Don't Debate This, Too

Discussions on mid/high powered model rockets using F powered motors and above.

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Passaretti
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Don't Debate This, Too

Postby Passaretti » Sat Apr 28, 2018 10:10 am

The goal is to fly and safely recover a minimum diameter CTI N5800 rocket in a single-use composite motor case.

The target launch is BALLS 27, 21-23 September (147 days left). This project has been in the works since returning from BALLS in 2014. I've been working on it on and off since then. Unfortunately, due to work, I haven't been able to commit to a BALLS launch, but that's FINALLY not the case this year. Work on the project has increased steadily this past year, thanks to a more manageable work schedule and having a home workshop. I feel I am on a good pace for September. The stars are aligning!

Why am I doing this?
A) It's cool (Does there need to be more of a reason?)
B) It's not going to be easy,
C) I think there's quite a bit more altitude that we can squeeze out of this motor.

A big question has always been - How high or fast can the N5800 go? Well, the answer is, relatively speaking - Really high and really fast ... Exactly how much? Well, at least 63,818 ft AGL, that much we know. But the consensus seems to agree there's at least a little more left :) After my flight in 2012, Nic's in 2013 and Matt Orsak's (big props to this guy!) just this past year it's clear that what once seemed impossible, is totally possible, repeatable and there's still room for improvement. Matt's most recent flight, a more optimized version of Nic and my project squeezed out another 9,000 feet or so. Is that the limit? Of course not. So let's take what we've all learned and push that limit.

The question I asked myself following my 2012 flight was - What could be gained by eliminating my fin can? Most cans have at least 0.125" or better wall thickness. The total cross-sectional area of the rocket is increased significantly by a can or a body tube. In this case for a 98mm motor, you're talking about 10-12% reduction in area by eliminating this part which will have a significant effect on total drag. But to get rid of the can or body completely, you are committing to attaching your fins directly to the motor case. More on this in a later post ...

The second question I asked myself was, where could I shed weight - lots of weight. It's no secret that I was conservative with my 2012 design. I knew I was heavy, and I didn't care. It was never about speed or altitude for me. I wanted the damn thing to work and knew that I could make improvements later. So here I am. Where could I lose significant weight, bearing in mind the ever important rule of M2+ flights - If you don't respect and manage your CP shift - You go sideways. And nothing survives. And it's sure fun to watch. Oh, is it fun to watch. More on this in later posts ...

To answer both of these questions, I started looking hard at the aluminum motor case. If I switched to a composite case, I could drop significant weight. CF composite is about 40% less dense than Al6061. A composite case also opens up a number of possibilities for directly attaching much higher performance fins. But ... if you'd don't know what you're doing. You're asking for a world of hurt. So, I've been doing a lot of reading, and a lot of material and construction tests ... a lot!

To make life simpler a little bit simpler, I decided very early on that this was all going to be a one-shot single-use project. That may sound crazy to some, but I asked myself, how often do I really get to fly these types of projects? So far, on average the answer is it's once every two years. A lot can change in two years, so why try to get a second shot out of it? There's a really good chance I'm not going to see the rocket again, and an even better chance it's not going to be in flying condition if found. So to me, it was a no-brainer, single-use it is. That's my logic at least, and it changed the way I think about the design. Reusability features add dead weight...

I've got a lot of catching up to do in terms of explaining what I've been doing so far, so please bear with me over the next few weeks. I promise to share as much as I practically can, but will likely focus on the areas that I think I have the most issues or questions. My progress has increased significantly because I now have a home workshop that I've steadily been building out over the past year. So here are a few pictures of where I'm doing most of my work. In my next post, I'll get into the details of the overall rocket and where I'm focusing my time.

Mike

My ever densifying single car garage ... This was before the lathe and dust collector were added. The addition of the CNC router has changed everything ...
20170528_213610_.jpg


20171021_182749_.jpg


20171021_182812_.jpg


Turning a hard maple NC positive. This is a 7.5:1 Ogive.
20180115_210758_.jpg


Post-layup (wet lay-up followed by vacuum bag), post-grinding and sanding, getting ready to separate from the mold. The NC is made up of about 15 layers of 6oz PW E-glass, cut in long tapering strips, laid at 45/-45 for conformability. I'm using a 309 deg F Tg laminating resin. Super slow cure and can be post-cured to about 300 deg F per the manufacturer. This is not the resin I will use for the final parts; I think I've found something better, especially for the motor case. A lot more on this later ...
20180422_145957.jpg


First NC shell and set of test CF fins. The fins were vacuum infused and consist of 9 layers of 6 oz PW 600KSI CF alternated 0,90;45/-45 + Soric Core + 9 more layers of CF.
20180422_210458.jpg


More details to follow, but this is the general design.
Screencap.JPG
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Re: Don't Debate This, Too

Postby SpaceManMat » Sat Apr 28, 2018 11:28 am

8) awesome, can’t wait for this to fly.

