Someone from Aerotech can correct me if I an wrong.
24/60 was created to fit the new Estes E9 sized models but provide 2.5 times the total impulse.
The F35 has more total impulse than the F39. Look at the actual test data.
The 24/60 has thicker walls and stronger threads so that it can withstand greater pressure for different reloads and higher efficiency.
Additional reloads were stalled by a number of factors, including the California State Fire Marshal having to enforce an old law that prohibited metal casings and handling of ingredients for "Model Rocket Motors". This was the result of an enforcement action against someone who then said something to the effect of: if you are going to enforce law X on me, then you need to enforce law Y on everyone else. This was in approx. Oct 2010.
As a result any new reload was Classified as a High Power Rocket Motor and couold only be launched at high Power Rocket launches with the appropriate permits and licensed Pyrotechnicians present. Insurance would be VOID if launched at a Model Rocket launch since the permit would not cover HPR.
With the help of many, I was able to get that law changed. That took effect Jan 1, 2016.
With the law changed, Aerotech was able to resume development of additional RMS reloads for this and other casings. They were going to have additional 24/60 reloads years ago, but that got derailed by the law situation in the large CA market.
(Cesaroni was also affected and all their motors are Classified as HPR in California. They were going to work on new Classifications in 2016 but the accident derailed that. They will likely resume pursuing Reclassification soon).
The big delay is NAR S&T. There is no official word from the NAR as to WHY there is a delay. If it is NAR testing facilities, it would be nice to let everyone know. If it is a paperwork submittal situation from the manufacturer, then they shold know and tell us (and not blame NAR). I have no firm idea what is the root cause of the delay, but it is annoying for TARC. The 24/60 is a great TARC motor casing - especially with more motor options (someday).
I'm trying to find a compelling reason for that case to exist. First, the case only has 3 reloads available, all of them being F-35W which come in a 2 pack. The only difference in all three is the delay time. Second, the 24/40 case has 2 slightly more powerful motors available which come in a 3 pack for only a few dollars more. Its like getting a full reload for around $3 if you go the 24/40 F39 route. I've compared the thrust characteristics of both options and they are similar. The main appreciable difference is that the 24/60 uses the White Lightning and the F39s in the 24/40 are Blue Thunder. What else am I missing? I'm new at all of this so there has got to be a reason for its existence that I'm not seeing.
Thanks.