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Minky

Afterburner Considerations

First can I please just check my understanding..
To be effective the afterburner flywheels need to exceed the primary flywheel motors RPM, or else they are bringing little to the party.
In order to ensure a 'clean' pick up of the dart by the secondary fwc there should be a guide tube to feed them directly into both flywheels.
The gap between first and second fwc should be as small as possible without allowing a dart to be gripped by both sets of flywheels at the same time.

Does all that sound correct?

If so are 2s specced motors overvolted to 3s the best bet for the second stage?
Would the 2nd set of motors have to be as Torque-y as we'd expect a primary set to be?
Would one fly back diode protect a mosfet from both FWC's back EMF?
Do we need to spec a battery to handle the stall of both sets of motors (plus pusher if required) or is the chances of experiencing stall on all of them so remote as to make it safe to spec off the three thirstiest motors (in a 5 motor set up)?
Baldrick

I would personally not use 2s motors overvolted to 3s due to the reduction in the lifespan of the motors, the other questions I have the answers to are the highest torque required will be for the first set of motors (taking a dart from near rest to high speed requires more torque than acelerating a dart from high speed to higher speed), the motors need to be higher rpm the further along the sequence they are (highest rpm closest to the muzzle), generally accepted is that the best distance between flywheel sets is just more than one dart length.  I don't know whether feed guides are necessary, but accurate alignment of the flywheel sets is critical for an afterburner set up.  I can't answer the fet protection question as I have not used any fets in my builds yet (I always replace my switches [so far, at least]).  On the battery spec, I would personally think it better to allow for the stall currents in all motors (in a high dps blaster, you cold have darts going through all flywheel sets simultaneously).
SSGT

There are actually a few schools of thought with this. The first is the classic "afterburner" setup where the second stage is usually faster than the first and normally located some way down from the first stage (often right at the other end of the barrel). Rather than a single initial burst of acceleration the dart is accelerated once and then accelerated again after it's travelled down a length of barrel (I guess it is actually a little bit like an afterburner in an aircraft gas turbine - you inject and burn fuel in the combustion chamber and then inject and burn fuel again in the exhaust stream).

The second is to move the flywheel cages as close to each other as possible so that more than one stage accelerates the dart at the same time. To do this the first stage actually needs to spin as fast (or at least nearly as fast) as the second stage as the first set of flywheels will still be in contact with the dart as it's passing through the second stage. If the flywheels in the first stage spin too slowly the second stage will effectively be dragging the dart through them which will rob you of velocity at the exit.

The spacing itself should actually have little to no effect on the final velocity as long as the motors selected are suitable for the relevant setup, choosing one method over the other comes down to other factors. The benefit of a true "afterburner" is that the different stages can be completely independent of each other (with the final stage possibly even being removable) - each stage has it's own critical speed and so can use different motors at different speeds. This may mean that you can get away with a smaller pack depending on the motors you choose (you could, for instance, use a 3S Hellcat first stage and a 3S Wolverine second stage as opposed to two 3S Wolverine stages). The downside is getting the alignment right. The benefit of a concurrent multi-stage setup is that the the first stage should do a better job of aligning the dart as it will still be in contact with the first stage (and held centrally if you're using concave profile flywheels) as it enters the second.

More recently it seems that people have started aiming for something in between i.e. as close to each other as possible whilst having as little overlap (or possibly even none) as possible. To be honest the best compromise might be having an ever so slight overlap - enough that the first stage can more easily guide the dart into the second but not so much that you need the first stage to be spinning as fast as the second (this may be feasible as there already is some overhead in that most commonly used flywheel motors already spin slightly faster than they need to for most single stage setups). You could add a tightbore barrel between the two but remember that any friction you add will effectively be taken off the force applied to the dart by the flywheels resulting in a reduction of velocity at the exit. It may make feeding more consistent and reliable but YMMV.
OldNoob

You should start by looking at the flywheel motor data sheets and look at the RPM's in there to consider how you will set up the two stages. Personally I think in brushed set ups a single high crush stage, which can achieve 150fps peak, is much more reliable and better than multi stage set ups.
If you must find out the hard way, then SSGT's considerations are the first thing you should look at.
The next thing you need to consider is powering them. If you go 180 then at best each stage is going to need 45A peak. That makes a whopping 90A, plus your 30% safety margin, to get out of a battery. That's going to need a very serious power supply. You will also not want that potential current anywhere near the grip.
The sensible option is to have the two stages wired to two 60A-80A peak packs and use a MOSFET for each, with only the control wiring shared. That way you only have signal current in the grip and need less enormous batteries. This method still assumes space for a proper pack or pair of packs in your project blaster.
All the ones I have seen were largely under specd on battery and  performance was meh. Eli gets great numbers from his brushless set ups, like 170fps, but at the expense of destroying flywheels almost every game.
Minky

I'm playing with the idea of a barrel attachment afterburner that can be fitted onto a 'fun' powered blaster to switch from kid friendly to adults only.

Won't having 2 packs and motor circuits mean I'll have two lots of switching gear for the Mosfets? Or maybe one and a double pole relay? I didn't think I could operate a Mosfet with it's Gate on a different pack to it's Source.
SSGT

Minky wrote:
I'm playing with the idea of a barrel attachment afterburner that can be fitted onto a 'fun' powered blaster to switch from kid friendly to adults only.

Won't having 2 packs and motor circuits mean I'll have two lots of switching gear for the Mosfets? Or maybe one and a double pole relay? I didn't think I could operate a Mosfet with it's Gate on a different pack to it's Source.


