CT Only: GravMoped

[atpollard]

It started with several recent topics on Grav Vehicles. That started me off on a "snipe hunt" for Civilian Grav Vehicles in Classic Traveller (like Striker rules make possible and "Civilian Striker Vehicles" by J. Andrew Keith suggest in JTAS 14). Then I kept seeing posts by @Grav_Moped ...

... which made me wonder if I could create a GravMoped. (Sure, why not?) So I offer for your general amusement, one GravMoped inspired by @Grav_Moped and Striker.

GravMoped​

Not every trip requires transporting four people and tons of cargo across hundreds of kilometers at half the speed of sound. Most of the time, you just want to transport a single rider a handful of kilometers through dense urban areas faster than walking. For those quick trips around Startown, there is the humble GravMoped.

• (TL 8) Total Volume: 0.1 cu.m.; Weight: 187 kg.; Price: Cr 821. Designed to accommodate a single rider (100 kg) straddling the vehicle at a Top Speed of 40 kph [216 km range] with no Avionics. Typical cruise speed is 30 kph [240 km range] with maximum efficiency at 20 kph [264 km range]. Included are Grav Modules (8 kg), a MHD Turbine Power Plant (32 kg) and 46 liters of Hydrocarbon Fuel (46 kg) all encased in a 2.5 mm Composite Laminate shell (AV 2) upon which the rider sits.

In case anyone was interested, there is also a 2 person GravScooter:

GravScooter​

• (TL 8) Total Volume: 0.2 cu.m.; Weight: 374 kg.; Price: Cr 1592. Designed to accommodate a rider (100 kg) and one passenger (100 kg) straddling the vehicle at a Top Speed of 40 kph [216 km range] with no Avionics. Typical cruise speed is 30 kph [240 km range] with maximum efficiency at 20 kph [264 km range]. Included are Grav Modules (16 kg), a MHD Turbine Power Plant (64 kg) and 92 liters of Hydrocarbon Fuel (92 kg) all encased in a 2.5 mm Composite Laminate shell (AV 2) upon which the rider sits.

And an Urban Delivery Scooter:

GravDeliveryScooter​

• (TL 8) Total Volume: 1.3 cu.m.; Weight: 487 kg.; Price: Cr 2038. Designed to accommodate a single rider (100 kg) straddling the vehicle and one cubic meter (100 kg) of cargo encased in a 2.5 mm Composite Laminate box (AV 2) at a Top Speed of 40 kph [216 km range] with no Avionics. Typical cruise speed is 30 kph [240 km range] with maximum efficiency at 20 kph [264 km range]. Included are Grav Modules (20 kg), a MHD Turbine Power Plant (83 kg) and 120 liters of Hydrocarbon Fuel (120 kg) all encased in a 2.5 mm Composite Laminate shell (AV 2) upon which the rider sits.

I hope you enjoy them as much as I did.

 

[Spinward Flow]

a MHD Turbine Power Plant

Ah yes, the (fabled) MHD Turbine, which actually has almost enough power density to make gravitics (almost) practical.
Problem is that MHD Turbines run at even higher temperatures than Gas Turbines ... which are basically "little jet engines" that produce rather hot exhaust products in order to achieve their better carnot cycle thermal efficiency than the humble internal combustion engine.

I submit to you, that for this specific application, an MHD Turbine is ... rather inappropriate.
For something of this scale, having an exhaust temperature of "a few thousand kelvins" would be ... less than ideal from such a small form factor.

However, what this means is that you are best served by switching power plant from TL=8 MHD Turbine to TL=8 Batteries.

For the exact same weight factor (32kg turbine + 46kg fuel = 78kg) you can store 1.25MWs/kg or have 78*1.25=97.5 megawatt-seconds (or 27 kilowatt-hours for those playing the home game). So long as you can keep the power consumption by the grav units down to something (almost) reasonable, you can get 1+ hours of endurance range. An additional benefit of switching from an MHD Turbine (an air breathing combustion+magnetic drive engine) to batteries is that the personal vehicle becomes "atmosphere agnostic" so you can take it anywhere with a gravity well.

Might I recommend that you "take your design back to the shop" and try again with a different choice of power source for your Grav Modules?

