Air/Raft Power

[Enoki]

One Kilowatt is equal to 3414 BTUs, and one BTU is equal to approximately 778 foot-pounds. Assuming not-quite perfect efficiency, then figure that one Kilowatt will lift 3,000 pounds to 750 feet above ground level on Earth. With an Air/Raft massing, loaded, at 8 metric tons or 17,636.981 pounds, to lift an Air/Raft to 750 feet will take about 6 Kilowatts. For every additional 750 feet, another 6 kilowatts will be required.

The forward speed of an Air/Raft is given at 100 Kilometers per hour, with some capable of 120 kilometers per hour, so about 62 to 75 miles per hour. That is about the top speed of some of the early biplanes, with a lot of drag producing wires, struts, and landing gear. The biplanes needed about 100 or so horsepower to achieve that speed. The Air/Raft might have a bit more frontal area for drag, so you might need somewhere around 150 or so horsepower to move it. One kilowatt is equal to 1.34 horsepower, assuming perfect efficiency. Allowing for inefficiency and drag, I will assume that an Air/Raft will need 150 to 200 kilowatts of power for forward motion.

It would appear that an Air/Raft will need a power plant capable of generating at least 24 kilowatts of power for every 900 meters of altitude capability, and at least 200 kilowatts for forward power. Drag will be less in thinner air and greater in denser air, so performance may vary based on the planet's atmosphere. For a 9000 meter altitude capability at 120 kph, it will require about 440 kilowatts of power. If you divert power from the forward propulsion plant to altitude, you can, of course, go higher.

I think it'd take a bit more than that.

For example, a Beechcraft Baron twin engine light aircraft (~2,000 kg) has a power output of 388 KW. It cruises at about 200 mph. It also generates its lift from the wings almost entirely. That is, it cannot climb vertically overcoming its own weight using the engines. Its stall speed is about 85 mph. I'm not sure how much power is required to maintain at least 85 mph but it would certainly be a small fraction of that its capable of generating.
But, it is also obvious that it needs most or all of that 388 KW to get airborne and up to speed.

With an air raft the vehicle has to generate sufficient acceleration, at a minimum to rise at 9.8m a second or about 35 Kilometers an hour (say 22 mph) right off just to oppose gravity on Earth. For 8 metric tons to rise at all requires about 79 (MV) metric tons per second of acceleration. That works out to about 78 KW just to get that weight off the ground and keep it there.
Now we need to get it moving forward at say 100 kph / 60 mph. Guesstimating drag and what not, and allowing for reasonable acceleration based on our airplane, lets say it takes a third of what the plane does. After all, the loaded air raft is nearly four times as heavy loaded but is only trying to go about a third as fast and most acceleration is a square function depending on how its being done. I think 100 KW is reasonable.
So, to give a bit of extra umpf here, let's say 200KW is necessary, at a minimum and with inefficiencies and a safety factor let's say 300 KW. Now, if the top speed were around 100 mph you'd need somewhere around 600 to 800 KW as a first approximation.
One thing is clear, the energy to overcome gravity is not insignificant and it is more efficient for a aircraft to have a higher speed since that doesn't require nearly as much energy as simply overcoming gravity.

Such a craft could run on a gas turbine or some future power plant but internal combustion isn't going to hack it.

 

[mike wightman]

How much energy does one null grav module require - an air/raft has 4 (A:12,SotA).

Each module costs Cr110,000, weighs 30 kg, and can lift 200 kg
against 2Gs.

Note no mention of energy input.

Does a null grav module produce "thrust" against gravity or does it neutralise gravity?

Put another way the null grav modules defy our understanding of physics so waving our physics formulae at them is unlikely to solve any problems. They work in the setting.

 

[Enoki]

230 kg accelerated at 2G (19.8 m/s) works out to 44.2 KW in terms of energy. Now, we know absolutely, you can't get something for nothing. That is you can't have a system that is more than 100% efficient when it comes to energy use. That's about 60 hp.

