damerell

I added to the linear relationship there a maximum torque - below the corresponding rotational speed torque does not increase and so power output drops. This seems to be typical in EV applications - presumably there comes a point where putting the motor's full stall torque into the gearbox will just cause something to break.

And this (still) makes no sense when the other 498 things aren't in any way something you're forced to use or significant consumers of resources or anything like that.

Thank you. I won't make any promises, since I'm lazy and have chronic health problems, but I'd like to take a look at it if I can.

You mentioned that you intended to relicence this under a Free licence. Is that still on the agenda, please?

I think you may have made an error there. WP lists the orbiter at 110,000kg, the external tank at 756,000kg, each SRB at 571,000kg. The SSME total thrust is 5,255 kN and each SRB gives 12,500 kN. That gives a takeoff TWR of 1.5. Of course, I may have made an error...

First stage, although of course any stage used low in atmo will want to have a respectable TWR. 1.1 is quite low for efficiency - consider that a rocket with a TWR of 1.0 would literally achieve nothing when first lit.

I have no idea why you suppose that is pertinent to my reply.

I wonder (assuming this analysis is correct) if the intention was to make the Elektron slightly lossy but it came out the wrong way?

A difficulty here is that most of the information about drivetrain losses pertains to motor cars, where it's typically expressed as a percentage of input power lost. (That would imply it's a force that does not vary with speed but, like rolling resistance, does more work at higher speeds). This is a perfectly sensible way to look at it in that context, where when the power is shut off most of the drivetrain isn't doing anything. In my proposed model the term proportional to the mass of the part added to the rolling resistance term is intended to cover these losses. Unfortunately that doesn't tell us much about tracked vehicles. However, as far as I can tell (see for example http://www.comw.org/pda/0007wheels.html section 2.1.2 (which is discussing total losses, I believe, given the discussion of fuel consumption)) it remains the case that most of these losses vary primarily linearly with vehicle weight and not with speed. ETA: Incidentally, it's also because of that document that I assess relatively similar rolling resistance penalties for wheels and tracks - while on-road wheels have an enormous advantage, it seems that off-road the two are remarkably similar, perhaps because tracks are much less prone to slipping.

Au revoir, I hope.

You've misunderstood me. Obviously, I know that the axle turns the wheel and the wheel pushes against the surface. What I mean by "The job of the torque isn't to rotate the wheel, it's to move the vehicle" is that the torque has no reason to vary merely because the moment of inertia of the wheel does, because the _purpose_ of applying the torque isn't to spin the wheel. If the dimensions of the wheel didn't change, once the vehicle reaches a steady speed on the flat, exactly the same amount of torque will be required to maintain that speed. It does, if the mass doesn't change, but of course the mass does change. I'm not sure what I can add here. If you suppose the new wheel is n times as large in all dimensions as the old one but of like density, you will find it weighs n^3 as much and so the moment of inertia is n^5 times as much. That graph is obviously bogus since it shows traction force constant with speed. That is extremely unlikely since it implies power output is increasing linearly with speed with no limit. The author is simply wrong about rolling resistance being proportional to speed (and it is because they are wrong about this that the RR vs drag curves are the wrong relative shape. Aerodynamic drag dominates much more at higher speeds because it is proportional to the square of speed [1] and rolling resistance is not.) The Wikipedia page mentions speed but does not discuss it in detail; most of the page discusses the usual approximation, where rolling resistance is proportional to the normal force. This is a perfectly good approximation for most purposes and it is a much better one than supposing it is proportional to speed. (For example, see http://www.analyticcycling.com who are happy to use this approximation throughout...) I increase RR in my proposed model above the part design speed not because that is realistic but because it's an easy way to reflect the part's supposed design speed in its performance, especially given that we don't have tools like increased maintenance, risk of throwing a tread, losses internal to the drivetrain, or whatever. [1] Someone's going to say, no, cube of speed. Power lost to aero drag follows the cube of speed because the amount of work done against the force is itself proportional to speed.

