[AR] Re: Tesla Thruster Specs?
- From: Peter Fairbrother <peter@xxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Sun, 22 Sep 2019 20:01:53 +0100
On 22/09/2019 17:08, Uwe Klein wrote:
Am 22.09.2019 um 09:29 schrieb Peter Fairbrother:
Hmm, maybe not - some rather ridiculous 0-60 mph bote numbers:
The drive motor doesn't show a perfect moment times rpm ~= constant
behavior.
( what do they use these days :: still a multiphase async engine?)
https://de.wikipedia.org/wiki/Drehstrom-Asynchronmaschine#/media/Datei:Kennlinie_Asynchronmotor.svg
Thus there is a limit to available acceleration.
Elon just said the non-rocket Roadster would do 0-60 in 2.1s, although
an earlier figure of 1.9s was touted about.
I calculated non-rocket mode at 1.78s assuming a delivered power of 1MW
and a coefficient of friction of 1.5, which is a measured average for F1
slick tyres and probably not obtainable with road-legal tyres.
Using a coefficient of 1.3 for road tyres and redoing the same
calculation [1] I get 2.06s.
On the other hand you have all the things available to
control moment/rpm to achieve best acceleration
Elon seems to have a handle on it for the vanilla Roadster.
Also, and perhaps most important, any acceleration limit as above is
most significant in the first few milliseconds when the motors and
wheels are moving slowest: but the power used then is much lower than
the maximum power, either because of traction limitations or because of
human acceleration limitations.
As I see it, it's pretty much a non-issue.
( at optimum slippage in the road to rubber interface.)
I used no slippage in the calculation, as the coefficient of static
friction is usually greater than the coefficient of sliding friction
(and it's kinder to the tyres); this is not always the case, but I don't
know enough about the real-world edge cases to use them.
Dragster slicks sometimes spin for a bit, then the tyres stick, wrinkle,
then awaay!. However the initial spin is to stop the tyres wadding up
during the wrinkle phase, and I don't think the spin actually increases
traction - probably the opposite.
A Roadster with modern low profile tyres wouldn't have that problem, and
as you say there is everything there to get the best control of traction
and slippage - which in practice, especially considering tyre wear, is
almost certainly zero slippage.
beware of the rim slipping versus the tire.
Superglue? I have some rubbery superglue which sticks like &#!^ which
might work, and which comes off easily at about 150-200C.
But bead locks should be OK on a production car.
Peter Fairbrother
[1] Incidentally that is a very easy calculation - the maximum force
obtainable from friction is the friction coefficient times the normal
force (the mass of the car times g). F=Cmg, so the acceleration = F/m or
Cg.
Time = speed / acceleration, so (60mph =) 26.8ms^-1 / (1.3 x 10ms^-2) =
2.06s.
We just have to check that the drive power can provide 1.5G, which it
can - at 60mph 1MW gives 1.86 G.
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