First self built rocket motor question (ARC team)

[SailBag]

We are a first year ARC team. We designed a rocket with a BT-70 top and BT-80 bottom section that is 29 in long. We estimated with open rocket that our design will be 305 g without motor (but with egg and payload) and using Openrocket are surprised how big of a motor we need. Two questions:
It seems really tough to find a motor that gets us really close to our desired altitude of 820 ft; I guess we'll need a bigger one and add weight?
To do that we seem to need an F size motor, which is the largest the competition allows. We thought we were making efficient design choices with the rocket and are surprised that we need the largest motor. Is this expected? Is our rocket heavy or poorly designed? Could we be doing something wrong in OpenRocket?

 

[mhanna]

I worked with 3 schools last year having a total of 8 teams involved. All of them ended up requiring "F" motors to get to the required altitude. Your calculations are probably correct.

 

[boatgeek]

It’s hard to go less than F power when the rocket is BT80 diameter. Most teams plan to add ballast to dial in the altitude. That also allows you to adjust altitude for the flights at nationals.

Note that your actual rocket will likely be a little heavier than OR estimates. You may also want to build with heavier but more durable materials like plywood fins. You’ll be doing at least 5-10 times and your life will be easier if you don’t have bits breaking off.

 

[neil_w]

Does any ARC team (it still takes conscious effort for me to leave off the leading "T") ever fly on less than an F? I would bet not. F is the norm.

 

[Art Upton]

Until you build it, you only have an estimate of the weight it will be. F is normal

 

[samb]

First time teams tend to spend too much time in the virtual design world. I think you should be flight testing a prototype and collecting real world data now. Stockpiling motors should also be a priority IMO. Good luck!

 

[WillMarchant]

First time teams tend to spend too much time in the virtual design world. I think you should be flight testing a prototype and collecting real world data now. Stock piling motors should also be a priority IMO. Good luck!

And you'll be acquiring useful building and operating experience at the same time. Remember that your team has multiple members and that you should think about doing things in parallel instead of just linearly.

 

[kjohnson]

I saw 2 teams fly ARC flights on E30 motors this month. One flew to 825ft but was long on time. Ready to fly mass was 341g. You can do this challenge on less than F power, just need to be very careful with your mass budget.

kj

 

[AtlPhilip]

Remember that motor classification by letter is a bit crude. F runs from 40 – 80 N-S Chose your thrust curve and total impulse accordingly.

I threw together a rough build based on what you said above. An Aerotech F62 gives a nice combination of rod velocity (55ft/sec) and Apogee (885ft).
The largest E (E28 @ 40N-S) only achieves 765ft If you can get the weight down to under 280gm, it should work.

And I would skip the transition unless it's required.

 

[samb]

@AtlPhilip, This year's challenge specifies an airframe made up of two different diameter tubes, implying a transition.

"The rocket must use body tubes of two different diameters, each no less than 150 millimeters long, with the smaller diameter tube on top. The upper tube must have a dimeter of no more than 57 millimeters and the lower one a diameter of no less than 63.5 millimeters. All parts of the rocket must return to earth connected/tethered together, under parachute. "

https://rocketcontest.org/faq/

 

[AtlPhilip]

The upper tube must have a dimeter of no more than 57 millimeters

That's going to force some interesting solutions. A typical egg of the mass required is pretty close to 57mm. That leaves precious little room for cushion. And pushing it lower into the BT complicates recovery and CG/CP relationship.

 

[neil_w]

I'm not sure there's any real reason to even *try* to squeeze into an E, other than the motors are slightly cheaper. Using an F gives you a lot of flexibility in motor selection and adjusting the mass of the rocket.

 

[boatgeek]

That's going to force some interesting solutions. A typical egg of the mass required is pretty close to 57mm. That leaves precious little room for cushion. And pushing it lower into the BT complicates recovery and CG/CP relationship.

I hope to see someone out with a normal-ish BT-80 rocket with a little BT-5 spike out the top, but I'm probably going to be disappointed. You can fit an egg into BT-70 tubing, but it's close. The team I mentor just had a lawn dart due to the complicated recovery issues. So a redesign is in progress...

Also worth noting that if you're doing reloads, the 24/40 case fits a pretty wide range of motors (E28, F24, F39) and there are several reasonable single-use motors (F30, F32, F41, F44), again with a fairly wide impulse range.

 

[Art Upton]

Seems correct that the Estes Pro Series Designer Parts kit was designed for this contest; it has the parts.

 

[Art Upton]

You can fit an egg into BT-70 tubing, but it's close. The team I mentor just had a lawn dart due to the complicated recovery issues. So a redesign is in progress...

If an Egg in Ziplock is a hard fit, put the egg in plastic wrap and tie one end off so it can't unravel.