This is something I've been considering recently aswell. Whilst you can't use a FET to switch a completely isolated circuit you should be able to use a FET to switch a circuit with a common source. This gives you a choice - you can either mount the second FET inside the blaster and send power connections to the barrel attachment, meaning you'd probably need a pack connector to connect the two, or you can put the second FET in the barrel attachment and simply send it signal connections (gate and ground - you should be able to tie the -ve terminals of both packs together to a common ground even without connecting their +ve terminals together) which could easily be handled by a JST or an even lower current connector. A relay (single pole would be fine) would similarly work and likewise you'd have the choice of keeping the relay in the blaster with high-current connections to the barrel attachment or putting the relay in the barrel attachment with (relatively) low current connections between the two just to energise the coil.

Again the issue with a removable afterburner is alignment both of darts through wheels and with the barrels themselves. Not only will the distance between the cages be quite large in blasters with longer inner barrels you may also find that the barrel attachment isn't physically stable enough and may wobble about (definitely an issue with the Stryfe and RS and possibly with a few others).

You can switch multiple FETs with one switch as long as each FET has a resistor on it's gate to prevent oscillation or "ringing" between the capacitive gates (you should really have a series resistor with a single FET it just becomes more important when you run multiple FETs from a single output).

I just realised I missed you're other questions so I'll get to them here:

Minky wrote:
Would the 2nd set of motors have to be as Torque-y as we'd expect a primary set to be?

Ideally yes although it's not so much overall torque as it is speed a given load/torque. You could have a motor that spins at 70,000RPM but if it can barely maintain 5,000RPM under very little load then it wouldn't perform as well as a slower spinning motor that produces more torque (and so can maintain a speed closer to it's no load speed).
Minky wrote:
Would one fly back diode protect a mosfet from both FWC's back EMF?

Depends how you wire it. If you're treating the cages separately with different switches/FETs then you'll need a diode for each cage (although even if you ran them from a single/parallel set of FET(s), more motors in parallel = potentially more current to dissipate through that diode so you'd quite likely at least need a higher current diode anyway).
Minky wrote:
Do we need to spec a battery to handle the stall of both sets of motors (plus pusher if required)...?

Definitely yes. If you turn on all motors at the same time you'll get a sudden demand for (close to) stall current for each one. It's also definitely possible to get a stoppage where both cages and pusher are stalled (if you get a stoppage at the second stage the first stage and pusher will keep on happily firing darts into the back of it).
Minky

The blaster I was thinking on using this with would have a combined (5 motor) stall of under 70A on 3s (1xRhino, 2x Barricade, and 2x Honey Badger) and I am potentially going to step down the current into both FWCs to protect the motors a little so I think I can use a single pack for that, though out of curiosity could I wire two packs into parallel and stack the discharge ratings?

The DRS cage I bought for the Typhon came with a long guide 'barrel' that when I tested the other day seemed to cause a pile up in the barrel until it shot the guide barrel straight out of the faux barrel. This being the case I'm tempted to cut down this and as such will have me some ideally sized pipe that I would fit out of the connection end of the barrel attachment and link up with the first stage fwc.
SSGT

If possible I'd probably go with 3 Rhinos rather than a Rhino and 2 Barricades. Barricades draw more current and produce less torque (at loads greater than 30gf.cm or so they'll actually be spinning slower than a Rhino). I believe they also use stamped copper brushes as opposed to carbon - if you do use Barricades I'd at least recommend swapping out the brushes for the sets from a pair donor FC130s.

You can put packs in parallel to sum the capacity and discharge rating (it's not essential for both to have the same capacity/discharge rating but is highly recommended - if you don't you effectively limit yourself to the lowest discharge "C" rating) as long as you connect them when they are both at close to the same voltage and you monitor both of them individually. If possible a single, larger, higher capacity pack would be preferable as the cells would have been specifically matched to ensure comparable internal resistance.

How are you going about stepping down the current?
Minky

I was just going to use a diode or two. The afterburner set up is still a 'maybe' at this stage I need to build the main blaster but I want to spec it so that I can add it in at a later stage. As the blaster I have in mind will feature a barricade fwc I figured I may as well make use of the motors with there being a Rhino shortage on (though I think that's coming to an end soonish). I want 3s to allow more LEDs in series in the huge amount of LED circuits I need to make but I know running the barricade motors on unadulterated 3s will likely kill them quick so I thought I'd drop the voltage for them on the motor circuit same logic for the honey badgers. I'd wire them on the positive pre flyback diode.

If I were to do this would I have to think about dropping the Gate voltage on any Mosfets also?

EDIT: Sorry, mixing my terminologies. I meant Voltage not current in my previous post, but I'm thinking good ol Ohm says that if I'm dropping the voltage I'm also dropping the current.. So maybe I would of got away with it Smile
Northwind

This is actually something I've been thinking about again because some of the new, effective darts are relatively slow- the very best results I've had from waffle tips are about 15% lower than koosh in FPS terms, but they're more accurate/consistent.

So til recently I thought, meh, you can shoot 140-150fps out of a single stage with hellcats and the right cage and flywheels, why would you bother with an afterburner? And much (not all) of the interest in afterburners has been about chasing numbers not really practicality, t's not so much effective as just a cool deviation, like overclocking contests vs everyday PCs.

But I think maybe shooting waffle tips out of a nicely designed afterburner could be a very good, practical combo.

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