I don't think wheel inertia should rise with the square of radius. It would do so if the mass of the part didn't change. However, the mass probably increases with the cube of linear dimension so the moment of inertia would follow the fifth power of radius. However, I don't think it has anything to do with torque. The job of the torque isn't to rotate the wheel, it's to move the vehicle! If one assumes a fairly naive scaling-up, the power output has presumably increased with the cube of linear dimension. However, wheel revolutions for a given speed will have reduced proportionately to the linear dimension. If a wheel n times as big is expected to go no faster, torque should increase with the square of linear dimension; if it's expected to go n times as fast, it should increase with the cube of linear dimension. [1] Linear scaling for a spring is surely wrong - even if the spring is an exact copy of the spring it replaced, it is fatter as well as longer. I would guess, cube of the linear dimension. [1] One of these cases looks like something for nothing - but that's because gearing, and the point where the power output can no longer provide maximum torque, aren't reflected in it. If rolling resistance is to be applied as a torque it should not vary with speed (aside from the reduction at very low speeds I outline). It will already drain more energy at higher speeds because more work is done per second against it.

Well, I appreciate that if I want something implemented I should be prepared to implement it myself, but... one advantage of the system I outline, I hope, is that it's almost entirely hands-off. Given the part's wheel radius, assign a nominal speed and all the other values just fall out deterministically. If you don't like the performance that gives you, assign a non-default power/mass ratio. (It's worth noting, as well, that motor RPM is a purely cosmetic value. It isn't _used_ for anything - the power tailoff is just based on the nominal speed, multiplied by gearing). I don't see any need for the user to change gearing in flight rather than in the VAB - certainly not with keybinds, and arguably if it can be done in flight, only when the part is not operating. Electric vehicles don't typically make use of gear shifting because of the way electric motors can already provide full torque at very low revolutions in a way that infernal combustion engines can't (which is also partly why diesel locomotives typically generate electricity to run electric traction motors). The proposed gearing feature is part of vehicle design - not just to up the torque if designing a very heavy crawler, but because it will make it easier to mix and match wheels - a wheel designed for a slower speed can be geared up and while it won't provide much torque it will be able to provide some at the vehicle's intended speed. It also is intended to reduce the need for the part designer to get the part "just right".

I think you have misunderstood. There are no "inner wheels" and "outer wheels" on a track; I'm talking about the line-astern wheels on one track unit, which all turn at the same speed. Addendum (again): One advantage (I hope) of the approach I outline is that the part data file need only specify the wheel diameter, power output (or perhaps power/mass ratio), and nominal maximum speed, unless it is desired to override some of the computed values like maximum efficient motor RPM or default gear ratio. Everything else can be computed. The advantage is that the potential for error - putting some value in the data file that's off by an order of magnitude, or something - should be reduced.

I think it would be best to try, as far as possible, free the player from the need to play a little tattoo on the brake button. :-) Addendum the second; this is all a bit more complicated when it comes to treads, and I'm not sure I know enough about the current internal model. Assuming they are still so-many actually independent wheels in line and the tread is purely cosmetic, I think the thing to do is perform the calculations as above then divide the total torque between the wheels. Force their speeds to be identical then dynamically change the division of torque to remove it from wheels which are trying to go faster than average and add it to slower ones. I don't know how feasible that is and it's definitely pretty vague.

One addendum. Maximum braking torque should probably need to be around the maximum drive torque at double gear reduction (the 22kN for the worked example above) but will need to be tweakable freely in flight - because KSP's brakes are either full-on or full-off.

No.

