Grav Vehicles, Grav Drives

[Grav_Moped]

This is based on a few assumptions. Unfortunately the conclusion is incompatible with the stated performance ("An air/raft can reach orbit in several hours (number of hours equal to planetary size digit in the UPP...)".

That's nearly 6 hours to put the Air/Raft into a Low Earth Orbit, but probably not the orbit you want it to be in. It's up there and will stay there, though.

If you want to be in a particular orbit (and why would you go into orbit if you weren't meeting something up there?), it'll take time to get to the appropriate starting location for the ascent. The "missing" two hours allow a 2000km radius to reposition (in vacuum or something very close to it) before going the rest of the way up. And even if you don't need all of that time, you'll need to wait for up to two hours (orbital period at LEO) for what you're meeting up there to be in the right place when you arrive.

Actual time to desired orbit and position is therefore 6-10 hours on this Size 8, Atm 6 world.
Call it Hours=(Size-2)+(2D/3) if you need to be picky*, Hours=Size if you don't.

I'd say that's close enough.

* CT: Pilot or Small Craft skill (not both) give DM-1 to the 2D roll per skill level (this is mostly about timing the takeoff for rendezvous). Time can never be less than (Size-2) hours.

 

[Enoki]

The way I see grav vehicles is that the one usually presented in the rules is the equivalent of a Toyota Hilux diesel pick up. It's not particularly fast, but it can haul a decent load and is highly reliable.

But, I figure that there are lots, and lots, of other types out there. I've put a few of these in the gallery here. My view is most are enclosed and even pressurized for comfort. They can accelerate, depending on design all the way to 6G (since that's the game limit) in some cases.

I see them in forms like ultra-sports car models (think a Ferrari or Lambo), the family minivan, or the SUV, among other types. I figure that people might have reasons to go into low orbit for a whole host of reasons, not all instantly apparent to us.

The big limitation is the size is always small. This means they are really unsuited to long trips into space because of the lack of facilities, and limited comfort. But, if it's all you've got, sure, you could drive one to the Moon but you better have some diapers or a place to do your business along with a big sack of munchies and be willing to sit for hours on end getting there.

This is where small non-starship spacecraft fit in. They are the semi-tractors, busses, and airliners of space. They get into orbit just as fast as an air raft can, but they're big enough that you can take one to satellites or other planets in the system without suffering the discomfort of driving the "family car" there.

 

[Grav_Moped]

Pretty much this.
I'm basically discussing TL-8 grav vehicles here, and see the basic Air/Raft as an early VW Vanagon/late Bus (67 HP, drag-limited to 70MPH at sea level) with a sunroof and the ability to levitate indefinitely.

 

[Grav_Moped]

And of course you wouldn't use a standard Air/Raft to get to and from orbit (or travel halfway around a world) unless it was all you had on hand.

I don't assume cabin pressurization, airframe hulls, or re-entry heat shielding. They're not typically operated at altitudes where oxygen or pressurization would be required, nor subjected to ballistic re-entry.

Basically, TL-8 gives you grav vehicles with the drive characteristics of an Air/Raft (0.1G discretionary thrust, plus weight cancellation) in a variety of sizes. Some are Air/Rafts by the book, some are airplanes, some are even space shuttles. But you're still talking subsonic speeds except in the near-vacuum above the mesosphere.

The problem with aerobraking (which is really helpful in cutting down intercontinental transit times, by the way) is that hypersonic re-entry means means sonic booms in addition to the flaming contrails. Locals tend to frown on such things, most places.

 

[AnotherDilbert]

As far as getting to orbit,the first half of the ascent to orbital altitude (not velocity) takes 2.25 hours : 1 hour to get to 30km altitude, 1.25 hours to get to 1000km altitude. The second half of the ascent has the 0.1G that was used for ascent thrust redirected laterally to achieve orbital velocity.** After another 1.25 hours of this, the craft is at the 2000km orbital altitude but at only half orbital velocity (about 4km/sec). At this point the antigravity is dialed back to about 0.5G and progressively reduced to zero over the next hour of continued lateral acceleration. 55 minutes later it will have achieved 7.8km/s orbital velocity at 2000km altitude.

