Mach Goon build thread

[The EGE]

Because all other Goonies, while surely very cool, are not fast enough. Not nearly fast enough. I've seen scale Goonies, retro Goonies, and even a few with 24mm motor mounts. But, I have never, ever, seen one with a 38mm motor before.

It's time for that to change.

I took an ordinary Baby Bertha kit (fig A.). I took the body tube and nose cone (fig B), and put the rest of the parts back in the bag for use later.

I trimmed much of the shoulder off the nose cone so I can fit a recovery system inside it, then selected an eyebolt that I'll glue into whatever nose weight I add.

I cut six fins from a clementine box and coated them on both sides with wood glue. They passed the driveway test (thrown into my driveway from a second-floor window) with flying colors.

I cut a six-inch piece of LOC 38mm tubing for a motor mount and peeled off a few layers to fit the BT-60, except for the last 1/2" which I left as a thrust ring. It's designed to fly on a CTI 38-1G G115WT or G185 Vmax, but it'll fit an AT 38/240 case.
Fig. C shows the parts; Fig. D also shows the original plan with larger fins (I originally planned for 3 larger fins rather than 6 smaller) and the 38/240 casing.

I cut 3' of 1/8" kevlar string for the shock cord. I'll tie it to the eyebolt in the nose cone and embed the other end in a blob of epoxy clay attached to the inside of the body tube.

 

[powderburner]

What's a clementine box? Are they made out of titanium or something?

And what has throwing them onto the driveway got to do with anything?

Explain, please

In the mean time, have you done any calcs to see if that big fat nose cone can be pushed anywhere past about M 0.85 or 0.9 with anything short of infinite thrust? Keep in mind that most of the simulation software packages out there do OK for subsonic flight, and some do OK for supersonic flight, but few come anywhere close to reality for transonic. Weird things happen in this speed regime and that blunt nose is not going to be the best way to slip through to supersonic flight. And if that thin plastic gets warm, it gets soft, and what little streamlined shape you start with will be gone.

Either way, this is going to make for one MONSTER Goon!

 

[Pem Tech]

A 38mm MM in a BT-60 tube?
Insane...
Oh so insane...

But I like it.
Be sure to get all your pictures BEFORE you hit the launch button.

 

[The EGE]

What's a clementine box? Are they made out of titanium or something?

And what has throwing them onto the driveway got to do with anything?

Explain, please

In the mean time, have you done any calcs to see if that big fat nose cone can be pushed anywhere past about M 0.85 or 0.9 with anything short of infinite thrust? Keep in mind that most of the simulation software packages out there do OK for subsonic flight, and some do OK for supersonic flight, but few come anywhere close to reality for transonic. Weird things happen in this speed regime and that blunt nose is not going to be the best way to slip through to supersonic flight. And if that thin plastic gets warm, it gets soft, and what little streamlined shape you start with will be gone.

Either way, this is going to make for one MONSTER Goon!

Clementine boxes are made from pretty cheap 1/8" plywood, 3-ply. It's not spectacularly strong wood, but with a layer of wood glue over it it becomes very strong - I've used it on a previous Machbuster, and it came back with all fins intact.

The driveway test is my way of testing components and rockets, especially ones like this that have to survive huge aerodynamic forces. If they can survive a 15-foot drop onto asphalt, then they are more likely to survive forces in flight. It makes more sense for the built rocket than individual components, but on my last machbuster, it broke a fin with a hidden knot that might have failed in flight, and didn't damage the 3 fins that worked perfectly.

As for the nose cone, it's not as bad as you might think. According to this site, blunter cones like elliptical cones are actually pretty good in the transsonic range, which is where the Mach Goon will spend much of its brief powered flight. It's not going to get very not in 1.4 seconds of powered flight, and I'm planning on coating the entire rocket with wood glue anyway. Poor man's phenolic.

 

[JRThro]

Clementine boxes are made from pretty cheap 1/8" plywood, 3-ply.

Is a Clementine one of those little oranges?

 

[mach7]

OK...OK...

I know I should lose some weight, but I'm not a Goonie!

And NO way are you going to shove a 38mm where the sun don't shine! :y::eyepop:












sorry

Mach

 

[The EGE]

JRThro: Yes, clementines are the mini oranges.

Mach7: That was the hardest I have laughed in at least a week.

 

[The EGE]

All six fins are now glued on. Tacked with CA and then fully glued on with wood glue. Now I just get to add lots of fillets....

 

[New Ocean]

Should be a nice one!

 

[The EGE]

Thanks New Ocean.

Sorry I don't have pictures, folks. I really need to get my own digital camera.