You can have one that is very efficient, but not one that creates energy out of thin air. So, that grav plate will have to overcome the equivalent of 44.2 KW of energy somehow.

 

[BlackBat242]

How is an air-raft maintaining a set distance from the ground somehow accelerating at 2G away from the ground? I could see 1G "acceleration" (actually, 1G repulsion, exactly countering the 1G attraction of the planet).

Conversely, if it is maintaining a set forward airspeed, what is its acceleration? That's right, its acceleration is 0G! You need to use the steady-speed energy use.

 

[mike wightman]

It reverses the polarity of the higgs boson and causes gravitational interaction to cease - by fluctuating the polarity of the higgs boson you can direct the gravitational energy in any direction thus generating buoyancy and 'thrust'.
This is done using tiny magnets directed by electric motors powered by hamsters running on wheels.

We don't know how Traveller gravitics works - the null grav modules ripped off an air/raft to make you custom grav belt don't even need a power source - none is mentioned in the adventure.

Striker is the first place energy input is mentioned:
0.02m^3 (0.04t) generate 1 ton of thrust and require 0.1MW

I wonder if the A:12 air/raft heavy duty null grav module is a combination of grav unit and fuel/energy cell?

 

[aramis]

How much energy does one null grav module require - an air/raft has 4 (A:12,SotA).

Note no mention of energy input.

Does a null grav module produce "thrust" against gravity or does it neutralise gravity?

Put another way the null grav modules defy our understanding of physics so waving our physics formulae at them is unlikely to solve any problems. They work in the setting.

Striker, Book 3, page 8:

K. Grav Generators: A grav vehicle requires grav generators installed in its chassis. Each .02 m3 of grav generators produces 1 ton of thrust and requires .1 megawatts of power from the power plant. They weigh 2 tons and cost Cr100,000 per m3​

So Cr100,000, 2 tonnes, 50 TT and 5 MW per m³. This is MT's Standard grav. 1TT=1 tonne * 1 G, or 9.8kN
Striker, Book 3, page 11:

H. Grav Vehicle Thrust: A grav vehicle generates one ton of thrust for each .02 m3 of grav generators it has powered by .1 megawatts from the power plant. If a vehicle has energy-consuming weapons, it has two thrust values: one for when the weapons are firing and one for when they are not.​

MT adds two more TL improvements to grav.
Putting them into per m³ units:
Std Grav 1m³ = 50 TT, 2 T, 5 MW, Cr100,000
LP H-Grav 1m³ = 20 TT, 0.6 T, 0.1 MW, Cr100,000
LP L-Grav 1m³ = 33.333 TT, 0.666 T, 0.333 MW, MCr10

 

[kilemall]

Inefficiency inherited from Striker. It's the 250MW = 1 EP specified in striker that really kills things... not because it's any the worse than HG, but that more stuff uses power...

Actually I've been attending to that with the HG2/Mayday reboot I have been working on, and resolved all that. The weapons make a lot more sense- and are explodey.

What's fabulous is that a whole scout ship does 20 mps accel with that weight on 500 mw constant.

 

[kilemall]

How much energy does one null grav module require - an air/raft has 4 (A:12,SotA).

Note no mention of energy input.

Does a null grav module produce "thrust" against gravity or does it neutralise gravity?

Put another way the null grav modules defy our understanding of physics so waving our physics formulae at them is unlikely to solve any problems. They work in the setting.

I always figured it was repulsion, and the strength of repulsion possible depended on the grav field, + repulsor thrust- so a grav module rated at 1G is actually generating 2G against a 1G planetary field, 3G against a 2G planetary field, etc.

What I came to coming back to the game was extending this to the artificial gravity- no pull/tractor, it's more repulsor, pushing squishy people down since it's mounted on the ship ceiling, on the back bulkhead for countering thrust, and an extra set in the floor to push up to help with 2G planets.

 

[AnotherDilbert]

Sorry for the pedantry, but much of this thread hurts to read.