You are basing your calculations on thrust giving 0.1 g acceleration in addition to overcoming 1 g gravity for a total of 1.1 g as Striker specifies.

But available thrust is based on current local gravity:

The movement rate is determined as outlined in the vehicle design rules in Book 3, but instead of subtracting 1 from the G-rating of the vehicle's drives, the local gravity is subtracted instead.

Gravity at 1000 km height is about 0.74 g. So excess acceleration at 1000 km should be about 0.36 g.

But we also know that the effectiveness of grav vehicles drive decreases away from gravity sources (but we don't know how) since "... interplanetary travel in an air/raft is not possible".

So excess acceleration is probably less than 0.36 g?


The only thing we really know is that "An air/raft can reach orbit in several hours (number of hours equal to planetary size digit in the UPP ..." so 8 hours for a size 8 planet like Earth.

Either way we can't make precise calculations about how grav vehicles move outside atmosphere.

 

[AnotherDilbert]

But, I figure that there are lots, and lots, of other types out there. I've put a few of these in the gallery here. My view is most are enclosed and even pressurized for comfort. They can accelerate, depending on design all the way to 6G (since that's the game limit) in some cases.

Agreed there are many types of grav vehicles.

But the 6 g limit is for M-drives in spacecraft, not grav vehicles. The Striker design system specifies performance up to 8 g (Striker, Design Sequence Tables, p5).

 

[Carlobrand]

Old rusty math gives me a peak velocity up of 22.5 m/s, assuming a drag coefficient of 1.28 (which is to say, a plate). So, you lift off, and at some point between a half minute and a minute later you top out at 22.5 m/s, then very gradually increase velocity as the air gets thinner. I could have done something wrong in the math there, but I think I got that right.

So, yeah, an hour or a bit more to 100 km altitude - but at that point air resistance is pretty much inconsequential and you're accelerating up at 0.1G; you'll reach 1000 km altitude something under 20 minutes later and you'll have a hefty vertical velocity vector when you get there, and that's being lazy and not figuring the declining value for gravity as altitude increases. If you want a stable orbit, you'll need to put some power toward horizontal movement (and braking that upward vector), but you can wait that until you've cleared the atmosphere and friction's no longer slowing you down.

At 0.1G it's 4 hours 20 minutes to the 10 diameter limit where performance falls off, for an earth-size world. If you want a truly different form of suicide, you'll be outbound at well over 15 kps at that point (again, being lazy). Aim well, and you can make a lovely splat on the surface of the moon about 7 hours later and get a crater named after you.

I frankly haven't got the foggiest idea where canon drew that "number of hours equal to planetary size digit in the UPP" idea from. Most likely the game designers needed a number and that just looked easy; that was long before they gave us design systems and graphs correlating Gs with velocities. That doesn't even make sense when you consider a Size-4 world might have a surface gravity of a half G or so; why would it take 4 hours for an air/raft to make orbit from a half-G world with a thin atmosphere? It's hard to justify saying a craft that's pushing straight down with 1G can't push straight down with that final 10th of a G when it wants to, but I think that's what they were thinking - something that simply neutralizes gravity and then gives you some forward motion, but pushing up was a good deal harder. That's a piece of history I retired once the design system showed up and gave us something approaching a coherent picture of the technology.

 

[AnotherDilbert]

Old rusty math gives me a peak velocity up of 22.5 m/s, assuming a drag coefficient of 1.28 (which is to say, a plate). So, you lift off, and at some point between a half minute and a minute later you top out at 22.5 m/s, then very gradually increase velocity as the air gets thinner. I could have done something wrong in the math there, but I think I got that right.

Top speed for an air/raft horizontally is about 100 km/h ≈ 28 m/s. It has to be considerably smaller vertically?

 

[Enoki]

And of course you wouldn't use a standard Air/Raft to get to and from orbit (or travel halfway around a world) unless it was all you had on hand.

I don't assume cabin pressurization, airframe hulls, or re-entry heat shielding. They're not typically operated at altitudes where oxygen or pressurization would be required, nor subjected to ballistic re-entry.