I have been making some progress on the Mach Goon, though. I roughed up the nose cone with 60-grit sandpaper, and that got the wood glue to stick pretty well.

Currently, I'm waiting for epoxy clay to dry. Some of it is holding the eyebolt to the inside of the nose cone, while the other half attaches the shock cord to the inside of the body tube. Far stronger than a paper mount.

 

[Grif Ingram]

The EGE,
I've looked at https://mcfisher.0catch.com/other/mach1/mach1.htm and I find it puzzling. This chap states that for supersonic speed, you need a supersonic exhaust, as a rocket cannot go faster than it's exhaust - but that's obviously wrong! Does he mean MPR rockets, with low mass ratio?
Grif

 

[New Ocean]

You will have a hard time getting a rocket past the speed of sound, in the atmosphere at least, without supersonic exhaust. Lucky for us, our rockets exhaust pretty fast indeed, right?

I was just reading "Retro rockets", and they discussed the New York Times article about Goddard that states clearly that rockets will never fly to the Moon because there is no air in space to push off of, and any high school student knows that one needs something better than a vacuum to push against. In other words, it is always more popular to take away from a plan than to support it.

I see this article talks about the mach buster. I think my G55 flights went fast, but I did also fly that rocket on F101s and I bet those were mach 1.2 or so. How I wish I just got 10 of those kits, now it is too late.

I dont know if even a G Vmax will do it in your case, the rocket is just very wide. However, if you could get an H400 in there... I bet that could do it. An I800 should do mach in a construction boot with fins.

 

[Grif Ingram]

Well, if it's some sort of atmospheric thing, I can just about accept it - a friend of mine knows more about aerodynamics than me, I'll ask him & report back. There's another bit in that article about fins having to have a tip chord inside the root chord, which made me think about: What if you need more fin area and can't do it without lots of sweep? Then it occurred to me - this must be the reason for the X15 wing shape, you just put the extra area as a sort of reverse sweep on the trailing edge, if you see what I mean! I wish I could think of a clearer way of phrasing that...
Grif

 

[The EGE]

Surprisingly, both the G115 White Thunder and the G185 Vmax make it to around Mach 1.15. There's enough space to fit an Aerotech 240Ns case, but prolly not a Pro38-2G which is 1.2" longer, so I'm going with the Pro38-1G which is cheapest and lightest.

It should be very stable - it only needed 4 fins to be stable, and I've got six.

 

[powderburner]

There's another bit in that article about fins having to have a tip chord inside the root chord...

I think what you are referring to is wing (or fin) taper ratio:

TR = (tip chord)/(root chord)

The reason for making the tip chord shorter has to do with wing three-dimensional effects and finding minumum drag. You can optimize a wing planform (the shape, as viewed from above) by using the same airfoil from root to tip and changing the chord to an elliptical spanwise distribution (like the WWII Spitfire). You can optimize a wing for minimum drag by using a constant chord from root to tip and changing the airfoil to achieve an optimum spanwise load distribution. You can optimize a wing for minimum drag by using a constant chord and airfoil from root to tip and twisting the wing to an optimized shape.

The simple way to get most of the benefit of minimum drag is to use a single airfoil, no twist, linear taper in the planform (so you have a straight leading edge and trailing edge), and size the tip chord so it is 30 to 50 percent of the root chord. The "sweet spot" is a tip chord around 35 to 40 percent. You can look this up in textbooks on wing theory under "span efficiency factor."

An interesting variation of this was an experiment to deliberately reverse the taper ratio on the XF-91 test aircraft. This experiment was not about efficient wing design but an investigation of airflow interference effects at the wing root. Actually, there are more important reasons NOT to do this, but again, it was an aerodynamic test.

Rocket fins work the same way as wings, so the same theory applies. Having straight leading and trailing edges makes it easier to sand an airfoil across the span (if you are not using an airfoil, and if you are just leaving the edges square, you don't need to worry about any of this). For best effectiveness, the thickness of the fin also needs to taper from root to tip at the same time.

These principles are purely for optimized rocket performance, as for altitude competition. Fins will still work to stabilize a rocket if they have a different planform, if they have square edges, or if they are constant thickness. They just won't work as well, and you would need a larger fin to provide the same stability.

 

[rocketman]

:shock:Any idea how high that'll go?

 

[The EGE]

:shock:Any idea how high that'll go?

On a G115WT, roughly 3400 feet and Mach 1.15
On a G185Vmax, roughly 2900 feet and Mach 1.19

Higher and faster if I use an H242T, which will only happen if it turns out over 5.5 oz.

