Why so many case sizes

[m85476585]

I'm just wondering why multiple reload case sizes are necessary. The AT 29/40-120 case can take grains of different lengths (or two grains), but all other hardware that I know of requires a different size for each reload size group. Would it be possible to make one or two cases that can take a wide variety of reloads? For example, with 38mm motors there could be a "small" case that takes 1-3 grains and a "large" case that takes 4-6 grains. If there are not enough grains to fill the case, some kind of filler or plug could be used. Of course it would be a little heavier than individual cases, but that's a trade off for saving some money.

 

[Chrisn]

cheaper for extra aluminium tubing than all the other material and machining required such as a "plug" you propose is used, along with decreased performance/lengths of rockets etc.

 

[terryg]

I like having the different case lengths, rather then an all in one solution. For the 29-40/120 case I almost never use the smaller reloads which cost as much or more then the more popular and discounted larger reloads (best bang for the buck). I find it much cheaper to use the 24 mm reloads for smaller flights. Some of the smaller reloads are harder to light due to the extra spacers needed.
I am hoping that aerotech will add extra cases for the new 24-60 engine to give us options for 24-100 and 24-120.

 

[bobkrech]

I'm just wondering why multiple reload case sizes are necessary. The AT 29/40-120 case can take grains of different lengths (or two grains), but all other hardware that I know of requires a different size for each reload size group. Would it be possible to make one or two cases that can take a wide variety of reloads? For example, with 38mm motors there could be a "small" case that takes 1-3 grains and a "large" case that takes 4-6 grains. If there are not enough grains to fill the case, some kind of filler or plug could be used. Of course it would be a little heavier than individual cases, but that's a trade off for saving some money.

A critical concern with any rocket motor is the start-up time. You want your motor to come up to pressure quickly so that it smoothly and rapidly accelerates off the pad. This happens when the motor casing has a minimum dead volume.

While one could in principle load and fire a single grain reload in a 3 grain casing, you have to fully pressurize the casing before the motor develops peak thrust. The extra volume in the larger casing must be filled with gas by burning the propellant grain, so the time to full thrust is extended, sometimes significantly, and the motor performance suffers.

Additionally the thermal loading to the casing is greater because in the void sections, you do not have the thermal insulation of the unburned grain to prevent the hot gases from transferring heat to the casing.

And lastly, the extra casing length adds weight, reducing the lifting power of the reload.

Bob

 

[MarkH]

I like having the different case lengths, rather then an all in one solution. For the 29-40/120 case I almost never use the smaller reloads which cost as much or more then the more popular and discounted larger reloads (best bang for the buck). I find it much cheaper to use the 24 mm reloads for smaller flights. Some of the smaller reloads are harder to light due to the extra spacers needed.

Yep. The E16W for the 29-40/120 case is nearly impossible to light with a copperhead, and I almost always get a bonus delay with this load too. Add in the weight penalty of the 29 mm case, and the 24 mm E18W is always the better choice.

 

[MarkII]

I believe that Aerotech high power (and high power-style) RMS cases are closely matched to the reloads that are used in them, in order to obtain optimum performance. That is why you will only find at most just a small handful of reloads for any given case, and sometimes just one or two.

I suspect that Aerotech's hobby-line 24/40 and 29/40-120 were designed to take a wide variety of reloads in order to bring AP to the fliers of large low-power rockets and mid-power rockets while keeping the cost of entry down. Also, the issues that Bob mentioned are probably less of a problem with motors in those sizes, especially the 24/40. Furthermore, the hobby-line cases are relatively lightweight in construction, because the pressures that they are designed to contain are relatively low. Some unused dead space in them doesn't really add much weight. When you get into the larger motor sizes, though, the cases and closures get much beefier, and any unneeded length adds quite a bit of of dead weight to the motor, weight that the reload has to hoist.

While one could imagine a set of 29mm or even 38mm motors that were designed with the flexibility of the 29/40-120 motor, what would happen if the concept was extended to the 75mm and 98mm lines? I don't think that Aerotech would find too many buyers for a K reload that was intended for use in an M case. I don't think that I would want to be around when someone tried to fire that setup, either. And even in the 29mm and 38mm sizes, I can't see the concept working for the longer cases.

