OpenRocket engine offset suggestion, offer

[WizardOfBoz]

Hi. I've been using OpenRocket for a few months - very nice. Thanks to all those contributing.

One suggestion involves how rocket motors are offset, specifically for lower-mid power engines. These engines typically use an engine block on the front of the motor tube. Small differences in engine length (for example in the Estes and Quest C6 - D21 range) SHOULD result in the engine extending out different lengths. Only a quarter inch variation or so. But the way engine position is specified seems to fix the engine's aft end with respect to the motor tube. Simulations are proably close, but .... (I'm a simulation scientist and I sweat details like this professionally - its hard to let go!) The workaround in inelegant: Different ork files for different motors. Note the gap between the forward end of the motor and the motor stop. What I'm asking for is a check box on the motor tube "Align motor front with engine block". Alternately (and probably better) would be the two check boxes on each motor spec. There would be a default motor position (specifed on the MTT), a motor specific offset override, or the abovementioned "Align with engine block".



I'll offer to help. I could do demos, give feedback, write documentation now. Right now I'm familiarizing myself with the dev environment. Later I may be able to help with code projects if that's ok.

 

[neil_w]

That would be possible, but it is also (to my knowledge) contrary to the way most rockets (and rocketeers) work. Normally, the motor block is either placed for the longest possible motor (e.g. 24mm "E" mounts, with a spacer up front for shorter motors) or omitted entirely (standard in the MPR/HPR world). To have, say a 2.75" motor mount and then stick a 3.75" E motor in it, sticking way out the back, is in my estimation extremely unusual. And it would mess up your motor retention.

 

[WizardOfBoz]

Thanks, Neil. I'm specifically looking at the Estes Patriot using either Estes or Aerotech 18mm motors (which are standard length), or Quest motors (which are not). The MTT for that kit is 2.76" long (that is, exactly the length of an Estes 18mm engine. If one puts the engine hook and block in as directed stnndard Estes engines extend about 0.26". I was thinking of making an improved Patriot and a longer MTT is on the list of revisions (along with adding the side conduit detail and subbing a motor retainer for the clip). But 1) there's no extra length available in the stock MTT and 2) I already made the rocket and glued the engine block in. So I'm stuck simulating what I have. But thanks for the idea of putting in a longer tube and using spacers.

Here's the problem. According to the data in OR motor lengths are:

Estes motors (all 2.76" long) B4-4 B6-4 C6-5
Aerotech motors (also all 2.76" long) D13W-7 (RMS), D21T-8 (SU), D10W-7 (SU)

Quest motors B4-4 (3.09" long), D16-6 (3.09"), D20W-6 (3.19")

I've got the .ork file (attached) set up to replicate the engine block position shown in the instructions. That is, standard length motor sticks out 0.25 inches from from MTT. Works fine for those motors. But for the non-standard Quest motors, the fixed extension doesn't mimic what actually happens (that is, the motors extend through the engine mount and preserve the 0.25 extension out the back, rather than sticking out an additional 0.43 inchs). I can manually change this for the three motors (two different changes) and keep 3 versions of the same model or... Hence my request.

And again, thanks for the guidance and ideas.

 

[neil_w]

The extra length of the quest motors is at the front (the red plastic ejection charge cap); they should have the exact same overhang out back when mounted in the rocket.

Unfortunately there’s no spec that gives “motor mount length” for each motor, so you end up with some confusing weirdness like this.

 

[Charles_McG]

Also, try not to get too hung up on things like this. There are more lurking in the multivariate space of what you can enter into OpenRocket. It's not a physics sandbox, and it's not CAD. It has a fairly fixed notion of what parts do what in the calculations, and it cares little/none about the rest. You can make a perfectly valid sim with only the outer surfaces and no internals, with the airframe set as a motor mounts and an 18mm motor floating freely in 60mm tubing. As long as you override the mass and Cg to match the as-built, it works.

 

[neil_w]

Also, try not to get too hung up on things like this. There are more lurking in the multivariate space of what you can enter into OpenRocket. It's not a physics sandbox, and it's not CAD. It has a fairly fixed notion of what parts do what in the calculations, and it cares little/none about the rest. You can make a perfectly valid sim with only the outer surfaces and no internals, with the airframe set as a motor mounts and an 18mm motor floating freely in 60mm tubing. As long as you override the mass and Cg to match the as-built, it works.

Well, that's all true, but it would be relevant to loaded CG calculation if OR were not positioning the motor correctly in the mount. But it generally *is* doing it correctly, so there's no issue here as far as I can tell.

 

[Charles_McG]

Well, that's all true, but it would be relevant to loaded CG calculation if OR were not positioning the motor correctly in the mount. But it generally *is* doing it correctly, so there's no issue here as far as I can tell.

But it's not doing it by having a 3D CAD model of the motor and automatically positioning it flush against other surfaces - like an engine block centering ring, or the aft end of the motor tube and the motor thrust ring - if the motor has one.

