Motor Dimensions

[mtnmanak]

A while back, there was a thread in which I discussed an issue with short liners in some AT motors and recently I saw a thread about liners that are too long. I had posted this data somewhere earlier, but figured it may be useful to get it in one place.

If you are flying commercial motors, I do recommend you check your liner lengths.

The data to make the below tables was relatively easy to harvest from the manufacturers websites, but I have yet to find a place to get the CTI data easily. I could scrape the PDFs, but that is time consuming. Does anyone know if there is a CTI table somewhere?

 

[mtnmanak]

Aerotech:

MOTOR	Case Length (in)	Case Length (mm)	Case Length w/ closures (in)	Case Length w/ closures (mm)	Liner Length (in)	Liner Length (mm)	Grains
24/40	2.718 in	69.037 mm	3.598 in	91.389 mm
24/20-40	2.718 in	69.037 mm	2.843 in	72.212 mm
24/60	4.701 in	119.405 mm	5.581 in	141.757 mm
29/40-120	4.874 in	123.800 mm	5.889 in	149.581 mm
29/60	3.055 in	77.597 mm	3.864 in	98.146 mm
29/100	4.055 in	102.997 mm	4.864 in	123.546 mm
29/120	5.062 in	139.700 mm	5.871 in	149.123 mm	3.548 in	90.119 mm	2
29/180	6.812 in	173.025 mm	7.621 in	193.573 mm	5.298 in	134.569 mm	3
29/240	8.572 in	217.729 mm	9.381 in	238.277 mm	7.058 in	179.273 mm	4
29/360	12.291 in	312.191 mm	13.100 in	332.740 mm	10.780 in	273.812 mm	6
38/120	3.250 in	82.550 mm	4.198 in	106.629 mm	1.875 in	47.625 mm	1
38/240	5.125 in	130.175 mm	6.073 in	154.254 mm	3.750 in	95.250 mm	2
38/360	7.000 in	177.800 mm	7.948 in	201.879 mm	5.625 in	142.875 mm	3
38/480	8.875 in	225.425 mm	9.823 in	249.504 mm	7.500 in	190.500 mm	4
38/600	10.750 in	273.050 mm	11.698 in	297.129 mm	9.375 in	238.125 mm	5
38/720	12.625 in	320.675 mm	13.573 in	344.754 mm	11.250 in	285.750 mm	6
38/1080	18.250 in	463.550 mm	19.198 in	487.629 mm	16.875 in	428.625 mm	9
54/426	5.000 in	127.000 mm	6.042 in	153.467 mm	3.620 in	91.948 mm	1
54/852	8.448 in	211.328 mm	9.362 in	237.795 mm	6.950 in	176.530 mm	2
54/1280	11.758 in	295.910 mm	12.692 in	322.377 mm	10.280 in	261.112 mm	3
54/1706	15.105 in	380.492 mm	16.022 in	406.959 mm	13.610 in	345.694 mm	4
54/2560	21.765 in	549.148 mm	22.662 in	575.615 mm	20.270 in	514.858 mm	5
54-2800	23.579 in	598.907 mm	24.621 in	625.373 mm	22.084 in	560.934 mm	6
75/1280	7.800 in	198.120 mm	10.089 in	256.261 mm	5.750 in	146.050 mm	1
75/2560	13.100 in	332.740 mm	15.389 in	390.881 mm	11.062 in	280.975 mm	2
75/3840	18.420 in	467.868 mm	20.709 in	526.009 mm	16.375 in	415.925 mm	3
75/5120	23.720 in	602.488 mm	26.009 in	660.629 mm	21.688 in	550.875 mm	4
75/6400	29.040 in	737.616 mm	31.329 in	795.757 mm	27.000 in	685.800 mm	5
75/7680	34.350 in	872.490 mm	36.639 in	930.631 mm	32.312 in	820.725 mm	6
98/2560	9.750 in	247.650 mm	11.913 in	302.590 mm	6.625 in	168.275 mm	1
98/5120	15.813 in	401.650 mm	17.976 in	456.590 mm	12.688 in	322.275 mm	2
98/7680	21.875 in	555.625 mm	24.038 in	610.565 mm	18.750 in	476.250 mm	3
98/10240	27.938 in	709.625 mm	30.101 in	764.565 mm	24.813 in	630.250 mm	4
98/15360	40.063 in	1017.600 mm	42.226 in	1072.540 mm	36.938 in	938.225 mm	6
98/20480	52.178 in	1325.321 mm	53.841 in	1367.561 mm	49.053 in	1245.946 mm	7