You don't need reentry heat shielding if your reentry is non-ballistic in nature. This is only required when you don't have the means to slow your reentry using the vehicle's engines, or don't have the means to make one that is non- ballistic.
Pressurization is required (by Earth standards) above about 30,000 feet and best if above 20,000. Above about 15,000 you need supplemental oxygen. So, to go to any sort of altitude, you need pressurization or will have to wear pressure or vac suits. If the vehicle isn't intended for operation in these sorts of conditions then the pressure / vac suit would be the only option, and maybe even the vehicle could be compromised otherwise if used in such a manner.
For example the electronics and electrical system might not be rated for use above 15,000 feet (Earth equivalent) so you can't go into orbit simply because the electronics on the vehicle will fail if you do.

 

[Enoki]

Agreed there are many types of grav vehicles.

But the 6 g limit is for M-drives in spacecraft, not grav vehicles. The Striker design system specifies performance up to 8 g (Striker, Design Sequence Tables, p5).

That's fine. It only proves my point. Different vehicles have to exist. If you have the equivalent of a modern super car you can get into orbit in a matter of minutes.

 

[Grav_Moped]

You are basing your calculations on thrust giving 0.1 g acceleration in addition to overcoming 1 g gravity for a total of 1.1 g as Striker specifies.

But available thrust is based on current local gravity:



Gravity at 1000 km height is about 0.74 g. So excess acceleration at 1000 km should be about 0.36 g.

But we also know that the effectiveness of grav vehicles drive decreases away from gravity sources (but we don't know how) since "... interplanetary travel in an air/raft is not possible".

So excess acceleration is probably less than 0.36 g?


The only thing we really know is that "An air/raft can reach orbit in several hours (number of hours equal to planetary size digit in the UPP ..." so 8 hours for a size 8 planet like Earth.

Either way we can't make precise calculations about how grav vehicles move outside atmosphere.

The reason I assume you only get the 0.1G lateral thrust rather than "1.1G total, minus local gravity" is that the top speed in a standard atmosphere is a constant rather than varying by world size, and is clearly drag-limited due to the vehicle description.

Anyhow:
Part of the problem here is that the SF/Sci-Fi conceptions of flying cars, small craft, and space-/star-ships (as reflected in the game) are pretty much each in their own silo, without much thought given as to how they might overlap. Flying cars are basically helicopters without rotors. Small craft are how you cross planetary and maybe interplanetary distances. Beyond that you use spaceships.

This overlooks that an Air/Raft's stated capability to get to and from orbit is equivalent to what the first manned rockets could do, but in slow motion and without the fireworks -- and those rockets were basically repurposed ICBMs that could hit any point on the planet in under an hour. That's covered by the "intercontinental passenger rocket" idea ("small craft" in Traveller parlance), but if you're still thinking "flying car" it won't occur to you that an Air/Raft can do that too (only slower), because cars (flying or not) obviously have to deal with aerodynamic drag and can't go that fast. Hence the LBB3 "Speeder" grav vehicle, which can almost break the sound barrier with its 2G drive (Striker design tables).

It looks like it really never occurred to them that one could use a grav vehicle to transit sub-orbital space for on-world point-to-point travel -- you're either in the atmosphere, in orbit, or en route from one to the other.

I'll digress for a moment to note that this mindset caught me up in it, too. While I've been going on (and on, and on -- yes, I know) about aerobraked re-entry to save time on the deceleration/descent phase for a while, it hadn't occurred to me until just now that one could build an Air/Raft derivative that looked like a tailsitter rocketship to cut drag (and thus save time) getting out of the lower atmosphere in the first place! I was thinking in terms of abusing flying cars by using them as small underpowered spacecraft -- not in terms of designing small underpowered spacecraft in the first place. This reminds me of the discussion about Grav Tanks and what they look like...

 

[AnotherDilbert]

The reason I assume you only get the 0.1G lateral thrust rather than "1.1G total, minus local gravity" is that the top speed in a standard atmosphere is a constant rather than varying by world size, and is clearly drag-limited due to the vehicle description.