As luck would have it I haven't been around the last few days, so I'm just downloading KSP 1.2.1 now. :-/ There is a worked example at the end of this post. There are a number of fudge factors in the post itself which could be tweaked if it turns out to produce results which are comedically bad. I don't think max torque is a useful place to start. Max power is, which should be roughly proportional to part mass - changed if we feel less or more of the mass of a part might be the motor (for example, track vs. wheel parts). Electric vehicle motors are approaching 10 kW/kg _but_ that is the motor alone. A Telsa S has a maximum of 568kW power output on circa 460kg of drive bits (motor, inverter, differential, wheels, brakes, suspension, rack and pinion but _not_ the battery since that's a separate part in KSP terms too). Now our parts are engineered for aerospace, but they're also engineered for off-road travel with superbly robust suspensions and parts; we might perhaps get 1 kW/kg on wheels and 0.75 kW/kg on tracks. (What we decide a kW costs in ElectricCharge is another question, but in terms of determining part performance we need to know kW). Next is the motor's maximum efficient RPM. Larger motors typically have a lower maximum RPM because the centrifugal force generated increases with the size of the rotating parts. A Tesla motor runs at a maximum 16,000 RPM, but that is high for EVs and that is the absolute maximum. I would suggest that a part the diameter of an ordinary motor car wheel (70cm) should run at 8,000 RPM and then making it scale inversely with part linear dimension - twice the size, max 4,000 RPM, and so forth. Now we can determine the maximum motor torque. The maximum motor torque is that delivered at 3/4 the maximum efficient RPM and full power. (Where does the figure of 3/4 come from? I have just eyeballed some torque v. speed curves and made it up). Next is to decide a nominal maximum operating speed. If the part isn't tiny (smaller than the wheels on a real-world 4x4), this should be more based on perceptions of the part's intended usage than anything else - tracks generally slower than wheels, etc. If it is tiny the maximum speed should probably be further reduced. (This is the only really subjective part of this process). Next is to determine the default gear ratio. We know the nominal maximum operating speed and the diameter of the wheel, so we can tell the wheel RPM at maximum speed. The default gear ratio should be such that at the nominal maximum speed, the motor is running at its maximum efficient RPM. Now we know the default gear ratio we can divide the maximum motor torque by that to determine torque delivered to the wheel. The user should be able to alter this default gear ratio in the VAB, down to about half or up to about double the default value. This will either increase the maximum torque but decrease the effective maximum speed, or decrease the maximum torque but make it more viable to exceed the nominal maximum speed. Motor performance is as follows (based on the actual gear ratio): If we are below 3/4-maximum-RPM, deliver maximum torque. Calculate power used based on RPM. If we are above 3/4-maximum RPM but below motor max efficient RPM, deliver full power. Calculate torque based on motor RPM. If we are above motor maximum efficient RPM, multiply maximum power by (1-(2* excess RPM / maximum RPM)), minmum 0. At 150% of nominal maximum RPM, the motor can deliver no power. What about rolling resistance? I wrote about this before. A potted summary is I think the force at the wheel contact point should be 0.08 times the weight on the part (ie, the current force on the suspension, but read on), 0.1 times if the part is tracked. Add to the current force on the suspension 10N per kg of the part to account for losses internal to the part - even on Gilly, grinding the tracks around takes work. If the springs are bottomed out, double the rolling resistance force (we can't know exactly how much mass they are supporting, but it's probably too much). If part speed is below 1 m/s, multiply the rolling resistance force by the speed. (This neatly means 0 force at 0 RPM). This is the only fudge for speed - normally it's not necessary because work done against the force will rise proportionately to speed anyway. This just prevents phantom forces at very low speeds. If the part is going above the nominal maximum speed, increase rolling resistance by multiplying by (1+ excess speed/nominal maximum). Given wheel diameter this can be turned into a torque term to be applied counter to the direction of movement. Here is a worked example (intermediate values have been rounded). Suppose I have a 200kg part with a wheel diameter of 1m. (Rather like the old KF medium wheel, in fact). Since this is a wheel not a track and doesn't seem to have any inordinately heavy accoutrements, I will give it 1 kW/kg. Maximum power is 200kW. It's made for roving not racing so I will assign it a nominal maximum speed of 12 m/s. (This is not so