1 KW = 1 Kelvin-Watt, I have no idea what that is. K/W is thermal resistance.
1 kW = 1 kilowatt, measurement of power, not energy.
1 kWh = 1 kilowatthour = 3600 kWs = 3600 kJ = 3,6 MJ, measurement of energy.

 

[aramis]

Actually I've been attending to that with the HG2/Mayday reboot I have been working on, and resolved all that. The weapons make a lot more sense- and are explodey.

What's fabulous is that a whole scout ship does 20 mps accel with that weight on 500 kw constant.

it's 500 MW, not KW.

 

[AnotherDilbert]

What's fabulous is that a whole scout ship does 20 mps accel with that weight on 500 kw constant.

m/s is a velocity, m/s² is an acceleration. (kW, not kw or KW, and as aramis said it should be MW.)

But, yes manoeuvre drives are rather "magical".

 

[Reban]

Last question, why is it not offered in an enclosed version?

I'd assume that the air/raft does come in enclosed versions, but they get called things like air/car, air/vans and for speed orientated versions; air/speeder or just speeder.

As to why an air/raft is unenclosed I'd posit that as they are offered as an auxiliary or runabout for spacecraft they are unenclosed to allow crew and passengers in vacc suits and other types of bulky environmental protective gear to enter and exit in all types or atmosphere and gravity.

Imagine trying to sit into a normal car with a spacesuit on. Now imagine trying to do the same if they car is under water and you've got diving gear on. Finally try opening a door when the external atmosphere is greater than that of the interior, or vice versa. The unenclosed version seems to make sense for a much wider range of conditions if the crew and passengers have personal protection in the form of suits.

Also, consider the utility of an unenclosed floating platform for performing inspections and maintenance of a spacecraft hull. Just float beside the hull and lean over the side (i'd assume that in addition to safety restraints they have safety line attachment points for a tether).

So unenclosed air/rafts have the most utility to a Traveller, although undoubtedly enclosed versions are available.

 

[kilemall]

it's 500 MW, not KW.

Ya I had it MW and changed it for unknown reasons to KW.

Late night posting can get ugly.

 

[kilemall]

I'd assume that the air/raft does come in enclosed versions, but they get called things like air/car, air/vans and for speed orientated versions; air/speeder or just speeder.

As to why an air/raft is unenclosed I'd posit that as they are offered as an auxiliary or runabout for spacecraft they are unenclosed to allow crew and passengers in vacc suits and other types of bulky environmental protective gear to enter and exit in all types or atmosphere and gravity.

Imagine trying to sit into a normal car with a spacesuit on. Now imagine trying to do the same if they car is under water and you've got diving gear on. Finally try opening a door when the external atmosphere is greater than that of the interior, or vice versa. The unenclosed version seems to make sense for a much wider range of conditions if the crew and passengers have personal protection in the form of suits.

Also, consider the utility of an unenclosed floating platform for performing inspections and maintenance of a spacecraft hull. Just float beside the hull and lean over the side (i'd assume that in addition to safety restraints they have safety line attachment points for a tether).

So unenclosed air/rafts have the most utility to a Traveller, although undoubtedly enclosed versions are available.

Short answer- space jeep.

A big class of antigrav vehicle I have is the grav truck. Since I'm envisioning the classic 5-ton and 10-ton cargo lots to correspond roughly to space equivalents of 20-ton and 40-ton containers, a cheap interface method would be to have a cab and power unit hooked to a grav sled that the container would be mounted on. Then the grav truck could go straight between the highport and the surface customer, no intervening handling. Perfect for high value high speed lots with less capital entry into the business then small craft.

Since a lot of my planets outlaw reaction drive craft, the side effect is another boost to that vehicle segment.

 

[boomslang]

I always figured it was repulsion, and the strength of repulsion possible depended on the grav field, + repulsor thrust- so a grav module rated at 1G is actually generating 2G against a 1G planetary field, 3G against a 2G planetary field, etc.

