how much can we trust Openrocket; or supersonic on an Estes motor

[lr64]

I'm wondering just how much we can trust Openrocket to predict speeds. I managed to come up with something on Openrocket that just BARELY breaks Mach 1 with an Estes F15 motor.
One element of doubt is the use of a boat tail that goes past the end of the motor, though I think the aft diameter is still large enough not to actually be blasted by the exhaust. (There must be a better word.)
Another is that you only have three lousy choices for the fin cross section. Plus I don't know how to get fins to mount on a boat tail. I guess there's a paper out there someplace describing just how Openrocket does its simulations, but I haven't seen it. If memory serves, I managed to find a motor that wasn't from Estes, without much, if any more impulse, that would break Mach 1 in an Alpha, at least inside Openrocket. I've attached two ork files. One with a minimal diameter, just breaks Mach 1. The other, with a larger diameter, but a somewhat curvier shape, comes close. I think I may have had some tweaks that got the second design a lot closer to Mach 1.


I didn't bother with opulent luxuries like electronics and recovery, but perhaps a streamer could be put in the nose cone. Rear eject might make for less drag by eliminating a discontinuity in the surface. I guess for the first design you'd have to glue an attachment for the shock cord (aka tree snagger) to the back of the motor.

. I'm assuming one is willing to do a bit of polishing. Figuring out the actual speed might be tricky without a radar gun or something. Or maybe someone standing behind a rock on top of a steep hill that's very close by, listening for the sonic boom.

 

[neil_w]

I'm wondering just how much we can trust Openrocket to predict speeds. I managed to come up with something on Openrocket that just BARELY breaks Mach 1 with an Estes F15 motor.

I tend to doubt you're really going to hit Mach 1 with a black powder motor, but the overall sim results you're getting should at least be close for simple rockets like that. Always interesting to compare against what RASAero says.

One element of doubt is the use of a boat tail that goes past the end of the motor, though I think the aft diameter is still large enough not to actually be blasted by the exhaust. (There must be a better word.)

Cooked? Fried? Barbequed? Also: Krushnic. That first rocket isn't going anywhere with the motor recessed like that, and I'd have some doubts about the second one as well.

Another is that you only have three lousy choices for the fin cross section.

What else do you need? The other most likely fin profile (tapered front/back) would probably sim almost identically to "airfoiled".

Plus I don't know how to get fins to mount on a boat tail.

Select the boattail and then click on "Freeform Fin".

I guess there's a paper out there someplace describing just how Openrocket does its simulations, but I haven't seen it. If memory serves, I managed to find a motor that wasn't from Estes, without much, if any more impulse, that would break Mach 1 in an Alpha, at least inside Openrocket. I've attached two ork files. One with a minimal diameter, just breaks Mach 1. The other, with a larger diameter, but a somewhat curvier shape, comes close. I think I may have had some tweaks that got the second design a lot closer to Mach 1.

I didn't bother with opulent luxuries like electronics and recovery, but perhaps a streamer could be put in the nose cone. Rear eject might make for less drag by eliminating a discontinuity in the surface. I guess for the first design you'd have to glue an attachment for the shock cord (aka tree snagger) to the back of the motor.

Stability is marginal on both of those. A little bit of equipment in the nose would help on that front. I don't know if, by the time you equip it to be safe for flight, whether it would still be able to make Mach (even in simulation) with the extra weight.

Also, you *have* to have some sort of recovery, and without electronics (a) you won't know how fast you went, and (b) you'll never see the rocket again.

It is interesting as a theoretical exercise but I'm doubtful you could actually build a rocket to match what you're showing in OR.

 

[lr64]

I've already added a bit to the nose cone weight already, so that I could make the fins smaller. Some of that weight could be a very thin streamer, rolled up. It wouldn't have to be very large for a rocket that might weigh less than 3 ounces after the fuel burns off. Or maybe a very long, thin Kevlar shock cord might provide more drag per gram or cc.

The boat tail could also have a tube or cone inside it. Obviously of some kind of light, heat resistant material. If the nozzle in the motor doesn't already expand to the optimal diameter, it might even add a little thrust.

Speed measurement would probably need to be by some external means.
A microphone in an RC glider at 1,000 feet? The maximum speed is supposed to occur at less than that altitude. Radar?

A more optimized BP motor could be made.

 

[Raythain]

It's an interesting exercise.

To pour even more rain on the parade, I'd be really concerned with flutter on either of those fin designs going up to Mach. Even with recovery gear, that's probably coming down in more than one piece.

 

[KenECoyote]

Or maybe someone standing behind a rock on top of a steep hill that's very close by, listening for the sonic boom.

I don't believe that's ever audible. The motor and coast noise is just too great and any "boom" is likely a tiny pop that's drowned out.

 

[lr64]

Hypothetically, the rocket should outrun most of the other sounds, shouldn't it? Or maybe that only works if it's supersonic for a while. It's not something you're likely to hear on the ground. Also, the boom would be coming from much closer than the other sounds.

 

[lr64]

It's an interesting exercise.

To pour even more rain on the parade, I'd be really concerned with flutter on either of those fin designs going up to Mach. Even with recovery gear, that's probably coming down in more than one piece.

Carbon fiber tow is cheap. If I was going to build something like this, it would probably have a couple of carbon fiber spars and the rest would be balsa. These are small fins.

 

[Buckeye]

The velocity calculation is highly accurate. The mass, drag, and thrust inputs may not be.

 

[Chuck Rogers]

I'm thinking of a much more practical variation of this idea, a two stage rocket, an Estes F15-0 staged to an Estes F15-8.


A few questions:

1) I'll need the nozzle exit diameter for the Estes F15 motor. In fact, if someone has data for both the initial (not fired) nozzle exit diameter and the burned out nozzle exit diameter (so I can use the average), that would be great.

2) From those with experience with staging Estes F15 motors, how much of a time delay is there from what looks like the burnout of the F15-0 booster stage and the ignition of the F15 upper stage?

3) This rocket would really need an integrating accelerometer instrumentation package to actually measure whether it went supersonic or not. Any hope of an instrumentation package that would fit in a rocket this small?


Charles E. (Chuck) Rogers

 

[SolarYellow]

Blue Raven. For a 29mm motor, the bulkhead form factor is the way to go.

You'll need a tall tower. Initial thrust peak of the F15 is considerably less than the E12 and takes longer to arrive. Two-stage is really hard to get adequate exit velocity due to the high mass.

 

[neil_w]

E16-0 is better to get the first stage moving, but maybe that would limit achieved top speed.

 

[Art Upton]

When I back simed my 4" M-1400 supersonic rocket in my Avatar, the speed was reasonable to what the accelerometer put out back then. Back then of course I used RASaero as OR was not made yet.