So Soric core is used to lighten the fin. How thick is the core and how thick is the fin? Also I assume core is smaller than fin as you don’t want to expose edge of core to supersonic flow. Can you please explain their construction?

Composite case = experimental motor so unfortunatetley can’t do that in Aus. Without all that weight it’s going to go much faster but also making lots more heat, so those fins are going to take more punishment then ever.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

Passaretti
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Re: Don't Debate This, Too

Postby Passaretti » Sat Apr 28, 2018 10:08 pm

Hi Matt,

In this case, these are test fins and won't be going very fast (if at all). Their main purpose is to test infusion technique, measure how much the materials consolidate, be used for mock-ups of the rocket and possibly for low-altitude test flights to test the recovery system.

I haven't decided if I am going to use a core in the final version of the fins. It just might not be worth it and there may not be enough room considering all edges serve an important role and need to be structural. Namely the root edge (structural attachment point to the motor) and the leading edge which is going to be facing the "breeze". :)

My nominal approach for the fins is to go solid CF about 0.2-0.25" finished thickness, airfoil all exterior edges, use a thin SS sheet metal cap on the leading edge. Simple, effective and leaves me more time to focus on the biggest unknown, which is the motor case.

Cutting the 45/-45 layers of CF. 0/90 layers not shown.
20180420_174343.jpg


This is the Soric. I'm using 2mm the Soric. The uncompressed thickness was actually closer to 1.5 mm. However, this, like most materials compress under vacuum, which is another reason I tried it in this plate to measure exactly how much. It turns out that by putting a caliper on the dry material, and squeezing lightly, you can get a rough idea of how much it's going to compress in the part. This is obviously a bit subjective, but it gets me pretty close. Based on this, I assumed about 30% compression for all materials (18 layers CF @ .0115" thickness each + 1 layer Soric @ .067 " thick). I was aiming to make a plate about 0.2 ish inches thick. Compressed, this ply stack should have given me a plate 0.19 inches thick or so.
20180420_182333.jpg


Post-infusion and initially curing at about 90F (using a heat mat) overnight.
20180420_201031.jpg


I just barely squeeze all four fins out of this plate. I make this mistake all the time!
20180422_205109.jpg


From the edge you can see the layers pretty well. The consolidation and finished parts come out fantastic with vacuum infusion. It's a bit of set-up but it's certainly worth it for certain parts. Very easy to do for plates like these. The final thickness came out a few thousandths under 0.19 inches, pretty much spot on with my estimate.
20180422_205125.jpg
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Re: Don't Debate This, Too

Postby OverTheTop » Sun Apr 29, 2018 5:24 am

Welcome back Mike! Hadn't heard from you for a while and I had wondered what you were up to.

Looks good so far. Looking forward to see where you get to.

Have you considered using tapered fins, thicker at the root and thinning towards the tip?
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Re: Don't Debate This, Too

Postby CATO » Sun Apr 29, 2018 9:07 am

Thanks for sharing Mike looks awesome, I'm sure it will be a great flight (as always). please keep us updated.....

Passaretti wrote:To answer both of these questions, I started looking hard at the aluminum motor case. If I switched to a composite case.


:D :D :D :D :D :D :D :D :D :D

2012-06-30 21.01.45_sm.jpg
2012-06-30 21.01.45_sm.jpg (59.63 KiB) Viewed 1797 times
"In thrust we trust"

AMRS 21 L3 RSO
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2019: 14,725 Ns (44% N)
Ns 18: 14,767; Ns 17: 5,973; 16: 34,558; 15: 35,955; 14: 6,016; 13: 10,208
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Re: Don't Debate This, Too

Postby SpaceManMat » Sun Apr 29, 2018 12:55 pm

OverTheTop wrote:Welcome back Mike! Hadn't heard from you for a while and I had wondered what you were up to.

Looks good so far. Looking forward to see where you get to.

Have you considered using tapered fins, thicker at the root and thinning towards the tip?


I’m with OTT on tapered fins. I think Ape offeres CNC cut diamond profile fins, of course no idea how it would hold up to Mach 3+ airflow.

