First scratch build --- planning.

[Babbaloie]

Hey all!
As I'm sure you can tell, I'm new to the hobby, and along with my kids, we've enjoyed getting started. We've taken very much a step-by-step, graduated approach to the hobby, and it's really paid dividends in terms of learning little bits at a time.

We kicked it off with the launch of a couple of RTF rockets, and then worked through a very simple little 18mm kit to learn the basics of construction. We learned the basics of operation, flight, and recovery of LPR rockets, and had a total ball.

Currently, I'm working on finishing out an Estes Big Bertha --- and if the Texas weather plays nice, we'll launch her on her maiden flight this weekend. Here, the focus has been on building it "right" -- taking care with the finish, and the construction, working to get a much nicer looking rocket onto the pad.

So the next step.

My first inclination was to go take a step up in power --- maybe work on building a mid-power kit and taking it out to a club.

But I think I'm going to work an intermediate step and take a shot at a low-power scratch build instead.

And being a Raytheon guy, I feel like I should build something iconically Raytheon --- a Standard Missile 3.

I've got a design I've put together, and mocked it up in RockSim (Thanks free 30 day trial!) --- it appears that it'll fly if I put some weight in the nose (Those small fins on the SM-3 seem to be problematic).

So my question is this --- other than what I've done to this point, is there anything relative to scratch building that I ought to be paying attention to?

What am I not considering??

Thanks in advance, and nice to meet all of you!

 

[neil_w]

Well, your #1 thing is stability, and you're already on the right road there. If you don't want to spring for RockSim there is always OpenRocket, a free and open source alternative (it's what I use).

With an LPR scratch build, you need to think about:
1) CG and CP. Be sure to recheck your CG after construction; you'll never get it exactly correct in advance in the SIM.
2) Choosing most appropriate motors and delays
3) Where to put the launch lug(s)
4) Paint scheme and/or decals
5) Acquiring and/or fabricating all the parts

How much weight are you putting in the nose on that thing?

 

[Babbaloie]

How much weight are you putting in the nose on that thing?

Thanks for the inputs. I'd gone with half an ounce --- truthfully, I didn't really iterate on a variety of amounts to get the CG where I thought it ought to be, I just went with a guesstimate and it worked.

Relative to motor delay --- is there a "right" way to think about how to make that decision?

 

[neil_w]

The easiest way is just to simulate. A model like that has no super-weird stuff to confuse the sim programs, so you should get pretty accurate results.

Given that it seems to be a typical geometry, drag-wise, you're probably talking the typical single-stage motors: A8-3, B6-4, and C6-5 (although that's probably heavy for the A).

 

[snrkl]

A few thoughts based on your diagram:

Your motor mount is further forward of where I'd be placing it - depending on your motor retention method, it could be tricky getting into the motor mount or putting a tape wrap over the end of the motor overhang.

I put some thoughts in a post that might help here: https://www.rocketryforum.com/showthread.php?t=140824

It's also hard to tell from your screen shot (made worse by my lack of understanding imperial units - I speak metric) how deep your engine block is.

I've always preferred to arrange my MMTs such that on a 70mm motor tube, my engine block is 10mm back from the fwd end of the tube. A/B/C motors being 70mm long, this means I get a good 10mm of over hang at the aft end.

I then either flush mount the rear CR so the MMT hangs10mm aft of the body tube or I recess mount it so that there is 10mm of recessed tube and 10mm of motor for a wrap of masking tape for motor retention on ejection.

Either way, it means I get 10mm of MMT and 10mm of motor for a tape wrap.

CR flush to the aft of the BT:


CR recessed to the aft of the BT (even though an engine hook was used, next time I'd leave it off and just use a tape wrap on the motor overhang)

 

[snrkl]

Relative to motor delay --- is there a "right" way to think about how to make that decision?

As Neil said, I just use openrocket. It will give you ideal delay times.

If it's close, like 4secs recommended and you need to choose between 3sec and 5sec, I look at the "velocity at eject" figures on the sim for 3secs and 5secs and choose the slower of the two - less velocity at eject means less strain on the shock cord and chute.

Looks like an interesting build - I'm looking forward to watching it - make sure you post regular updates!!

 

[snrkl]

Well, your #1 thing is stability, and you're already on the right road there. If you don't want to spring for RockSim there is always OpenRocket, a free and open source alternative (it's what I use).

