Engine Class Question

[Tweener]

I have always understood that the letter classification meant that the total impulse was twice the class before it for each class. Looking at the Estes engine chart, it seems that their engines follow that progression all the way through the D class (20 N-sec), but the E class seem as though they should actually be 3/4 E (30 N-sec). Looking at the Aerotech specs, most of their E's seem to be correct (40 N-sec), but the higher classes F and up seem to vary considerably from this progression. Anyone know what the standard for the letter class really is? :confused:

[stefanj]

Lettered classes are ranges.

In recent memory, Estes and Quest and so-on's A - D motors have had "claimed"* total impulses near the top of their ranges, so you could conclude that an E "should" have a total impulse of 40, but it doesn't work that way.

Once you venture above C, all sorts of other factors pitch in that result in barely-F motors, respectable-but-not-full G motors, etc.:

The E9's 30ns is just about as much as you can safely put in a 24 mm x 3.75" casing when you're working with BP.

Composite fuel lets you barely cram a 50 ns "F" in a 24 mm x 2.75" reload casing. It's a nice trick, really.

Legal limits on propellent mass limit commercially available G motors to 120ns.

Stefan

* Estes motors actually run a bit short. D12 are rated by the NAR at 18ns, C6 9ns, etc.

[Green Dragon]

the letters reference the next higher level can be UP TO twice the power, so given an 'A' is 2.5 nsec, then a B can be up to 5.0, C up to 10 Nsec, etc.

this isn't much of an issue in 'smaller' motors,until you get to G ( and some E-f in recent years ), G motros, due to the 62.5 gram propellant weight limit without being 'regulated' , are made at the legal limit of 62.5 grams prop, resulting in aprox 120 nsec.

the class 'range' is a real issue more with larger motros, as there's a LOT of difference in a 2700 n-sec "L" versus a 5000 N-sec 'L' ........

~ AL

[Tweener]

Thanks, guys. I've been developing an interest in mid-power lately, and that's how I came to notice the variation in the E-G (and higher) ranges. I never really thought about it when I was a kid, just stick a recommended engine and ignitor in it and send it off. But I'm starting to REALLY understand why something simple is referred to as "not rocket science" now that I'm starting to dig into it. :D

[Royatl]

Legal limits on propellent mass limit commercially available G motors to 120ns.

Stefan




And recent changes to the design of the single use Aerotech motors have led to them making 100ns motors that were formerly 120ns (G80 and G40). Same for the F25 and F50 (now 60-70, were 80)

[Green Dragon]

And recent changes to the design of the single use Aerotech motors have led to them making 100ns motors that were formerly 120ns (G80 and G40). Same for the F25 and F50 (now 60-70, were 80)

hmm. they de-rated the impulse, AND prices went up 60% ?

does not sound good to me . :-(

had not heard they were cut back that much - is it lower impulse propellant, or an actual design (grain ?) issue ?

~ AL

( will stick to pouring my own if this continues )

[Royatl]

hmm. they de-rated the impulse, AND prices went up 60% ?

does not sound good to me . :-(

had not heard they were cut back that much - is it lower impulse propellant, or an actual design (grain ?) issue ?

~ AL

( will stick to pouring my own if this continues )

Design. They switched from phenolic casings to one piece plastic casings. I think these casings are also going to be used for the "limited-use" reloadables, so they are probably a little thicker and therefore use less propellant. Gary gave me one to play with during the NARAM barbecue, but I didn't think to study it too closely! Anyway, I checked, and the F25 is 73ns, F50 is 70ns, and the two G's are 100ns. The single-use E motors have been in the plastic casings for almost 15 years now and so apparently won't change (except maybe for their delay housings)