I mentioned this in another post not too long ago, but a reloadable motor (in any size) does not make economic sense because of the variety of different reloads that can be used in it; it makes economic sense because of the number of times that you actually launch it using the reloads that are designed for it. For example, the RMS 29/60 motor has two reloads that it can use. If you happen to have a couple of rockets that you like to fly, and they fly really well on F62's, then you will get a lot of use out of that case, and it will quickly pay for itself. If Aerotech "simplified" its 29mm RMS case line, and sold a 29/60-180 case, for example, then you would have to fly that F62 reload in a case that was only one third full of propellant. And you would be forced to use a motor that was more than twice as long as it needed to be (7.25" vs. 3.5"). If you still just wanted to fly F62's, then you would pay for (and have a motor mount designed for) a lot of motor case that you never use.

MarkII

 

[mikeyd]

Thats why in another thread, I asked if this could not be done with the CTI Pro 38 Motors, Basically you would have one reload case, with hollow "slugs" of which could be plastic, or phenolic. You would load the appropriate "slug", then insert the reload. Thus the reload still only Pressures up the same volume it would in a smaller/shorter case, as the "slug" would be loaded on top of the forward delay assembly. The only force on the "slug", would be lengthwise, not outward pressure.

 

[jderimig]

A common trick some EX'rs do is to use a PVC pipe (or aluminum) spacer between the snap ring and the forward closure to take up the space when using a case with 1 or more grains short. Works perfectly and a grain length or two of aluminum case has negligible weight for these types of projects.

 

[jimzcatz]

A critical concern with any rocket motor is the start-up time. You want your motor to come up to pressure quickly so that it smoothly and rapidly accelerates off the pad. This happens when the motor casing has a minimum dead volume.

While one could in principle load and fire a single grain reload in a 3 grain casing, you have to fully pressurize the casing before the motor develops peak thrust. The extra volume in the larger casing must be filled with gas by burning the propellant grain, so the time to full thrust is extended, sometimes significantly, and the motor performance suffers.

Additionally the thermal loading to the casing is greater because in the void sections, you do not have the thermal insulation of the unburned grain to prevent the hot gases from transferring heat to the casing.

And lastly, the extra casing length adds weight, reducing the lifting power of the reload.

Bob

Bob,this is not how I would do it. Several years ago Synerjet marketed a 38/720 that could also be loaded with a 480 or 600. Starting on the aft end and going forward,you had a snap ring,nozzle washer, nozzle, propellant and liner, forward bulkhead,aluminum spacer, and finally the last snap ring. You had no empty spaces to pressurize. I flew this several times until Wayne went Tango Uniform. It was a great design,and never misfired.

 

[m85476585]

It sounds like a spacer above the forward bulkhead is the way to go.

I always build my rockets to take the longest motor I might ever want to fly in them, so extra length wouldn't be a problem. And a few ounces of extra weight in a rocket that weighs a few pounds isn't a big deal unless you are doing some kind of competition, which I generally don't do.

I mentioned this in another post not too long ago, but a reloadable motor (in any size) does not make economic sense because of the variety of different reloads that can be used in it; it makes economic sense because of the number of times that you actually launch it using the reloads that are designed for it. For example, the RMS 29/60 motor has two reloads that it can use. If you happen to have a couple of rockets that you like to fly, and they fly really well on F62's, then you will get a lot of use out of that case, and it will quickly pay for itself. If Aerotech "simplified" its 29mm RMS case line, and sold a 29/60-180 case, for example, then you would have to fly that F62 reload in a case that was only one third full of propellant. And you would be forced to use a motor that was more than twice as long as it needed to be (7.25" vs. 3.5"). If you still just wanted to fly F62's, then you would pay for (and have a motor mount designed for) a lot of motor case that you never use.

The 29/40-120 is my most used case, with probably 20-30 flights on it. It has paid for itself many times over (compared to SU motors) because I can use it in a wide variety of situations and in most of my smaller rockets. If it's windy, I can fly an E or an F, and if it's a good day or I have a heavy rocket, I can fly a G. If there was a 38/120-720 case, I would probably have almost as many flights on that as I do on the 29/40-120, and I would have only spent a little more than the cost of the 720 case plus closures. Instead, I have 6 38mm cases and a set of closures. (It would probably be more practical to split the 38mm line into two cases, though, since a 6 grain case would be quite heavy for a 1 grain motor.)