But Striker specifically says top speed is dependent on local gravity, and the 1.1 g drive performance of the air/raft is from the Striker tables. If you use one, I think you should use the other.

Part of the problem here is that the SF/Sci-Fi conceptions of flying cars, small craft, and space-/star-ships (as reflected in the game) are pretty much each in their own silo, without much thought given as to how they might overlap.

At least MT and TNE has unified design systems; vehicles and spacecraft are designed using the same system. The overlap is clearly defined.

While I've been going on (and on, and on -- yes, I know) about aerobraked re-entry to save time on the deceleration/descent phase for a while, it hadn't occurred to me until just now that one could build an Air/Raft derivative that looked like a tailsitter rocketship to cut drag (and thus save time) getting out of the lower atmosphere in the first place!

Of course we can build sub-orbital or orbital grav vehicles, but we end up with something less efficient than a small craft. Once we have M-drives they are superior for space and orbital interface.

 

[Grav_Moped]

But Striker specifically says top speed is dependent on local gravity, and the 1.1 g drive performance of the air/raft is from the Striker tables. If you use one, I think you should use the other.

That's a defensible position. Since Striker grav vehicles are handled differently than CT grav vehicles, I'd prefer to use only the minimum necessary and most relevant info from the latter source.

At least MT and TNE has unified design systems; vehicles and spacecraft are designed using the same system. The overlap is clearly defined.

Useful info, thanks! They may be designed with the same system, but I was more concerned with how people think of them and how that shapes expectations of what they can do.

Of course we can build sub-orbital or orbital grav vehicles, but we end up with something less efficient than a small craft. Once we have M-drives they are superior for space and orbital interface.

True, but that's TL-9+ and an entirely different matter. Still, four Air/Rafts have more payload capacity (by mass) than a 20-ton Launch and take up 20% less space, while costing MCr 2.4 instead of MCr 14. You give up a lot of flexibility and the ability to operate beyond planetary orbit in exchange for the low-budget price tag.

That said, I was pointing out that even while trying to push the envelope of what a lowly Air/Raft is capable of, I was still thinking in terms of it just being a car that flies. For example, why does it have to stay horizontal like a car? Could you just point it straight up so drag is less of a factor in a vertical ascent? Maybe not, but it's noteworthy that I didn't even think of it until then even when I was trying to eke out every advantage for it.

The funny part is that I've thought of pointing them nose-up for aerobraking in normal flight (120kph airspeed, meet barn door!) as well as heat-shielded re-entry. Doing the same for vertical ascent didn't occur to me because cars don't do that.




Addendum: In previous posts, I miscalculated the time to descend from 30km to the surface (and thus to descend from 100km).
Ascent is at 30kph in the lower atmosphere (up to 30km) because it's at 0.1G. Descent through that zone is at up to 1G and at the craft's terminal velocity.
Top speed is power/drag, drag is square of airspeed (all else equal, low subsonic regime).
Descent speed is sqrt(10 times the acceleration force)*(the 30kph ascent speed): approx 100 kph. Expect a turbulent ride down, but it's plausible.
With 0.1G available for arresting the fall, it'll take 30 seconds to come to a stop from 100kph (that is, come to rest in a hover).
So, it takes an hour to get to 30km altitude, but only 18.5 minutes to get back down through that again.

Getting from 30km to 100km is D=AT^2 where A=0.1. Getting back down is similar, but requires speed<100kph at 30km altitude. Acceleration down is at 1G, deceleration to atmosphere contact is at 0.1G. This is slightly different from orbital re-entry, as that will be at 0.1G deceleration all way down.

It's 0200 local and I'm too tired to do the math right now. Goodnight!

 

[AnotherDilbert]

That's a defensible position. Since Striker grav vehicles are handled differently than CT grav vehicles, I'd prefer to use only the minimum necessary and most relevant info from the latter source.

Striker is CT. It's just a more detailed view than LBB3. You can of course use whatever you want in your game.