 

[New Ocean]

Surprisingly, both the G115 White Thunder and the G185 Vmax make it to around Mach 1.15. There's enough space to fit an Aerotech 240Ns case, but prolly not a Pro38-2G which is 1.2" longer, so I'm going with the Pro38-1G which is cheapest and lightest.

It should be very stable - it only needed 4 fins to be stable, and I've got six.

But the case can hang out the back a bit right? Id say it can hang out the back a CM or so if needed, would that be possible?

It is funny how everyone says fins should never go past the airframe end for supersonic rockets, or have a long sweep, but many sounding rockets do and they do just fine.

A rocket built like the estes gnome will go supersonic if you do it right.

 

[The EGE]

But the case can hang out the back a bit right? Id say it can hang out the back a CM or so if needed, would that be possible?

It is funny how everyone says fins should never go past the airframe end for supersonic rockets, or have a long sweep, but many sounding rockets do and they do just fine.

A rocket built like the estes gnome will go supersonic if you do it right.

My fins are forward of the aft end of the body tube by about a centimeter. There's a step down by 0.02" from the Baby Bertha tube, so I had to mount the fins entirely on the larger tube.

The problem with fin sweep is that fin flutter occurs really easier unless all of the fin is directly onto the body tube with no sweep. Sounding rockets have thick metal fins; we do not.

A Gnome can hit Mach, on a 13mm EX D40. Somebody posted about it on TRF 1.0.

 

[kjohnson]

So what model is this a goonie version of? Or are you just calling it a goonie because you started with a Baby Bertha kit?

kj

 

[The EGE]

So what model is this a goonie version of? Or are you just calling it a goonie because you started with a Baby Bertha kit?

kj

Just because it's made from a Baby Bertha, and is not a 'normal' 3FNC rocket. It's taking a Baby Bertha and changing it into something that shares the BB shape, but is no longer related to it.

 

[The EGE]

The Mach Goon is now finished.

I've got three layers of fillets on the fins, so they should be rock solid. I glued on a thin 1/4" launch lug into one of the fins roots and filleted it.

I also attached the streamers - two 48" streamers that should bring it down at around 20 fps - a hair faster than the usual 10-15 fps under a chute, but slow enough to avoid damage and danger.

If the weather gets above 50 someday soon, I'll paint it red for visibility.

 

[The EGE]

Well, amazingly, it made it over 50 yesterday, so I went out and painted it.

It was already very smooth, and I didn't want any extra weight, so I didn't prime it. Instead, I sprayed on a light coat of yellow, a light coat of red, and then a little yellow on the nose cone - allowing all the colors to run together a tiny bit. Makes it very visible in the sky or on the ground, without sacrificing weight.

Final weight is between 80 and 90g - roughly 3 ounces - and the Cd is prolly down near 0.5 or 0.55. On a G115WT, which is 195g loaded, that means that about 70% of the rocket will be the motor. Almost 20% will be propellant alone.

Unfortunately Openrocket doesn't have CTI motors, but I'm currently creating Pro38-1G files for my RASP program, and I'll have those sims up soon.

 

[New Ocean]

The problem with fin sweep is that fin flutter occurs really easier unless all of the fin is directly onto the body tube with no sweep. Sounding rockets have thick metal fins; we do not.

I dont think fin flutter on 38mm rockets or smaller is going to be a huge issue. The rigidity of fiberglass is more than enough to handle anything mach 1.x

 

[The EGE]

I dont think fin flutter on 38mm rockets or smaller is going to be a huge issue. The rigidity of fiberglass is more than enough to handle anything mach 1.x

I don't have fiberglass. Hence why I used clementine boxes...

 

[Stymye]

say goodbye to those fins...heheh

 

[The EGE]

say goodbye to those fins...heheh

Hey. I've sent clementine box fins to Mach before. And I got them back intact.

Turns out, the 1/8" ply from the boxes is actually decent quality, and for smallish fins, the aerodynamic forces aren't that ridiculous.

If I'm saying goodbye to the fins, then I'm guessing the rest of the rocket is coming with them...

 

[UPscaler]

:y: And you think my rocket design is crazy.


This is nuts! Do you plan on recording the flight? I want to see this!



This has to be one of the best ideas ever!:clap:

 

[The EGE]

:y: And you think my rocket design is crazy.

Correct. I do believe mine is crazier, though.


This is nuts! Do you plan on recording the flight? I want to see this!

My dad's camera has video recording. Only problem, this will be traveling so fast It'll be hard to track, even with the White Thunder propellant.

This has to be one of the best ideas ever!:clap:

Thanks. I try to have good ideas. Don't always happen.:eyepop:

 

[UPscaler]

How is this thing coming?