 

[Grav_Moped]

Problem is that MHD Turbines run at even higher temperatures than Gas Turbines ... which are basically "little jet engines"

Reminds me of the custom motorcycle using a BMW drivetrain behind a Mazda Renesis Wankel rotary. Over-powered but a bit toasty...*

I hope you enjoy them as much as I did.

Really nifty! Wow.

‐--------------
*also reminded that one of the car magazines did an article on testing several sports-cars' engines to destruction by running them flat-out at redline. The failure mode of the mazda rotary was that the exhaust manifold melted.

 

[Spinward Flow]

The failure mode of the mazda rotary was that the exhaust manifold melted.

 

[Grav_Moped]

The rest generally had their exhaust valves fail. Wankel: no valves, just ports on the sides of the chamber.

 

[atpollard]

Ah yes, the (fabled) MHD Turbine, which actually has almost enough power density to make gravitics (almost) practical.
Problem is that MHD Turbines run at even higher temperatures than Gas Turbines ... which are basically "little jet engines" that produce rather hot exhaust products in order to achieve their better carnot cycle thermal efficiency than the humble internal combustion engine.

I submit to you, that for this specific application, an MHD Turbine is ... rather inappropriate.
For something of this scale, having an exhaust temperature of "a few thousand kelvins" would be ... less than ideal from such a small form factor.

However, what this means is that you are best served by switching power plant from TL=8 MHD Turbine to TL=8 Batteries.

For the exact same weight factor (32kg turbine + 46kg fuel = 78kg) you can store 1.25MWs/kg or have 78*1.25=97.5 megawatt-seconds (or 27 kilowatt-hours for those playing the home game). So long as you can keep the power consumption by the grav units down to something (almost) reasonable, you can get 1+ hours of endurance range. An additional benefit of switching from an MHD Turbine (an air breathing combustion+magnetic drive engine) to batteries is that the personal vehicle becomes "atmosphere agnostic" so you can take it anywhere with a gravity well.

Might I recommend that you "take your design back to the shop" and try again with a different choice of power source for your Grav Modules?

Oh ye of little faith.

• Are there batteries in Striker? (I did not see them in the vehicle design rules, but I did not go searching for them.)
• I WAS curious what the heck a MHD Turbine actually WAS. Researching it to try and find a picture for graphic inspiration when I get around to drawing up a Grav Vehicle, I learned that MHD generators are either open or closed cycle … so there technically need be no Jet Exhaust … just a heat capture and recirculating system for the plasma. It could theoretically be 100% self contained just like an internal combustion engine. (I have the diagrammatic sketches from the research papers and photos of the experimental models.)

Traveller probably still got details of real world physics wrong, but I just wanted to use the Striker rules … not build an actual prototype.

SOMEHOW, the Air\raft works at TL 8 … so the problem must have been solved without batteries or melting bystanders faces off.

 

[Grav_Moped]

And back on topic....

The simple way to do up a G-cycle is to rule that it's a grav belt in a different form factor. Adjust cost relative to performance, if you're commited to the idea of a literal moped or motorscooter analog.

Mind you, that's not nearly as much fun to detail, or as likely to generate a long discussion thead...

 

[Grav_Moped]

Mind you, that's not nearly as much fun to detail, or as likely to generate a long discussion thead...

...or incinerate bystanders with the exhaust, depending on how one interprets the rules.

Which is generally accepted as a desirable feature.

It's all good.*


------------------
*Except the charred bystanders -- but they're fictional so, like, whatever.

 

[Grav_Moped]

Adjust cost relative to performance, if you're commited to the idea of a literal moped or motorscooter analog.

Or adjust it in the other direction if you want an antigravity superbike. That said, if you're looking for speeds over a couple of hundred kph in a standard atmosphere, you'll probably want something enclosed rather than something the operator sits astride.

The slower ones (moped analog) may be allowed to operate indoors if the're running on batteries.

 

[Grav_Moped]

And for a lot of situations, something smaller than an air/raft would be quite useful. Air mobility is handy, but is it worth giving up 4Td of cargo space for? A couple of grav-bikes in 0.5Td, or even a 2Td 2-seat flying car/pod would be nice to have. (The latter can be specced as "half an air/raft".)

 

[kilemall]

I like cheating and using LBB8 robot power for small CT equipment design. A moped is right in the sweet spot.