With an LPR scratch build, you need to think about:
1) CG and CP. Be sure to recheck your CG after construction; you'll never get it exactly correct in advance in the SIM.
2) Choosing most appropriate motors and delays
3) Where to put the launch lug(s)
4) Paint scheme and/or decals
5) Acquiring and/or fabricating all the parts

How much weight are you putting in the nose on that thing?

On motor selection, I also learned the hard way about MRLOW values for motors ( max recommended lift off weight).

Check your constructed weight (sims tend to calculate them in the light side - I'm convinced they don't take glue into account) and make sure your selected motor has an MRLOW to suit.

Estes B/C motors MRLOW is 113g from memory. I have a chart if you want I can dig up from my web archive.

Choosing a motor that doesn't have enough thrust means your rocket won't be travelling fast enough off the rod and will likely turn immediately leading to low flying lawn darts and ejection charges in the dirt.

Openrocket gives you figures for velocity as it leaves the rod - fins only become useful at 30km/ (~8m/s) so that what I aim for.

 

[Babbaloie]

I totally agree relative to the MM location --- it's just an error in the rocksim plans. I'll have to make that change.

That said, I did some research on the build, and buying all of the parts open stock seems like a pretty spendy way to build the thing. I think what I'll likely do is buy a kit (either an Estes Alpha or an Apogee Avion) and then a different nosecone and some extra balsa for the fins/strakes. I think the result will be far more cost effective.

Also, any suggestions for what to use as Nosecone weight?

 

[snrkl]

Have you looked at the Estes "Designer's Special"? I just got one and I'm having a blast. It's just a good assortment of parts that end up way cheaper then buying individuals.

Oh, and it includes awesome nose weight clay.

 

[neil_w]

That said, I did some research on the build, and buying all of the parts open stock seems like a pretty spendy way to build the thing. I think what I'll likely do is buy a kit (either an Estes Alpha or an Apogee Avion) and then a different nosecone and some extra balsa for the fins/strakes. I think the result will be far more cost effective.

Speaking for myself here: in general, my scratch builds cost quite a bit more than buying kits, by the time they're all finished. As time goes by, I am accumulating more parts that will make future builds cheaper (because I'll already have parts on-hand). When I order parts now, I try to strategically order extras that I believe will be useful in the future (like engine mount parts, etc.).

An Estes designer special (as snrkl suggested) will also stock you up for a while. I didn't go that route because it has a lot of parts that I am not too interested in. Although if I did buy one, I'd probably come up with designs to use those parts, in the classic SPEV tradition.

Also, any suggestions for what to use as Nosecone weight?

Non-drying modeling clay. The stuff I purchased is Crayola modeling clay. The pack I purchased for a few bucks should last me the rest of my life I think.

 

[Pem Tech]

Speaking for myself here: in general, my scratch builds cost quite a bit more than buying kits, by the time they're all finished. As time goes by, I am accumulating more parts that will make future builds cheaper (because I'll already have parts on-hand). When I order parts now, I try to strategically order extras that I believe will be useful in the future (like engine mount parts, etc.).

An Estes designer special (as snrkl suggested) will also stock you up for a while. I didn't go that route because it has a lot of parts that I am not too interested in. Although if I did buy one, I'd probably come up with designs to use those parts, in the classic SPEV tradition.



Non-drying modeling clay. The stuff I purchased is Crayola modeling clay. The pack I purchased for a few bucks should last me the rest of my life I think.

" in general, my scratch builds cost quite a bit more than buying kits, by the time they're all finished. "
When first returning to the hobby I found this to be painfully true. The old BNC-20 left in my old range box that used to cost .25 cents was now over $2!!! :y: Very quickly I found that buying closeout kits and mixing parts was the cheapest way to go until you build a small backlog of parts. By that time you should be over the sticker shock and just be willing to cough up the dough for the parts you want.

 

[neil_w]

To be honest I haven't worried *too* much about my scratch build costs. Each one tends to be a bit involved and takes me months to work all the way through, so by the time I'm finished I feel like I've gotten my money's worth.

For quick and simple scratchers, though, I might well be deterred if the cost were disproportionately high.

Certainly some of the Estes closeout kits ($3 Photon Probe I'm looking at you bub) can be fantastic sources of parts, although it's obviously hard to control exactly what you're gonna get.