First Attempt at 2 Stage HPR

[cthunter01]

I've built LPR multistage rockets that use Estes black powder motors before, but never anything at the mid or high power level that utilize composite motors. I enjoy launching multistage rockets, and enjoy building and flying HPR rockets, but have never combined the two before and would like to attempt that this year.

I've been working on a design for a 3 inch diameter 2-stage rocket with 38mm motor mounts in both stages, and would appreciate some feedback on what I've come up with so far. I'm attaching an OpenRocket design file with a few motor configurations as well. My local club has a waiver to 12k ft, and there are a few clubs a little further away with higher waivers if lower flights are all successful and I'm feeling adventurous.

Here are some extra data points about this design:

1. I would like to use a small BP charge to separate the stages about 1 second after 1st stage burnout. I would do it at first stage burnout except that I want to account for any residual thrust from the booster motor so as to not run into the sustainer after separation. I plan on using a Missileworks RRC2L as a timer (because I already have a couple, but thoughts on something better are welcome) in the booster to control that separation, as well as deploy the booster parachute at apogee. Motor deploy can be a backup to deploy the parachute if something happens with the electronics.

2. The sustainer will be have a redundant DD setup for recovery. I plan to use the Aux channel on a Missileworks RRC3 to ignite the sustainer motor a few seconds after the booster burns out and separates. Then use the main and drogue channels for DD. The backup will be a Missileworks RRC2.

3. I initially thought about using drag separation, but after searching the forums it didn't seem reliable. So I decided to use a small BP charge to separate them, and can use some shear pins to hold the stages together during boost.

4. The sustainer can be flown on its own.

5. The parachutes in the design file are just to get the descent speeds approximately right. If you have thoughts on specific parachutes or whatnot that would be great.

6. I'm using lipo batteries for the electronics.

7. Do you have any pointers on specific igniters?

 

[cthunter01]

To add, I have a JLCR I can put on the booster parachute so it doesn’t drift too far. I may need to upsize the sustainer main chute to account for the possibility that the booster doesn’t separate and the whole assembly comes down in one piece safely

 

[boatgeek]

Reviewing the Eggtimer Proton/Quantum airstarts guide is pretty helpful even if you're not using their products. They have some pretty good guidance on motor selection, settings, etc. My advice below is based on relatively limited experience, but it worked for me. For my first couple of flights, I wasn't really seeking altitude so I had the separation charge at 0.5s after burnout and sustainer ignition 0.5s after that. My booster motor was a fast-shutoff motor that didn't have a long tail on the thrust curve so I wasn't too worried about thrust continuing for longer.

1. I think the separation charge is a good idea. I didn't get drag separation on mine before the separation charge went.
2. Sounds good.
3. I wouldn't include shear pins mainly because that's one more thing to do at the pad and the consequences of early separation are minimal. You won't be able to put your igniter in until you get out to the pad area (or other remote prep area), so you'll need to take your booster off for prep. It's always a hassle getting things lined up in the field. Worst case is that you do get drag separation and the separation charge doesn't do anything. If you don't have shear pins, the separation charge can be very small as well.
4. That's a nice feature.
5. What's your ground hit speed for both stages separately and the stages together under the main?
6. Seems legit.
7. I used CTI motors, so the supplied ematch was plenty.

 

[mikec]

7. I used CTI motors, so the supplied ematch was plenty.

+1. There are many ways to make AT motors work (you probably can't use stock AT igniters because they use a lot of current, so you need something lower-current but with enough oomph to work) but the CTI motors just come that way standard.

 

[waltr]

Reading the Eggtimer air start manuals is a good idea since Cris goes over a lot of what if's and how to's that can be applied regardless of which computer used.
Agree with every thing baotgeek posted.

Also, there are a number of good threads discussing air starts here in this forum topic. All worth reading and many cover various safety aspects of setting up an air start 2 stage at the range.

I have been using AT motors so make igiters using 40ga nichrome and Quickburst Procast. A 2S LiPo easily fires these but did have one flight where the sustainer took 4 seconds to come up to pressure, By them the rocket peaked and nosed over. Sustained properled the down down through a tree. Not pretty. Velocity, altitude or tilt lock-outs would not have prevented this since the igniter fired at the correct time with rocket pointed up.

 

[cthunter01]

Thanks for the feedback and info!

I'm checking out the Eggtimer documentation, which is quite good. I may look into actually getting a Proton in the future, as it looks pretty capable, and the the wifi arming is quite nice. And I like the idea of soldering it together myself, sounds fun.

