That Silverliner V acceleration spec

In that long thread on the Silverliner V, Mr Mitchell quoted the original specified acceleration: ramp up to 3 mph/sec in 3 seconds, then maintain that to 30 mph or maybe a bit higher-- so, reach 30 mph in 12 seconds from the start. Then reach 50 mph in 11 seconds more.

That's with 111 people (or some such?) on each car-- so a gross weight of maybe 70 tons per car if they hold to their specified weight for married pairs.

Nobody commented on how startling such performance would be. Getting from 30 to 50 mph in 11 seconds would demand 1300 hp per car-- and if they really did maintain 3 mph/sec right up to 30 mph that would require 1600 hp per car at 30 mph.

What I think this tells us is that holding the weight down will matter, because those figures sound unlikely enough at the specified weight. I'd say it's a safe bet that if weight increases 10% the acceleration will drop proportionately-- the available power won't increase.

I don't suppose the S-Vs will have regenerative brake? If not, that wonderful acceleration can only come from greater kilowatt-hour consumption. (Compared to a more sedate design.)

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That's with 111 people (or some such?) on each car-- so a gross weight of maybe 70 tons per car if they hold to their specified weight for married pairs. </i>

Yes.

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Nobody commented on how startling such performance would be. Getting from 30 to 50 mph in 11 seconds would demand 1300 hp per car-- and if they really did maintain 3 mph/sec right up to 30 mph that would require 1600 hp per car at 30 mph. </i>

Yes. They would be the highest powered EMUs since the Metroliners (1200hp/car) the LIRR's MUs are capeable of 1080HP per car, but tuned down.

<i>What I think this tells us is that holding the weight down will matter, because those figures sound unlikely enough at the specified weight.</i>

Yes. Septa's weight limits are a bit higher than they should be. But it's nice to see that they'll put their foot down at a certain amount and NOT accept a car of more than a certain weight, though their specs are way heavier than the existing equipment.

<i> I'd say it's a safe bet that if weight increases 10% the acceleration will drop proportionately-- the available power won't increase. </i>

Yes, this was the LIRR's experience. The M-7s can't generate 1000 hp/car without popping substations. After entering service, their acceleration was dropped significantly.

<i>I don't suppose the S-Vs will have regenerative brake? If not, that wonderful acceleration can only come from greater kilowatt-hour consumption. (Compared to a more sedate design.)</i>

They may. They'll certainly regenerate to make HEP for the train. Generally, regeneration only works if the catenary can accept it. If it rejects it, you need to dump the DC link somehow else.

The real answer is the weight will have to be dropped as low as possible.

What do the HP numbers work out to be for a 100,000 car?

Anyway, one mitigating factor is SEPTA runs short trains. But weight still matters.

I did make one mistake: The required acceleration to 30 mph in 12 seconds doesn't necessarily require 3 mph/sec all the way to 30 mph-- just almost (4.5 mph to 30 mph in 9 seconds). So we have to whittle down that 1600-hp-per-car estimate a bit.

The necessary power is almost directly proprtional to weight, since maybe 97% of the total resistance is inertia.

You got a quick / dirty formula for doing the accelertion w.r.t. weight? I got the basic idea, but looking for a prefab, rail specific formula

I do, BTW, have the aceleration curves for GE's proposal for the Arrow Is, which was the SAME system as the Silverliners. I'm guessing the data will be very close to an SL II/III. anyone who's curious - say so - I'll scan it.

BTW, Westinghouse got the Arrow I order. I don't have that proposal

When you're figuring the power needed to accelerate at 3 mph/sec the "rail-specific" part of the calculation is a tiny fraction of the total; at least 90% of the total is straight high-school physics (assuming we have the correct weight for the train). And we can make a pretty fair guess at the other 10%.

As you probably know, the 90% is translational inertia. 3 mph/sec is 0.1368 times gravitational acceleration, so accelerating a 70-ton car at that rate requires 19140 lb, which (if the car could produce that at 30 mph) would be 1530 horsepower.

We augment that by a few percent for the rotational inertia of the wheels and traction-motor armatures (?)-- what do you suppose one of them weighs?

If we're calculating the average power needed to accelerate from 30 to 50 mph in 11 seconds (including translational inertia only), it's 50 squared minus 30 squared equals 1600, times 70 tons is 112000, divided by 11 seconds is 10182, divided by the necessary constant of 8.2263 gives 1238 hp. Which we augment as before.

Then we add rolling and air resistance, less than 50 hp per car at 30 mph. If you want details...

timz wrote:I don't suppose the S-Vs will have regenerative brake?

Yes. According to the SL-V spec, regenerated braking energy is returned to the DC system for use on board, return to the catenary, or for dissipation.