Still, four Air/Rafts have more payload capacity (by mass) than a 20-ton Launch and take up 20% less space, while costing MCr 2.4 instead of MCr 14. You give up a lot of flexibility and the ability to operate beyond planetary orbit in exchange for the low-budget price tag.

That is a bit of apples and oranges.
The air/raft will lift 4 tonnes = 4000 kg.
The Launch will fit 13 displacement tons = 182 m³ ≈ 5 TEU. At a standard max of about 20 tonnes per TEU, that would be in the region of 100 tonnes.

Note that the launch can carry four air/rafts, presumably loaded, in its cargo hold.

Spacecraft, even small, are quite big and capable.

That said, I was pointing out that even while trying to push the envelope of what a lowly Air/Raft is capable of, I was still thinking in terms of it just being a car that flies. For example, why does it have to stay horizontal like a car? Could you just point it straight up so drag is less of a factor in a vertical ascent? Maybe not, but it's noteworthy that I didn't even think of it until then even when I was trying to eke out every advantage for it.

Why would we not be able to fly upside down? It's just inconvenient, especially in an open-topped vehicle.

The funny part is that I've thought of pointing them nose-up for aerobraking in normal flight (120kph airspeed, meet barn door!) as well as heat-shielded re-entry. Doing the same for vertical ascent didn't occur to me because cars don't do that.

It may be a lorry, but it's a flying lorry. It is manoeuvring in 3D like an aircraft or submarine:

This is supposed to show a helicopter looping...

Getting from 30km to 100km is D=AT^2 ...

D = at²/2 if we are accelerating fully without slowing down.

 

[Carlobrand]

Top speed for an air/raft horizontally is about 100 km/h ≈ 28 m/s. It has to be considerably smaller vertically?

Considerably larger horizontally, I think. The Striker G/velocity grid is straightforward, doesn't care what the size or shape of the craft is. However, those are key variables in the drag equation. Bricks have a pretty high drag coefficient, but it doesn't look like the air/raft is presenting a very high cross section to the air. Could be increased drag due to being open-top but, again, the system doesn't look at shape except at higher speeds. Even aircraft use the same table; they only get drag for weapon mounts.

Striker didn't care about shape, and MegaTrav handled it by capping speed at 300kph for nonstreamlined craft. Whatever's limiting the air/raft's horizontal speed does not appear to be related to drag. Or, we try to forget Striker/MT and just assume something else is going on, which would be unfortunate since we then have no cohesive system for designing the other vehicles that would fill our worlds.

 

[AnotherDilbert]

The Striker G/velocity grid is straightforward, doesn't care what the size or shape of the craft is. However, those are key variables in the drag equation. Bricks have a pretty high drag coefficient, but it doesn't look like the air/raft is presenting a very high cross section to the air. Could be increased drag due to being open-top but, again, the system doesn't look at shape except at higher speeds. Even aircraft use the same table; they only get drag for weapon mounts.

I think the system assumes "reasonable" streamlining. So for an 100 km/h air/raft it's not major issue, but a hypersonic craft is assumed to be extremely streamlined.

A lot of things are not defined in the system.

 

[Straybow]

The air/raft will lift 4 tonnes = 4000 kg.
The Launch will fit 13 displacement tons = 182 m³ ≈ 5 TEU. At a standard max of about 20 tonnes per TEU, that would be in the region of 100 tonnes.

Note that the launch can carry four air/rafts, presumably loaded, in its cargo hold.

No, the a/r is 4 dtons, way more than 4 tons. If the 4 dT is the external size, then it is the size of the largest trucks one can rent without a class B license, a 26' truck with a capacity of about 45 m³ = 3 dT. That could easily be 30 tons by various Trav rules.

 

[Enoki]

I think the system assumes "reasonable" streamlining. So for an 100 km/h air/raft it's not major issue, but a hypersonic craft is assumed to be extremely streamlined.

A lot of things are not defined in the system.

A crappy little econobox with a 4 cylinder engine can usually manage to get up to 100 mph. Something with even a little umpf can manage more than that.
Even something as basic as a Cessna 150 can exceed 100 mph and cruise at nearly that in level flight.