What I came to coming back to the game was extending this to the artificial gravity- no pull/tractor, it's more repulsor, pushing squishy people down since it's mounted on the ship ceiling, on the back bulkhead for countering thrust, and an extra set in the floor to push up to help with 2G planets.

Possibly OT, I have a somewhat-convoluted rationale for this IMTU.

Spoiler:

In short, gravitics have two operational modes: antigrav (inertial) and direct thrust (inertialess). Antigrav works in the presence of an external gravity (even microgravity) field and the resulting interia, and "pushes" "against" it (action-reaction) up to 2Gs, giving the traditional stately air/raft performance envelope (~100kph for unstreamlined air/rafts and perhaps ~200kph for sleek grav sedans and maybe GCarriers and their ilk) (and allowing streamlined ships with only 1G drives and unstreamlined ships with any drive rating to land on and take off from Size-8+ planets very slowly without relying on atmospherics for aerodynamic lift).

This is also modified into the basic repulsor technology for artificial gravity that as you mention is ceiling-mounted (and also limited to a maximum of 2Gs). This gravity/antigravity operational mode is very energy-efficient; batteries and fuel cells can run it for long periods of time.

The direct thrust model is used to reactionlessly shove a drive around (in any chosen direction) without regard to local gravity fields, and requires large amounts of energy input (fusion is the preferred power source for anything larger than a missile). Being related to the same principles that make Jump drive possible, in vessels displacing 100dt or more, the M-drive can crete a field that cancels most inertia and accelerates everything within it; in vessels less than 100dt, this field cannot fully form, and so acceleration couches (which have little local antigrav generators built-in) are needed to cushion the acceleration effects on personnel somewhat since only the small craft drive unit itself enjoys the full benefits of inertialess acceleration. But in any vessel this inertialess acceleration can reach up to the gravitics-maximum of 6Gs, depending on how much vessel -- and in the case of big craft, displacement -- the drive is dragging along with it.

On the upside, since small craft cannot fully form the inertialess acceleration field, their drives do not require computers to control them (and any big craft trying to accelerate volumes beyond what its drives are rated for does not enjoy the benefits of acceleration compensation; i.e., you'd better strap in before you start to tow that huge disabled freighter up out of the gas giant's gravity well at a fractional Gee). In all cases, the engine's grav field, whether fully or partially formed, clings to the vessel's hull in such a way that small-enough pieces of matter (interstellar dust and cosmic rays, typically) are deflected as per Beltstrike. Furthermore, when operating a grav vehicle near a planet, Air/Raft or some similar skill is used (representing operating in antigrav mode), but under Zero-Gee conditions Pilot skill (or Ship's Boat as a reasonable substitute) is used (representing operating in inertialess direct thrust mode).

All this is a long way to go for a modest benefit, but it at least has the virtue of consistency. (And it lets unstreamlined vessels land on any world, since what is an open-topped air/raft if not an unstreamlined flying machine to haul astronauts around in?)

As always, YMMVIYTU, but in my experience this answers all my players' FAQs... usually.

 

[Enoki]

Short answer- space jeep.

A big class of antigrav vehicle I have is the grav truck. Since I'm envisioning the classic 5-ton and 10-ton cargo lots to correspond roughly to space equivalents of 20-ton and 40-ton containers, a cheap interface method would be to have a cab and power unit hooked to a grav sled that the container would be mounted on. Then the grav truck could go straight between the highport and the surface customer, no intervening handling. Perfect for high value high speed lots with less capital entry into the business then small craft.

Since a lot of my planets outlaw reaction drive craft, the side effect is another boost to that vehicle segment.

One that I have is the grav barge. This is a vehicle that is a cab and power plant with a large flat bed behind those. It is designed specifically to carry 8 to 12 standard cargo modules and is used to load and unload cargo from starships.
It is used in orbit for loading and unloading to / from an orbital platform to or from a ship and on the ground for the same purpose.
Say, you have a starship that needs unloading but can't physically land due to soft soil, a water surface, or the like. The grav barge can be used to unload and haul cargo in quantity from the ship to a pier, warehouse, etc.