Motor Delay for PSII MDRM

I'm just getting into composite motors, and couldn't resist picking up a Mega Der Red Max. I'm building it stock.

Im planning to order a couple of G77/78/79 motors from the LMS series (no RMS cases yet, and I want to avoid HAZMAT fees). I know the bag card recommends 7 second delays for Gs, but that seems awful long for a big draggy bird with a projected altitude around 500'.

I'm considering using the delay tool to drill a 7 down to a 5 second delay when assembling the motor - is that a good idea? What have the rest of you experienced? I know the "right" answer is to sim it, but again being new, don't quite trust my OpenRocket skills yet.

Welcome to the forum!

The MDRM flies to more like 800-900 feet on a G77/78/79, so the 7 second delay should work, although 6 would deploy closer to apogee. Having not used the LMS motors I can't say if drilling the delay is allowed, but 6 seconds should be ideal.

When you are ready to L1 BTW, these fly great on a small H for a perfect flight to around 1300-1400 feet. I've flown mine on CTI H54s twice and launched it on an H133 Blue Streak last weekend for a nice fast flight.

I think a stock MDRM is going to go higher than 500 feet on those motors. It's big and draggy, but I still think its likely to go higher. That said, I think it's probably true that 7 seconds is a bit long. Mine is pretty heavily modified, so it's not a good guide for others. Maybe someone who has flown their stock version on these motors can advise you better.

ThirstyBarbarian said:

I think a stock MDRM is going to go higher than 500 feet on those motors. It's big and draggy, but I still think its likely to go higher. That said, I think it's probably true that 7 seconds is a bit long. Mine is pretty heavily modified, so it's not a good guide for others. Maybe someone who has flown their stock version on these motors can advise you better.

Click to expand...

I agree, 7 is a bit too long. F50-4 deploys right at apogee though for a nice low flight, and most G's deploy at apogee with a 6 second delay. One other note, these rockets tend to descend on their side rather than going ballistic, so if the delay is a bit too long the rocket may still recover safely. I saw someone fly one on a G78-10 and it just fell slowly on it's side for a few seconds before deploying safely. Not ideal, but probably okay with a 7 second delay.

One of the best things about the stock MDRM is how the drag and relative light weight for such a big bird make for such nice slow flights. You might want to add a G40 to your list for a long slow boost. And like SC mentioned, an H54 would be a great motor for the MDRM.

ThirstyBarbarian said:

One of the best things about the stock MDRM is how the drag and relative light weight for such a big bird make for such nice slow flights. You might want to add a G40 to your list for a long slow boost. And like SC mentioned, an H54 would be a great motor for the MDRM.

Click to expand...

I've seen a bunch of them fly on G40-4s and it's a very cool slow boost. I really want to try an H53 Mellow in mine for a really slow climb to ~2200 feet.

Thanks for the replies! @ SCrocketfan - based on the instructions, the LMS motors use the same delay grains as the RMS - in fact the instructions mention using the RDK "reload delay kits" to swap delays, so I would think they'd drill the same. Plus there was another thread on here where a user emailed Aerotech about drilling LMS and got an affirmative.

All the LMS Gs come with 4, 7, and 10 second delays. Sounds like 7 is the closest to right, 1-2 seconds less would be perfect?

SCrocketfan said:

I've seen a bunch of them fly on G40-4s and it's a very cool slow boost. I really want to try an H53 Mellow in mine for a really slow climb to ~2200 feet.

Click to expand...

They are glorious rockets for that kind of flight, slowly growling their way up! It's too bad the Pod Rocket is too heavy for these kinds of long burn motors. I do have a spare MDRM in my stash of unbuilt kits, and someday I'd like to build one closer to stock for these kinds of flights.

Oberon said:

Thanks for the replies! @ SCrocketfan - based on the instructions, the LMS motors use the same delay grains as the RMS - in fact the instructions mention using the RDK "reload delay kits" to swap delays, so I would think they'd drill the same. Plus there was another thread on here where a user emailed Aerotech about drilling LMS and got an affirmative.

All the LMS Gs come with 4, 7, and 10 second delays. Sounds like 7 is the closest to right, 1-2 seconds less would be perfect?

Click to expand...

Yes, definitely try drilling a 5 or 6 second delay. 5 is probably a bit early but 6 should be good.

ThirstyBarbarian said:

They are glorious rockets for that kind of flight, slowly growling their way up! It's too bad the Pod Rocket is too heavy for these kinds of long burn motors. I do have a spare MDRM in my stash of unbuilt kits, and someday I'd like to build one closer to stock for these kinds of flights.

Click to expand...

You might just be able to fly an H54 in the Pod Rocket... It's a moonburner (offset propellent core) so it has quite a bit of initial thrust (105 n). The G54 red (159 ns so almost as much impulse as the H54) has a bit more peak thrust (119 n), so that might work too. It really depends on velocity off the rail...

I'm trying to decide what to do with my other MDRM... I might just build it stock, and fly it on the same motors I fly in my current one.

Thrustcurve.org is another resource you can use to get motor advice. Allows you to enter a smaller subset of info about your rocket than the sim programs. Here is a list of motors for a Mega Max entered by Kevin Katz:

https://www.thrustcurve.org/motorguide.jsp?rocket=2617

You'll want to check the weight of your model against the one Kevin entered.

samb said:

Thrustcurve.org is another resource you can use to get motor advice. Allows you to enter a smaller subset of info about your rocket than the sim programs. Here is a list of motors for a Mega Max entered by Kevin Katz:

https://www.thrustcurve.org/motorguide.jsp?rocket=2617

Click to expand...

Interesting, he has the dry weight at 44 oz, mine is around 38-40 so it sims out about 100-150 feet higher on some motors. Everyone builds differently, so it's definitely worth weighing and adjusting the sims accordingly.

When flying mr MDRM on G motors (usually the G77 RMS), I use about a 5 s delay. Drilling the LMS motors is fine, 1/32" per second. I wish LMS motors were more widely available, much prefer them over SU for just this reason.

More often than not, I fly it on the H165, and H210 redlines. Recently tried the H182 DMS also.

I usually end up with a dilemma with single use Gs on rockets I design. Ends up being I can use a G38-4 because its lower impulse and slower burn compensating slightly for the short delay, or a G80-7 because it has the most thrust and impulse, but every other G wants right between 4 and 7 seconds.

The G80 is now also avail in DMS version, actually lower priced than the regular.

SCrocketfan said:

I've seen a bunch of them fly on G40-4s and it's a very cool slow boost. I really want to try an H53 Mellow in mine for a really slow climb to ~2200 feet.

Click to expand...

are you going to LDRS this year? there will be a MDRM slow drag race on H53's. the goal is to get like 30 of them up at the same time! I don't think you'll see 2200 feet though. mine sims to about 1600 if I remember correctly.
https://ldrs34.org/der-redmax/

RKeller said:

are you going to LDRS this year? there will be a MDRM slow drag race on H53's. the goal is to get like 30 of them up at the same time! I don't think you'll see 2200 feet though. mine sims to about 1600 if I remember correctly.
https://ldrs34.org/der-redmax/

Click to expand...

I'm in CA so I won't make it, but it sounds great.