 

[Condottiere]

It comes down to cost, and that includes capital outlay for the production line.

Generally speaking, a scooter is going to be slow, short ranged, and battery powered, and likely has a passenger seat, though that might be more described as a long enough driver's seat that could have enough space for a passenger.

A cub is a more robust scooter, more into commercial usage.

Can you overload scooters?

Not really, unless you have tyres or skids to prevent it scraping on the ground.

And then there are grav skateboards.

 

[kilemall]

Tuk Tuks are the next level up.

 

[Condottiere]

We had grav chairs, at one time.

And you can imagine what the traffic in a WalMart in a technological level eight society is going to look like.

 

[Grav_Moped]

Can you overload scooters?

Depends on the rule version (or, more precisely, how antigravity/grav drives are supposed to work).

The LBB version (arguably) neutralized weight (perhaps inside a specific volume) and provided a little thrust beyond that. Striker and later rules provided a specific amount of thrust from which local gravity would be subtracted before determining how much was available for lateral or vertical acceleration. So, basically, the LBB version, if overloaded, would still hover but couldn't move under its own power. The Striker version would not be able to hover, but could slide across the ground if the ground was smooth and solid.

[Edited] Under the early rules, you wouldn't see the classic southeast-asian absurdly-overloaded bike (whole family on board, or baskets stacked a few meters high), while Striker would allow it.

[Edited] Or is it the other way around? LBB would have the vehicle hit its bulk limit before its mass limit. Striker would not limit bulk, but would definitely limit mass.

Can't link pics (on phone) but they should be easy to find.

 

[Condottiere]

As I recall, it's volume up there, and mass, dirtside.

The thing about ground vehicles is, you usually can divide/distribute weight over power, to get speed, and it's held up by tyres.

With grav motors, you sink, with nothing between the chassis and the ground.

 

[kilemall]

Depends on the rule version (or, more precisely, how antigravity/grav drives are supposed to work).

The LBB version (arguably) neutralized weight (perhaps inside a specific volume) and provided a little thrust beyond that. Striker and later rules provided a specific amount of thrust from which local gravity would be subtracted before determining how much was available for lateral or vertical acceleration. So, basically, the LBB version, if overloaded, would still hover but couldn't move under its own power. The Striker version would not be able to hover, but could slide across the ground if the ground was smooth and solid.

[Edited] Under the early rules, you wouldn't see the classic southeast-asian absurdly-overloaded bike (whole family on board, or baskets stacked a few meters high), while Striker would allow it.

[Edited] Or is it the other way around? LBB would have the vehicle hit its bulk limit before its mass limit. Striker would not limit bulk, but would definitely limit mass.

Can't link pics (on phone) but they should be easy to find.

The grav speeder definitely was a lot of thrust to get that speed. You could home rule it was some sort of rocket thrust, but lack of description strongly suggests integral grav thrust.

 

[atpollard]

There is room in the STRIKER rules for overloading Grav Vehicles. Let’s assume TL 14 avionics which provides 180 kph NOE speed. Per RAW, that optimized vehicle should have a Top Speed of 720 kph (0.60 G maneuver thrust or 1.6 G total thrust).

For discussion, assume a Grav vehicle with a design weight (vehicle, passengers & cargo) of 4000 kg … roughly the GVWR of a Ford F-250. The vehicle would still have a Top Speed of 120 kph with 0.10 G maneuver thrust or 1.10 G total thrust. Therefore, 4000 kg x 1.6 = 6400 kg actual thrust, and 6400 / 1.1 = 5818 kg. Since 5815 - 4000 = 1815 kg, the hypothetical Grav F-250 can be overloaded by 1815 kg and still fly at 120 kph instead of its 720 kph normal top speed. Bulk goods average 100 kg per cubic meter, so that 1815 kg equates to 18+ cubic meters piled atop the normal cargo load of 1800 kg (for an F-250). Visually, that equates to increasing the 4 meter tall stack of boxes in the pickup bed to 8 meters tall.

 

[atpollard]

Tuk Tuks are the next level up.

I have some preliminary designs for them, too. Based on the Piaggio Ape.

 

[whartung]

How does a GravBelt fit into this scheme?

Are they ungainly and clunky and "may as well use the GravMoped" or is a GravBelt and a small backpack a more common personal transit device?