Dilemma for a 2 stage HPR

[AstroAbaqus]

Dear all after my lvl 1 using a scratch build I started to work on my electronic bay and will do some other test launches using lvl 1 motors before attempting my lvl 2. In the meantime I started to put togheter ideas and parts for my little dream a 2 stage rocket.
My experience with SET my first HPR scratch build has been amazing, but also costly. For this reason I decided to use as much left over from previous project and to buy kits to be modified for this purpose. Here below you can see my idea, Lightining which is based on a 4inch booster LOC Expediter and a sustainer LOC Iris (or Sandhawk). The idea is to fly initially using at the maximum my CTI starter set using the 6 grain for the booster and the 3 grains for the sustainer to practice and collect data. But the final version will be a 54mm 4 grain and 38 6 grain. The Open Rocket sim is done using a K520 and J354.

My current main concern is about the finds of both sustainer and booster. I am quite sure the 3inch and the 4inch kit I selected have 1/8 inch thick fin and 1/4 inch thick CRs. I was thinking that would be better to laser cute new fins 1/4 inch thick. The tube will be fibreglassed and the fins will be fibreglassed tip to tip anyway ( plus internal fillet and internal fiberglass fin/mmt/fin. But I am not sure the 1/8 inch thick fin will be safe to fly with K motors and I am willing to ask advice. This will be a very slow pace project so I have no rush

 

[Steve Shannon]

I have flown 1/8 inch G10 fins in a 4 inch rocket on K motors with no issues and the fillets were just straight epoxy, no reinforcement or tip to tip.

 

[SparkyVT]

I concur with Steve about the fins. Even though you will be supersonic, you probably won't need tip-to-tip reinforcement if you have G10 fins. If you are using plywood, then then tip-to-tip is probably a good idea.

Looking at your screenshot, I'd think about your electronics next. With apogee over 4,000m, it will land pretty far away. I highly recomend a radio tracker and/or GPS of some sort. Also, you need to think about separation of the booster and ignition of the sustainer. What electronics will do this, and how will you route the wires? Will your electronics have tilt-detection that will stop sustainer ignition if an unsafe trajectory is detected? Your booster might also need electronics and a tracker too.

Just some things to think about.

 

[Tractionengines]

Agree with above... plus watch mixing 4-fins on sustainer, and 3-fins on booster. If there is any cross wind it could start coning. Doing the opposite usually works better, giving more fin area at the aft end.

Mike

 

[AstroAbaqus]

I am on bus right now but I would like to thank you all for this incredible number of points made. To only clarify the screenshot is the most powerful configuration, which now I cannot even fly. My idea is to built it for this but use 38mm motor to learn and practice.

I will buy for sure a staging timer from apogee which has tilt controller and will practice gps tracking with my workhorse before attempting any 2 stage launch. The booster will have a audio beacon at least at start the sustainer a gps and a beacon. The sustainer will be dual deploy but the staging computer will be in a slot that I plan to access via an hatch, following several build I saw on the forum.

I will post more details soon, but this is a slow and long term project which I am doing knowing that I need also other skills which I will master along the way. Again many thanks!

 

[AstroAbaqus]

Instead the 3 fins issue was a clear lack of forethought I will have to think on how to deal with it. I can cut away the pre slot section and cut new slots, which is fairly easy on Loc tubing. This will also make the booster more mass efficient and any electronic for it can be housed in a side hatch. Or found a 3 inch kit to be modiefied… caliper looks a good one but I think has a 54mm MMT.

 

[AstroAbaqus]

Ok after all your advices I am planning the following:

1. Switch to 4 fins, I have already the file for the laser cutter since my first rocket was a 4inch with 4 fin, all 1/4 inch thick so 6 mm and my workhorse was designed also looking at flutter velocity so should be pretty solid. I will go for tip-to-tip glassing since I have plenty of material for the next years xD and I love messing with composite.
2. I am going to evaluate volumes and will decide if I will just cut the pre slotted area or will add new slots and the fiberglass the tube. I think it will be fine but I ask for your opinion on this specific thing. Maybe I can use these slot to also add expanding foam to increase the stiffness of this part.
3. Sustainer/Booster connection. My idea was to use the 4 inch to 3 inch LOC reducer. However this will interfere with the sustainer MMT and aft CR. On Apogee Components there is a tutorial for the Terrier/Sandhawk kit. Tim in this video cuts away part of the fin tab and I think he do not use the aft CR at all. I think this is not a good solution for my situation. Any suggestion on this regard?

Thank you all for your inputs!

 

[Tractionengines]

Trimming the back of the fin tab shorter and moving the aft CR forward some to allow your interstage coupler to install is OK. Also, can trim the 3" coupler portion of the LOC transition a "little" too. Just need to add new front bulkhead, depending on you design.

Just make sure you have clearance for your retainer method of choice; and account for wiring access for stage separation charge (optional) and Sustainer starter, etc.

 

[waltr]

Yep, I did this a little differently moding a BMS 3" school rocket for 2-stage.
Moved the sustainer fins forward by lengthening the slots to get the rear centering ring forward enough for the ISC.

Since the BMS BT is thin (between Estes and LOC) I epoxyed a piece of split BT over the rear end to stiffen the BT where the ISC fits. Tip to Tip glass will also do this well. My idea was to ensure the back end of the BT deos nor warp and ensure the ISC always fits properly. Still fits nicely after four flights.

I also made an AV bay similar to what Apogee has for the Terrier/Sandhawk kit. There is an Eggtimer Quantum to do Airstart and Chute ejection at apogee. This ensure a chute even if 2-stage motor does not light.

 

[AstroAbaqus]

Yep, I did this a little differently moding a BMS 3" school rocket for 2-stage.
Moved the sustainer fins forward by lengthening the slots to get the rear centering ring forward enough for the ISC.

Since the BMS BT is thin (between Estes and LOC) I epoxyed a piece of split BT over the rear end to stiffen the BT where the ISC fits. Tip to Tip glass will also do this well. My idea was to ensure the back end of the BT deos nor warp and ensure the ISC always fits properly. Still fits nicely after four flights.

I also made an AV bay similar to what Apogee has for the Terrier/Sandhawk kit. There is an Eggtimer Quantum to do Airstart and Chute ejection at apogee. This ensure a chute even if 2-stage motor does not light.

Yes my idea was exactly this extend the slot, I will anyway fiberglass the airframe or buy another spare 3 inch loc tube. The retainer will be probably a slim one since there is not much clearance I think. I will start to put things on cad tonight as I did for my SET and I am sure other issues will rise xD.