AT 7680 Motors: 98mm vs 75mm with adapter and RAS?

[75Grandville]

Looks like my L3 kit from Wildman - Ultimate Darkstar - is going to be delivered today. <happy dance>

Next comes the selection of motors. According to preliminary sims, a DMS M won't have enough speed when it clears the rail, so going with RMS. That means I need to invest in new motor hardware, as the largest I have at the moment is a 54/1760.

I've been debating a 98mm 7680, with something like an M2100G, vs a 75mm 7680, with an M1500G using an RAS and an adapter. Total cost is about $1000 either way, assuming 20-25% discount on the hardware/reload, so it's not about initial outlay.

The advantages of the 98/7680 are clear, as there are lots of reloads that would make for some great flights. The downside is that I will have (to start with) exactly one rocket that I can fly those reloads in, and it's going to cost $4-600 per flight, depending on reload and whether I can preorder.

The advantages of the 75/7680 with the RAS are that I can fly any reload for 75/7680, 6400, and 5120. That also doubles the number of rockets I can use the hardware in initially. Good selection of reloads, but fewer that I can fly in the Ultimate Darkstar.

So, I thought I'd see what other people think. Although it's _so_ hard to get people's opinions from teh interwebs

In addition to your response to the poll, if you could expound a bit on your reasoning/experience, that would truly be appreciated.

 

[75Grandville]

One other helpful piece of information - UROC generally flies with a 10,000' AGL waiver. One launch this year at a new site with 100,000' waiver, but it's new to the club, so expecting recovery to be more challenging. That's a factor as well.

AT is what the club vendor sells, and is what I've got for 18mm through 54mm already. Not looking to switch to CTI or Loki. Yes I know that AT reloads are cross-certified for CTI cases, still not interested. Nothing against CTI - I got my L1 with a CTI reload. Just want to remain consistent.

 

[COrocket]

I voted for the 75 MM, the RAS is awesome for maximizing the amount of motors you can fly with a single hardware set. That said, based on altitude sims and budget, are you more likely to keep flying in the 5120-7680 range, or want to someday try the 10240-15360 reloads someday?

If you want to stay on the smaller end of the spectrum, the 75MM is nice because for an equivalent impulse, the 75MM 4 & 6 grain motors have higher average thrust than their 98MM 2 & 3 grain equivalents, which is good for a heavy rocket. Another nice thing is the 75MM reload MSRP is $43 cheaper in the 5120 case and $88 cheaper in the 7680 case, so there's some recurring cost savings as well.

If you think you want to fly 10240 or 15360 reloads someday, the 98MM set would be nice so you only have to buy a longer case in the future instead of an entire hardware set.

 

[n3tjm]

I would personally get the 10240 myself over the 7680

 

[mtnmanak]

I have at least one of every case Aerotech makes, but, for rockets in that weight class on fields with a 10K waiver, I keep going back to the M2050X. Just an awesome motor. I have flown my Ultimate DS on that motor a bunch of times and it has given me great flights every time. Scoots off the pad in a hurry, should give you about an 8 or 9:1 T:W and only goes up to about 6000 feet. I wish they made more motors with Propellant X.

 

[cbrarick]

Uggh, 75 mm would be more flexible because of the RAS.
Money? Well, if you're flying L3 it's never cheap, so I'd toss that out.
the real question is "do you have a favorite reload?" if so, then that's the correct hardware.

 

[75Grandville]

Uggh, 75 mm would be more flexible because of the RAS.
Money? Well, if you're flying L3 it's never cheap, so I'd toss that out.
the real question is "do you have a favorite reload?" if so, then that's the correct hardware.

I don't have a favorite reload (yet) for this level of flying. I do have some favorites (I357, J500, J315, K805), but I've had to do some flying to get there for L3 motors.

And I hear you on the $ - I've waited years until I was sure I had the $2-3,000 I figure I'm gong to need to step up my game. Part of it is just pure economics - would I rather have two flights on an M1500G, or one on an M2100G? Same cost for reloads. And it's one that I don't have a good answer for yet.

All this may very well be moot if I can get an acceptable velocity off the rod with a DMS. Right now it seems to vary according to my assumptions, so I need to go home and start weighing up components to make my simulations more accurate.

 

[jimzcatz]

I see no problem using the M1350DMS for this project but personally I would go with the 75/7680. Just my preference.

 

[Handeman]

I went with the Loki 76-6000 case and a M-1882-LW reload for my L3 cert. It was $630 back then, $770 now.

I did that because one of our club members had been mixing his own motor for 20 years and I intended to fly my own EX motors after the initial cert. I have 7 flights on that case, and only the first cert flight was a commercial load. The rest run me about $75 per motor for baby M EX motors that work really well at our field.

Just a consideration.