What was the complicated Recovery issue?

 

[boatgeek]

If an Egg in Ziplock is a hard fit, put the egg in plastic wrap and tie one end off so it can't unravel.

What was the complicated Recovery issue?

Let's say that the rocket stack looks like:
Small section smaller than BT-70
Transition
BT-80 with egg
Coupler
BT-80 fin can

You can end up with some challenges with getting the recovery harness connections past the egg and also having everything connected up top. Our team chose a solution that in retrospect was probably too complicated, leading to (probably) too sticky a connection in flight and no separation when the ejection charge fired.

 

[AtlPhilip]

Also worth noting that if you're doing reloads, the 24/40 case fits a pretty wide range of motors (E28, F24, F39) and there are several reasonable single-use motors (F30, F32, F41, F44), again with a fairly wide impulse range.

I'll give you three guesses what I have the forward and aft closures, plus a 4 or 5 reloads, for. But the casing itself is absolutely nowhere to be found.

 

[mhanna]

There are a fair number of Cesaroni motors in that range also. 24mm & 29mm. Availability is sometime difficult. The 3 teams I work with all prefer Cesaroni due to ease of use and adjusting delays.

 

[WillMarchant]

I'll give you three guesses what I have the forward and aft closures, plus a 4 or 5 reloads, for. But the casing itself is absolutely nowhere to be found.

https://performancehobbies.com/secure/store.aspx?groupid=1302

 

[boatgeek]

There are a fair number of Cesaroni motors in that range also. 24mm & 29mm. Availability is sometime difficult. The 3 teams I work with all prefer Cesaroni due to ease of use and adjusting delays.

We use Cesaroni too, but they may be a harder sell for a first time team because of availability and hazmat charges. The big draw for us is that we can do most of the motor prep in the nice warm classroom before heading out to the field. There's less chance of CATO on the 24mm and 29mm motors because there's less fiddling around with O-rings and the like.

 

[Tractionengines]

I'd cut the end off the transition and store the egg in the lower BT-80 area... but that's just a thought, as a starting point for "rapid testing" to dial in a design.

The Estes kit that seems geared toward this challenge; has a transition design, and included plywood bulkheads, that would make this quick to test.

 

[dhbarr]

My boys used e30's for FAI S2P but the mass budget was pretty tight. F42T, F52C, F67W had a lot more oomph to get off the rail, and they're all single-use.

 

[bjphoenix]

We had 5 ARC teams at our club launch 4 days ago. 3 of the teams used F42-8. One team used E35.
The fourth team used F23-7, F27-8, F32-4 and F44-8. It always amazes me how fast the F42 gets those rockets off the pad.

 

[boatgeek]

We had 5 ARC teams at our club launch 4 days ago. 3 of the teams used F42-8. One team used E35.
The fourth team used F23-7, F27-8, F32-4 and F44-8. It always amazes me how fast the F42 gets those rockets off the pad.

My team builds a little heavy for durability reasons. We really love the F70WT for a quick leap into the air. It’s a nice impulse range for the competition too.

 

[SailBag]

It’s hard to go less than F power when the rocket is BT80 diameter. Most teams plan to add ballast to dial in the altitude. That also allows you to adjust altitude for the flights at nationals.

Note that your actual rocket will likely be a little heavier than OR estimates. You may also want to build with heavier but more durable materials like plywood fins. You’ll be doing at least 5-10 times and your life will be easier if you don’t have bits breaking off.

So we flew, dialed in our drag in OpenRocket. With an F20 it looks like we'll need 5oz (140g) of ballast; which seems like a lot. The E30 that we also used was way too low. There just doesn't seem to be a good motor in between (we want a one time use aerotech, for simplicity). Does it seem ridiculous to add 5oz of dead weight to our rocket? Increasing the drag is also an option, but that seems imprecise and our team has limited time to characterize it.

 

[boatgeek]

So we flew, dialed in our drag in OpenRocket. With an F20 it looks like we'll need 5oz (140g) of ballast; which seems like a lot. The E30 that we also used was way too low. There just doesn't seem to be a good motor in between (we want a one time use aerotech, for simplicity). Does it seem ridiculous to add 5oz of dead weight to our rocket? Increasing the drag is also an option, but that seems imprecise and our team has limited time to characterize it.

This is a time when thrustcurve.org is your friend. Assuming you have a 24mm motor mount, then it looks like the only single-use motor between the E30 and the F30 is the Quest E35. That motor is made by Aerotech, so it should be available in the same places as the Aerotech-branded motors. The only issue you might have is that people have had trouble with motor reliability.

Personally, I wouldn't have heartburn over 140g of dead weight as long as you aren't close to the overall weight limits and it's secured well so it doesn't flop around the body tube in flight.