck above pull you up? Do they use a local inverse square effect? If so, then over a 3m deck height you will have quite a variation - light-headedness coupled with blood-pooling in your ankles! If there is no inverse square, why not?

Just to muddy the waters a little :smirk:

 

[Icosahedron]

Once you get sufficient lift to get off the ground (without a ground effect), you have sufficient to get airborne.

So either gravitics have a "surface effect" and sub-local thrust...

My Skimmers

or
they have sub-local thrust, used to reduce wing lift needed (and thus takeoff roll)

Possible, but I don't focus on STOL

or
They have local G's plus, and fly.

Two types IMTU - flying bricks that need a constant downward thrust but are compact, and lifting bodies that once flying can vector the initial downward thrust horizontally to increase speed, but require a much larger surface area to mass/volume ratio.

There are about 5 other scenarios, but of lower likelyhoods.

 

[BlackBat242]

This has intrigued me for years too. How DO those plates work? Why don't the plates in the deck above pull you up? Do they use a local inverse square effect? If so, then over a 3m deck height you will have quite a variation - light-headedness coupled with blood-pooling in your ankles! If there is no inverse square, why not?

Well, the way I do it, the plates are like magnets... no, not in how they work... they have a full polarity effect: one side attracts and one repells.

Therefore, a plate between decks both pulls toward the floor in the upper rooms and pushes away from the ceiling in the lower floor.

Cancellation grids in the hull (and in the walls, floor, & ceiling of cargo bays) provide isolation for areas where you don't want stray gravitational effects.



Grav vehicles use a larger, uni-directional field generator to lift the vehicle against the planetary mass, while skimmers uses a small unidirectional field generator to push the vehicle away from the local surface mass.



Of course, there is the neat little item in Paranoia Press's Merchants and Merchandise... Grav Cycles!

The Harley-Davidson-Indian touring cycle... great for roadless trips (not recommended for high-speed use in forests).

 

[Kharum1]

This has intrigued me for years too. How DO those plates work? :

If we knew the answer to this we would all be very rich men.

 

[Icosahedron]

I'm sure we would, Kharum, but surely someone has given this some thought over the years - I'm just collecting ideas here. I like BlackBat's polarized plates and cancellation grids, I just need to figure why these plates have only a local field - or go back to the handwave; there is at least a little more logic behind it now, thanks to the ideas posted here.

Brainwave whilst writing: so far the assumption has been that the inverse square law is applied to the distance from the planet core and the vehicle is lifted away from this, but if the distance is taken from the grav generator/plate and the planet is simply the object that is lifted away from the vehicle...
<hurriedly goes back to the drawing board>.