I'm targeting a ground hit velocity of between 4 and 5 m/s. Right now they're at 4.7 m/s or so each. That's nominal however, not if the booster is still attached to the sustainer. If that happens then for sure it'd hit faster, and I'll have to go back and sim that. I'll upsize the sustainer main to account for that possibility.

And good call on the shear pins with the interstage coupler. I don't think they're necessary, so I won't use them. I still do like the idea of the stages staying together until I intentionally separate them though. Maybe that's me being overly concerned. What are your thoughts on using some masking tape on the ISC to provide that fit, like you might do with a nosecone?

I have been using AT motors so make igiters using 40ga nichrome and Quickburst Procast. A 2S LiPo easily fires these but did have one flight where the sustainer took 4 seconds to come up to pressure, By them the rocket peaked and nosed over. Sustained properled the down down through a tree. Not pretty. Velocity, altitude or tilt lock-outs would not have prevented this since the igniter fired at the correct time with rocket pointed up.

That's my biggest concern honestly. And why I asked about igniters specifically. I plan on using CTI hardware, but I've still had motors take several seconds to come up to pressure on the pad on the ground, which I really don't want to happen in the air.

 

[waltr]

The best thing is to have plenty of air speed at 1st stage burn out. This helps ensure the rocket maintains an upward vector until 2nd stage motor comes up to pressure.
Starting with minimum delay is good until you get a good feel for what the rocket will do.
When you run simulations, try longer delays to 2nd stage ignition to check this. Look at velocity.

If the booster motor burns out then the stages can separate without issues. It is rare to drag separate so do not think this should be a concern.
Greater concern is ensure recovery deploys on each stage even when separation or ignition does not go as planned.
My last 2-stage flight the Proton did not fire the igniter but did fire separation charge. Booster deployed chute and the drogue then main deployed on the sustainer. Disappointing flight but all parts came down safely.

CTI motors are a favorite for upper stages since then use an Ematch and a starter pellet is in the propellant core.

I have been running a D15 in the sustainer, G74 in booster. We have a smallish field and my biggest worry is the sustain fires with rocket horizontal which then flies into the next county. Maybe run a very small motor in 2nd stage. since it already has air speed you don't need a lot of thrust in the 2nd stage. Also keeps peak altitude lower.
Again, play with this is the sim.

 

[boatgeek]

Thanks for the feedback and info!

I'm checking out the Eggtimer documentation, which is quite good. I may look into actually getting a Proton in the future, as it looks pretty capable, and the the wifi arming is quite nice. And I like the idea of soldering it together myself, sounds fun.

I'm targeting a ground hit velocity of between 4 and 5 m/s. Right now they're at 4.7 m/s or so each. That's nominal however, not if the booster is still attached to the sustainer. If that happens then for sure it'd hit faster, and I'll have to go back and sim that. I'll upsize the sustainer main to account for that possibility.

And good call on the shear pins with the interstage coupler. I don't think they're necessary, so I won't use them. I still do like the idea of the stages staying together until I intentionally separate them though. Maybe that's me being overly concerned. What are your thoughts on using some masking tape on the ISC to provide that fit, like you might do with a nosecone?


That's my biggest concern honestly. And why I asked about igniters specifically. I plan on using CTI hardware, but I've still had motors take several seconds to come up to pressure on the pad on the ground, which I really don't want to happen in the air.

The Proton is a nice unit. If you haven't soldered before, I'd suggest building a Quark first to gain skills. It's more or less the same price as a learn-to-solder kit and you get a working altimeter out of it. I used the Proton on my 2-stage--you might want to read over my flight programming details if you use a Proton. I had a few kinks that I needed to work out.

4-5 m/s is nice and slow. I wouldn't have too much heartburn about up to 7 m/s unless you're landing on hard surfaces like pavement.

I would use masking tape until it doesn't wobble, but I'm not that concerned about having it tight like a nose cone. If you do that, you'll definitely want to ground test the separation charge.

Somewhere on TRF, Jim Jarvis put together a list of which CTI propellants were easiest to light. IIRC, BT, WT, and RL were high on the list and WH, GR, and SS were lower. That said, 38mm motors seem to come up to pressure pretty fast.

 

[waltr]

A Quark is a good first build and could be used in the booster ISC to have electronic deployment at apogee. Later a Cable Cutter can be used for Main deploy at 600-300 feet.

Have you flown any DD? If not then I recommend build and fly DD the start learning about using electronics for deployment. There is a lot to go wrong with DD and a lot more with air start.