 

[mtnmanak]

Loki:

Motor Diameter	Case Length	Case Wall	Aprox. Total Impulse	Liner Length	Max Propellant Length	ID of Casting Tube	Commercial Nozzle throat	Suggested EX Nozzle Throat	Typical Bates Grain Geometry
29 mm	5.500 in	0.058 in	120 Ns	3.500 in	3.250 in			#13	Two 1-5/8" long grains with 3/8" cores
29 mm	7.000 in	0.058 in	180 Ns	5.120 in	4.880 in			#15	Three 1-5/8" long grains with 3/8" cores
29 mm	8.600 in	0.058 in	240 Ns	6.750 in	6.350 in			#18	Four 1-5/8" long grains with 3/8" cores
29 mm	11.800 in	0.058 in	360 Ns	10.000 in	9.750 in			#22	Six 1-5/8" long grains, four with 3/8" cores and two with 7/16" cores
29 mm	15.100 in	0.058 in	460 Ns	13.250 in	13.000 in			#25	Six 1-5/8" long grains, four with 3/8" cores and two with 7/16" cores, plus one 3-1/4" grain with a 7/16" core
38 mm	7.000 in	0.083 in	240 Ns	5.063 in	4.600 in	1.212 in	#16	#16	Two 2.25" long grains with 1/2" cores
38 mm	11.500 in	0.083 in	480 Ns	9.563 in	9.100 in	1.212 in	#19	#22	Four 2.25" long grains with 1/2" cores
38 mm	16.000 in	0.083 in	740 Ns	14.063 in	13.600 in	1.212 in	#22	#28	Two 2.25" long grains with 9/16" cores and four 2.25" long grains with 1/2" cores
38 mm	24.625 in	0.083 in	1200 Ns	22.625 in	21.750 in	1.212 in	#28	#28	Six 3.7" long grains with 1/2" cores. (Hold on tight)
54 mm	9.850 in	0.094 in	950 Ns	7.375 in	6.800 in	1.755 in	#19	#24	Three 2.265" long grains with 1/2" cores
54 mm	12.875 in	0.094 in	1200 Ns	10.375 in	9.810 in	1.755 in	#24	#29	Three 3.25" long grains with 5/8" cores
54 mm	15.875 in	0.094 in	1600 Ns	13.375 in	12.810 in	1.755 in	#26	#33	Four 3.2" long grains with 5/8" cores
54 mm	19.375 in	0.094 in	2000 Ns	16.875 in	16.250 in	1.755 in	#29	#36	Four 3.25" long grains with 5/8" cores and one 3.25" long grain with 3/4" core
54 mm	28.625 in (Gen 1)​	0.094 in	2800 Ns	26.05 (Std)	25.48 (Std.)	1.755 in	#42	#44	Six 4" long grains with 13/16" cores.
54 mm	28.525 in (Gen 2)​	0.094 in	2800 Ns	27.13 (Ext.)	26.68 (Ext.)	1.755 in	Single Use
54 mm	43.850 in (Gen 1)​	0.094 in	4000 Ns	42.375 in	41.920 in	1.755 in	Single Use		Gen2 - Snap ring end margins are 0.200" - Distance from front of thrust ring to back of case is now 0.425". This is now the same as all other current 54mm Loki Research cases.
54 mm	43.750 in (Gen 2)​	0.094 in	4000 Ns	42.375 in	41.920 in	1.755 in	Single Use
76 mm	14.500 in	0.116 in	2400 Ns	11.490 in	10.700 in	2.551 in	#28	#32	Two 5.34" long grains with 3/4" cores
76 mm	19.625 in	0.116 in	3600 Ns	16.937 in	15.750 in	2.551 in	#40	#40	Three 5.25" long grains with 3/4" cores
76 mm	25.000 in	0.116 in	4800 Ns	22.300 in	21.200 in	2.551 in		#48	Four 5" long grains with 7/8" cores
76 mm	30.875 in	0.116 in	6000 Ns	27.187 in	26.050 in	2.551 in	#52	#52	Four 5" long grains with 7/8" cores and one 5" long grain with a 1" core
76 mm	40.875 in	0.116 in	8000 Ns	37.187 in	36.050 in	2.551 in	#56	#60	Five 6" long grains with 1" cores and one 6" long grain with a 1-1/8" core