 

[neil_w]

But it's not doing it by having a 3D CAD model of the motor and automatically positioning it flush against other surfaces - like an engine block centering ring, or the aft end of the motor tube and the motor thrust ring - if the motor has one.

Correct, because it's doing it the way (almost?) all rockets are actually built: with a fixed overhang.

 

[WizardOfBoz]

Guys, this is great - like I'm back in engineering design talking with my friends (engineers).

So 1) the .ork I'm using , with the 0.25 motor extension specified, is representative of how the motors mount and so should simulate what actually exists (that is, the motor/engine block overlap is the forward, skinnier part of the motor overlapping and should not be a problem), 2) for most models, if you wan to change the engine extension its the same as adjusting the engine block (ignoring the effects of the minor positional changes of the block on Cg).

In looking at the 18mm engine options it seems like the standard engine block position is fine for the Patriot. One could tweak things and use a slighly longer MTT leaving, say, only an 1/8 of the motor hanging out. And one could use a more modern engine retainer rather than the engine hook. But it looks like OR does what is needed, and I needed an upgrade in understanding. Thanks!

Now all I have to do is to launch the thing!

Jim

 

[Art Upton]

I haven't put engine blocks in rockets very much at all once I found out about masing tape thrust rings in the 70s.
No use for engine clips either.

When Hi Power started, they didn't use engine blocks either and it was like wow.

Remember, Dr. Frank Kosden used a masking tape thrust ring and retention on an O motor at Black Rock decades ago.

 

[Charles_McG]

I don’t put engine blocks even in mini motor mounts. I have a box of 13mm B motors.

 

[WizardOfBoz]

So that makes me think. Do they make glue-on thrust rings for 18mm motors? Less ad hoc than making tape but easier (unless you have 3/16 or 1/4 wide tape.

 

[Art Upton]

So that makes me think. Do they make glue-on thrust rings for 18mm motors? Less ad hoc than making tape but easier (unless you have 3/16 or 1/4 wide tape.

Masking tape is easier then glue on. I even leave over hang of the tape past the motor, form the thrust ring, then use the Xacto to trim the excess off the motor. Still faster then gluing. Used by winning contesters long before HPR.

A NAR article in 1974 "In defense of masing tape"

 

[neil_w]

I generally use hooks and blocks in my LPR builds, but acknowledge that making tape rings is *really* easy.

 

[WizardOfBoz]

@Art Upton if you finessed the tape a bit you could fabricate a thrust nozzle skirt! I'll proably limit my tape to ensuring that the motors a snug in the MMT, even though I have a block in front and the clip in back.

 

[neil_w]

I'll proably limit my tape to ensuring that the motors a snug in the MMT, even though I have a block in front and the clip in back.

There’s no reason to add friction if you have a block and a hook. Why would you want to?

 

[Art Upton]

@Art Upton if you finessed the tape a bit you could fabricate a thrust nozzle skirt! I'll proably limit my tape to ensuring that the motors a snug in the MMT, even though I have a block in front and the clip in back.

I have a few times in the far past...

 

[WizardOfBoz]

There’s no reason to add friction if you have a block and a hook. Why would you want to?

I. Hate. Wiggle.

In the world of decision-making, there are optimizers, and satisficers (Nobel Laureate William Simon). If a satisficer is cold, he goes to the nearest clothing store, buys whatever seems to keep him warm, and goes home. An optimizer who is cold researches types of sweater material (wool? alpaca? cashmere?) and then analyzes the use cases for the sweater, as well as life-cycle analysis and understanding the implications of disposing of a worn out sweater at the end of life. Three weeks later, he has a sweater.

If you ask me what I want to drink, I'm a satisficer. "What are you drinking? That's fine". If I'm doing a technical design... Well, I've been known to do comparative analysis on the lubricant for my nose-hair trimmer. (Oster Blade Lube, if you are interested).

The rocket motor slides around a bit in the MTT and the retaining clip is a bit longerer than motor's 2.76 inches. So there's a bit of wiggle that half a wrap or so of tape removes. Hey, at least give me credit for not ordering Kapton high-temp tape! Though, come to think of it...

Tim Milligan used some tape to snug up a rocket engine, even though he was adding his special motor tape strips to hold the motor in securely. Video here.

 

[lr64]

Isn't Kapton tape kind of slippery?

 

[WizardOfBoz]

Kapton(R) is a harder polymer. You may be thinking that its similar to Teflon (polytetrafluoroethylene) since its a Dupont invention. It's different and has different properties. It's claim to fame is that it's thermally stable (up to about 750°F) and it's electrically insulating. It is slipperier than the crepe used in masking tape but not as slippery as (for example) Teflon(R) plumber's tape. Kapton is a polyimide, not a perfluoro compound:

If you wanted to ensure an electrical barrier between two (for example) circuit boards you could use Kapton. It is slippery enough that it might work as a wear service on (for example) a body tube coupler. It's quite durable.

 

[lr64]

Actually, I'm somewhat familiar with Kapton tape, having seen it sometimes at work. Didn't there used to be something called friction tape, not so different from what people wrapped on the ends of their hockey sticks?