 

[bobkrech]

Bob,this is not how I would do it. Several years ago Synerjet marketed a 38/720 that could also be loaded with a 480 or 600. Starting on the aft end and going forward,you had a snap ring,nozzle washer, nozzle, propellant and liner, forward bulkhead,aluminum spacer, and finally the last snap ring. You had no empty spaces to pressurize. I flew this several times until Wayne went Tango Uniform. It was a great design,and never misfired.

Jim

You can certainly plug a reload casing, however it is not efficient from a mass standpoint so why would you want to.

The density of aluminum (2.7 g/cc) is 60-70% higher than a propellant grain (1.6-1.7 g/cc). The mass of a 120 Ns propellant grain is about 60 g. where as an aluminum dummy grain (with bore) weighs 100 grams.

The loaded weight of a typical AT 38/720 motor is approximately 630 g. while the loaded weight of a typical 38/48 motor is 450 g. The propellant mass in a 720 Ns casing is approximately 360 g. and a conventional 480 Ns reload weighs 240 g. but if you fill in the void in a 720 Ns casing with aluminum spacers, the "propellant" mass increases to 440 g.

A 38/720 casing loaded to 480 Ns would weigh about 830 g compared to a 38/480 casing loaded to 480 Ns weighing 450 g. so by using dummy aluminum grain plugs you almost double the motor mass. To me this is not a good trade-off. You have greatly reduced the performance of the rocket and have added significant mass that will negatively influence flight performance and increase recovery stresses.

Bob

 

[Huff360]

CTI reloads do fine in longer cases than they were designed for. The particular test I saw was with a 1 grain reload in a 3 grain case. The ejection charge was removed, the case was packed very tightly with paper towels (yeah, paper towels) and the the reload was put in the case.

The assembly was fired and all was well. You do have to be sure that the front of the case is packed TIGHTLY as the packing has to hold the forward closure on the reload.

Disclaimer: This violates rules, laws, common sense, yadda yadda yaada... Don't try this at home, at the field, at the bar, yadda yadda yadda... I take no responsibility for any loss, damage, injury, or yanking of certification should you try this yadda yadda yadda...

 

[MarkII]

Somehow I doubt that you will ever see CTI endorsing that arrangement.

You removed the ejection charge? Great, so what happens if you want to use motor-initiated ejection?

The differences between CTI reloads and Aerotech reloads are too numerous to go into here. But to modify your experiment to make it even slightly resemble loading an AT 38/120 reload into a 38/360 or 480 case, try it again, only this time, snap off the delay and ejection module from the Pro38 1-grain reload, place it in the front end of the 3-grain case, pack paper towels in behind it, and then screw the 1-grain reload into the aft end of the case. How well do you think that will work? :shock:

I always build my rockets to take the longest motor I might ever want to fly in them, so extra length wouldn't be a problem.

So you'll put that 14" long motor tube into your Jackhammer so that it can hold your 38/480-720 motor, and...

MarkII

 

[m85476585]

So you'll put that 14" long motor tube into your Jackhammer so that it can hold your 38/480-720 motor, and...

MarkII

I assume you are talking about a Binder Design jackhammer.

I wouldn't put a longer motor tube in it, but I would make sure there is no bulkhead limiting motor length, and if a motor I wanted to fly in it was long enough to hit the bottom of the nose cone, or if there wasn't enough room for the chute, I would cut off the bottom plastic bulkhead and attach something farther up (Gorilla glue works great to attach stuff to the inside of plastic nose cones). In fact, I put the parachute in the nose cone of my Estes Big Daddy even though it only flies on stock 24mm motors because there is not enough room for a good amount of wadding in the stock kit. I built my PML Cirrus Dart with no coupler in the middle so I can fly it on a J570 (which would take it to 13000 feet) if I want. The coupler only served as an attachment point for the piston system, which I don't use in that rocket.

 

[MarkM]

You removed the ejection charge? Great, so what happens if you want to use motor-initiated ejection?

I've seen this happen...accidentally. A 5-grain reload was placed in a 6-grain casing. Obviously without all the packing, etc. The flight up was perfectly fine. However, since the ejection charge well was not sticking out the top of the casing due to the wrong reload-casing match, the result was a weak deployment of the recovery gear with mainly only the nose cone popping off. The parachute didn't exit the body tube so it didn't deploy, and the rocket came down hard although not ballistic. No damage to the casing, but the rocket had some, but not irrepairable, damage.