 

[Handeman]

The good thing about Loki and CTI is they use o-rings that seal radially between the closure and case so if the liners are short by a couple hundredths or more, it doesn't really matter much. You just end up with a little less liner on the nozzle and fwd closure shoulders.

With AT, since they use compression of the o-rings when the fwd and aft closures are closed to seal the motor, a short liner can easily cause too little o-ring compression and a CATO. I don't know what the tolerances are for their liners.

I've been flying AT reloads for 21 years and I have never had an issue with a too short liner. My small sample size of several hundred motors is probably not significant, but I don't see liner length as being a significant issue with AT motors. I don't intend to bother checking liner length on AT reloads unless I can close the aft closure without feeling the o-ring compression during the last couple of turns as it closes.

YMMV

 

[AeroTech]

The good thing about Loki and CTI is they use o-rings that seal radially between the closure and case so if the liners are short by a couple hundredths or more, it doesn't really matter much. You just end up with a little less liner on the nozzle and fwd closure shoulders.

With AT, since they use compression of the o-rings when the fwd and aft closures are closed to seal the motor, a short liner can easily cause too little o-ring compression and a CATO. I don't know what the tolerances are for their liners.

I've been flying AT reloads for 21 years and I have never had an issue with a too short liner. My small sample size of several hundred motors is probably not significant, but I don't see liner length as being a significant issue with AT motors. I don't intend to bother checking liner length on AT reloads unless I can close the aft closure without feeling the o-ring compression during the last couple of turns as it closes.

YMMV

Actually the forward and aft O-rings on the 75 & 98mm reloads are sealed radially too. The problem is if the liner contracts in length during operation or if it is cut too short, the seal disk can be pushed out of the liner and leakage can occur between the liner and the case.

 

[mtnmanak]

It is also helpful to know these dimensions if you need to cut your own liners. For EX, this is inherently obvious, but sometimes, things happen and you need to cut your own liners, even for AT. Over the winter, I did an inventory of all my motors on-hand and found that, through numerous moves and reorganizations, etc, I was missing the liners to four of my AT motors. It was more economical to buy full sticks of the liners from RCS than to get a bunch of cut pieces (in the sense that the full sticks gave me some spare liner material to use in the event I needed them and from a shipping cost aspect) and cut the pieces off I needed for the specific motors. I have the charts above printed out and stashed in my motor range box and on the bench I normally use to work on my motors in my shop. The chart was very convenient when cutting the liners.

As for the possibility of short liners, it is rare, but it can happen. This run of short liners (https://www.rocketryforum.com/threads/aerotech-open-thread.125657/post-2162578) did cause a CATO on one of my rockets (which was covered by @AeroTech, thank you!) and Karl ensured that the other affected liners I had were taken care of. Since then, I do check my liners. It takes less than 5 seconds to check the length of a liner. For me, that is time well spent.

For snap ring cases, I often cut liners to fit, and the charts are convenient for that as well.