I think it's reasonable that a very basic air raft could exceed 200 mph in flight and probably do upwards of 400. Something streamlined with more power and less weight (think something akin to Bugatti Veyron or McLaren) would easily exceed the speed of sound by at least double. The upper limit for some (as I call them) gravcoupe would be skin heating from atmospheric friction, not drag, just as in the SR 71 or X-15.
I would think it not unreasonable for a "sports" model with insane power and not a lot of weight to be able to exceed 50,000 feet a minute in climb. Some jet fighters can manage that or more. Even the old 1950's F4D Skyray managed to climb 49,000 feet and change from a standing start in 2 minutes 36 seconds.

I think the rules as presented are far too conservative about what's possible with grav vehicles.

 

[Carlobrand]

No, the a/r is 4 dtons, way more than 4 tons. If the 4 dT is the external size, then it is the size of the largest trucks one can rent without a class B license, a 26' truck with a capacity of about 45 m³ = 3 dT. That could easily be 30 tons by various Trav rules.

Given Cr600,000 to work with, I can design something that hits around 30 metric tons payload, but it flies like a bat outta hell when it's not loaded.

Edit: Ooh, 50 tons!

 

[Grav_Moped]

A crappy little econobox with a 4 cylinder engine can usually manage to get up to 100 mph. Something with even a little umpf can manage more than that.
Even something as basic as a Cessna 150 can exceed 100 mph and cruise at nearly that in level flight.

I think it's reasonable that a very basic air raft could exceed 200 mph in flight and probably do upwards of 400.

The original frame of reference was "car with antigravity", and in 1977-82, 120 kmh was significantly faster than cars could legally travel in the US.

This is because at the time they wrote the game, the US national speed limit was 55 MPH (88 kmh). It wasn't raised again until the mid 1990s.

Something streamlined with more power and less weight (think something akin to Bugatti Veyron or McLaren) would easily exceed the speed of sound by at least double. The upper limit for some (as I call them) gravcoupe would be skin heating from atmospheric friction, not drag, just as in the SR 71 or X-15.

The canonical CT vehicle you're discussing is the Speeder (TL 8, like the Air/Raft), and, surprisingly, even it wasn't supersonic.

But (dead horse abuse alert!) once you get above the atmosphere, neither drag nor frictional heating is a constraint.

I would think it not unreasonable for a "sports" model with insane power and not a lot of weight to be able to exceed 50,000 feet a minute in climb. Some jet fighters can manage that or more. Even the old 1950's F4D Skyray managed to climb 49,000 feet and change from a standing start in 2 minutes 36 seconds.

An canonical Air/Raft can easily exceed that 19,000 fpm (100 m/s) in vertical ascent in a vacuum. In the lower atmosphere it's drag-limited to 8m/s if ascending flat, 33m/sec if nose-up, because it has the aerodynamic characteristics of a heavily-starched flying carpet. Streamlining can enable far better performance than that, but as-written they're not particularly streamlined.

I think the rules as presented are far too conservative about what's possible with grav vehicles.

I agree, but for slightly different reasons. They're only "realistic" if you're operating in the lower atmosphere with vehicles that look like TL7 ground cars and have 0.1G (Air/Raft) to 1G (Speeder) thrust after neutralizing their own weight.

They're far too conservative because they do not account for aerodynamic optimization, nor do they even consider exo-atmospheric operation aside from "going to orbit".

Basically, given the stated capabilities of an Air/Raft, it could reach any point on Earth (20,000km distance) within 4 1/2 hours: 1 hour and 15 minutes to 100km altitude, 3 hours above the atmosphere (would be 2 1/2 hours but escape velocity limits peak speed), and 15 minutes getting back down again. Average ground speed is 8000kmh. Shorter trips would have lower average ground speed since the climb/descent would be a larger portion of the trip.

A speeder built like a scaled-down X-15 could easily go Mach 3+ in atmosphere. But then, when an Air/Raft can effectively travel at Mach 6+ over the longest distances, why would you need one?

And all of this discussion is about TL 8 grav vehicles that don't have much extra thrust left over after hovering -- once you get to TL9 and up designs, lack of thrust is no longer an issue.