restricting; by gearing it up, the user will be able to go at 24 m/s at full power and get somewhere between there and 36 m/s. IME the old KF parts topped out at about 20 m/s.) Our wheel is 10/7 as big as the typical motor car wheel so the maximum motor RPM will be 5600. The maximum motor torque is delivered at or below 4200 RPM, corresponding to 9 m/s with default gearing. The torque at the motor will then be 454 N m. [1] A 1m wheel at 9 m/s makes 9/pi revolutions per second, 172 RPM. The default gearing is a circa 24:1 reduction. The maximum torque at the wheel axle in the default configuration is 11111 N m. (This seems like a huge number at first, but it's not for a heavy offroad drivetrain, which is what we've got - and we're turning large wheels reducing force at the contact patch. It's huge because we have a powerful motor geared down to provide high torque at low speeds. Four of these beasts (800kg) would accelerate a 5 tonne vehicle at just under 9 m/s/s from a standing start [2]). In this default configuration, from 0 to 9 m/s the part delivers 11111 N m. Power consumption rises from 0 to 200kW linearly with speed. Above 9 m/s, power consumption stays at 200kw but torque drops off to (200kW / wheel revolution rate) [3], to 8333 N m at 12 m/s. Above 12 m/s power will start to also drop off - at 15 m/s power is only 100 kW and torque is now (100 kW/ ((15/pi) revolutions per second), 3333 N m. If the user had doubled the gear reduction in the VAB, they'd get an impressive 22222 N m of torque... but only up to 4.5 m/s, where torque would start to drop off and power will start to drop off at 6 m/s. If they'd halved the gear reduction, they get 5556 N m. However, that is delivered up to 13.5 m/s, and power only starts to drop off at 24 m/s. They do however take an increased rolling resistance term from 12 m/s upwards. What about rolling resistance? Suppose four of these are mounted on a 5 tonne vehicle, but we are on Tylo, where the surface gravity is about 8 N/kg. If the mass of the vehicle is currently evenly distributed, 1250 kg rests on the spring on one part, producing a force of 10kN in the spring. We add to this the gravity-independent term of 10N per kg of the part itself - 2kN, for a total of 12kN. We multiply by 0.08 to get 960N. At speeds above 1 m/s and below 12 m/s (the nominal maximum speed), this will apply a torque of 1920 N m counter to the direction of movement. This seems to be of about the right order of magnitude. It won't trouble the default gearing producing 11111 N m, but the maximum-speed gear reduction is clearly going to struggle as it approaches 24 m/s (4167 N m torque) with the rolling resistance term doubled for overspeed to 3840 N m. [1] This is the least obvious derivation. Here is a perfectly good calculator; or one can type "You have: 200 kW / (4200 revolutions per minute)" "You want: N m" into the UNIX program "units"; or one can know that 1W at 1 revolution per second is 1/(2 * pi) Newton metres of torque so 200kW at 4200 RPM (70 RPsecond) is 200000 / (2 * pi * (46 2/3)) N m = 454 N m. [2] An implication is that there may want to be a user facility - not restricted to the VAB - to cut maximum torque at low speeds, so that keyboard users don't constantly do wheelies because they have no choice but to floor the throttle. What I'd suggest is that the user can input a limited torque for use at 1 m/s and below, and torque then rises linearly between 1 m/s and 10 m/s. [3] At first glimpse this doesn't make sense because torque seems to be independent of gearing. It is - in the range where the motor is operating at full power and performance is limited by that. Gearing changes where that power limited range starts and where the motor power starts to drop off.

ElectricCharge, assuming you are back in 1.0.x where they work at all.

1) Why on earth wouldn't you be able to (in a modded attempt)? I brought KIS/KAS spares every time. 2) This question makes no sense to me.

MechJeb's Rover Stability Control does this (but also controlling roll, and with regard to the shape of the ground underneath you). The downside is that it does it even when all your wheels are on the ground, which tends to eat electric charge spinning your reaction wheels.

I must say, that looks very encouraging. I'm on holiday at the moment but I look forward to perhaps having a poke at the power and torque side of things when I return.

I'll be honest and I say I don't. Holding Mod makes it possible, not easy. I'm not clear on why it's so difficult in some installs and not in others.

How launch-y are we talking here? Vessel does a wheelie, or vessel fired into orbit? The former seems consistent with the idea that if you hit a bump at speed with the suspension bottomed out, bad things will happen.

In which you write "When acceleration/decelleration was needed, the reaction wheels were toggled off, to prevent it from flipping itself." It's possible (and highly desirable) to bind the rover movement commands to different keys to the rotation commands.