Level 1 attempt motor choice - here are my options

So finally (finally) getting around to attempting my L1 in October with a PML 1/4 Patriot. Overbuilt to the max as set it up for dual deploy when I feel the need to.

Here are the options I've narrowed the attempt down to. I've got a Pro 38 3G case. SOrry couldn't post screenshot, don't have internet in new house yet so had to take a photo and bring it to work.



My thoughts: H125 looks low and slow and safe. But I'm thinking too slow? Especially if a windy day. On the other side of the spectrum, I170 is a lot higher but what if my rocket falls apart at that speed (but no I'm just worried, I don't think it really will fall apart, I just want the safest L1 option).

Also, my ground hit velocity - is it too high? Might need to upgrade chute

Thanks!

With an H125, you'll have an average thrust to weight ratio that sits just under 5. You would also need a grain spacer since that's a 2G motor. Generally, a T/W ratio over 5 is recommended, which the I170 would achieve, but for a cert flight, I prefer to aim toward the lower end of the spectrum (ie - an H motor). If you have a CTI grain spacer, the H143, H152, H225, and H400 are all suitable. Looking at the speeds, I don't think you should be too worried unless you forgot to glue something in. If you epoxied your fins in and used decent fillets, there's no reason the rocket shouldn't hold up. You're well below Mach on all configs and nearly in the incompressible flow regime (that's a good thing), too.

jmattingly13 said:

With an H125, you'll have an average thrust to weight ratio that sits just under 5. You would also need a grain spacer since that's a 2G motor. Generally, a T/W ratio over 5 is recommended, which the I170 would achieve, but for a cert flight, I prefer to aim toward the lower end of the spectrum (ie - an H motor). If you have a CTI grain spacer, the H143, H152, H225, and H400 are all suitable. Looking at the speeds, I don't think you should be too worried unless you forgot to glue something in. If you epoxied your fins in and used decent fillets, there's no reason the rocket shouldn't hold up. You're well below Mach on all configs and nearly in the incompressible flow regime (that's a good thing), too.

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Thanks. Yes I have a space for 2G.

Brb while I Google incompressible flow regime...

Can you do a cert special and get a free case ?

If so you can do the ideal motor for your rocket ...adding a grain may get your desired profile with a bit of extra umph .

If so 5G i216 or if the vendor will let you pay the difference and get the 6g case as you already have a spacer .

If sticking to 3G case you may want to get that heavier rocket moving with a I345 White Thunder...low and slow is nice, but some punch can bring the shock and awe.

Kenny

ArthurSull said:

Also, my ground hit velocity - is it too high? Might need to upgrade chute

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Since this rocket is set up for dual-deployment, a larger main 'chute makes sense in general. For your cert flight it sounds like you're going to use motor ejection so it might drift a bit more, but if you fly low anyway I think it makes sense to go with a larger 'chute.

Thanks.

Kenny I live in the UK and have never seen a cert special advertised.

Will stick with 3G for now anyway as all I can justify spending currently. Just simmed an I345:



I'm seeing a Max 16 sec delay but on open rocket it's telling me 15 second. Went with 16 and put delay down to 11 - is 16mph too high a velocity at deployment?

John, whilst it's DD capable - for cert will just being single chute motor deployment. Got the stock 48in chute but 15mph seems awfully fast. Fins are steadfast and filleted with Fix It clay however.

I'm following thus thread in earnest... I'm currently building my Madcow Super Batray for my level 1 and have been pouring over the motors specs all day. Still can't decide. I'd love to cert on a skidmark...

Trust the thrust if an I-170 is to much something is wrong

The question really comes down to what is your flying field like? How high are you allowed to go and how large is the recovery area? What speeds are the normal winds at?

If you can fly to 2000 ft with motor deploy and land well within the field, by all means, go with the Pro38 3G I motors. I prefer the AT White Lightning so I would recommend you do the I345 White Thunder.

I was going ask the same about field size. What I will say is that my 4" Super DX3 in DD mode has virtually the same mass as the OP's Pat (DX3 is a bit longer though and has 3 fins). An I170 would be my choice field permitting - the H's I have flown it on have all been underpowered IMO.

ArthurSull said:

Brb while I Google incompressible flow regime...

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Basically below Mach 0.3. Your drag coefficient is unaffected by Mach number, which is nice because it keeps your drag forces down, reducing stress on various rocket appendages.

ArthurSull said:

I'm seeing a Max 16 sec delay but on open rocket it's telling me 15 second. Went with 16 and put delay down to 11 - is 16mph too high a velocity at deployment?

John, whilst it's DD capable - for cert will just being single chute motor deployment. Got the stock 48in chute but 15mph seems awfully fast. Fins are steadfast and filleted with Fix It clay however.

Click to expand...

Your deploy velocity should be fine. If you're really worried about it, I'm pretty sure there are delay grain adjustment tools for CTI motors (I've got the 54mm one. Can anybody confirm that a 38mm one exists?). Basically drills out a small layer of the delay grain, which can also be accomplished with your everyday drill and drill bit (and a bit of precision).

I like to shoot for touchdown speeds of 15-20 ft/s. You're coming in about 22 ft/s tops, which shouldn't be a huge deal, especially if you say you've beefed up the rocket. A 48" chute already seems kinda big for this rocket (I launched a slightly lighter, but same dimensions Patriot with a 42"), so I don't think I'd recommend going too much bigger.