Speaking of which you mentioned that you would go with protected leading edges ala croscum style. However Matt’s flight did not use this method. Given the lack of damage to Nic’s paint I’m thinking it would standup to this level of punishment.


CATO wrote:Thanks for sharing Mike looks awesome, I'm sure it will be a great flight (as always). please keep us updated.....

Passaretti wrote:To answer both of these questions, I started looking hard at the aluminum motor case. If I switched to a composite case.


:D :D :D :D :D :D :D :D :D :D

2012-06-30 21.01.45_sm.jpg


Here’s an eye opener of test to consider..
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

Passaretti
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Re: Don't Debate This, Too

Postby Passaretti » Tue May 01, 2018 8:00 am

What can I say - Some people are just ahead of the curve ... It might take me a while, but I eventually come around :)

CATO wrote:Thanks for sharing Mike looks awesome, I'm sure it will be a great flight (as always). please keep us updated.....

Passaretti wrote:To answer both of these questions, I started looking hard at the aluminum motor case. If I switched to a composite case.


:D :D :D :D :D :D :D :D :D :D

2012-06-30 21.01.45_sm.jpg
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Passaretti
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Re: Don't Debate This, Too

Postby Passaretti » Tue May 01, 2018 8:20 am

Thank you OTT.

I have so much to share, so I'll be playing catchup for a while ... Hopefully, I'll get caught up on the forum by September!

I have considered tapered fins, mainly for the benefits of reducing mass and increasing the stiffness of the whole fin. At this stage, it's certainly on the table but I'm not convinced it's necessary. Right now, the designed semi-span of each fin is about 3.5". The fins should be 0.2-0.25" thick nominally. The structural fillets will be at least .75" or so high which should make for a very very stiff fin. I haven't run it through FinSim yet, but I plan on doing so as soon as I can figure out how to use the program correctly. :)

It certainly seems reasonable, but for now, the baseline is solid carbon w/ metal leading edge. Nice and easy and lets me focus on the motor case with is a whole different ball game for me.

OverTheTop wrote:Welcome back Mike! Hadn't heard from you for a while and I had wondered what you were up to.

Looks good so far. Looking forward to see where you get to.

Have you considered using tapered fins, thicker at the root and thinning towards the tip?
Mike Passaretti, TRA 5369

Passaretti
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Re: Don't Debate This, Too

Postby Passaretti » Tue May 01, 2018 8:54 am

I have seen this video, Matt. In fact, I received quotes from several composite driveshaft manufacturers who are set up well to engineer CF Filament Wound tubes. I'm just about convinced that FWCF is the way to go to properly make rocket motors, however, the upfront costs are too high for me. So it's time to get creative ... Much of the tests I've been doing have been in this area.

Here is a really important question that I don't know the answer to and feel is critical to the path I need to take. This one has been keeping me up at nights, and I've been thinking about it from all the angles I can think of. It would be great to find some literature on the subject ... but I haven't been so lucky yet. Let me know what you guys think...

Q: Does the rocket motor casing need to be gas-tight AND withstand the forces exerted by the motor, OR does it only need to withstand the latter?

The N5800 (according to CTI) has a chamber pressure of 800 PSI. From here, you can calculate the hoop stress and axial stress created by this pressure in a thin wall pressure vessel. From there, you need to ensure that the material strength of pressure vessel (the motor case) exceeds that by an appropriate margin. Relatively easy math to do, and in practice, there are other things to consider, but let's just call this Step 1. Can my tube hold the forces generated by the pressure?

CompCalcs.JPG


However, in practice building, a composite pressure vessel that is also gas-tight is a totally different challenge. If the case only needs to withstand the forces and not be gas-tight at the motor pressure, that changes things drastically and for the better in terms of the construction.

Thoughts?



SpaceManMat wrote:2012-06-30 21.01.45_sm.jpg


Here’s an eye opener of test to consider..
[/quote]
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Re: Don't Debate This

Postby SpaceManMat » Tue May 01, 2018 10:38 pm

Passaretti wrote:Q: Does the rocket motor casing need to be gas-tight AND withstand the forces exerted by the motor, OR does it only need to withstand the latter?


Ok here’s my thoughts on this. The gas inside the the motor is being expelled at a rapid rate. So at least in part the thrust being applied to the motor is being physically supported by the pressure inside the motor. As the gas exits the motor it undergoes expansion and this expansion results in thrust being applied to the nozzle as well. However the internal pressure trying to push the rear closure down greatly exceeds the external pressure of the thrust being applied to the nozzle, so the net stress on the rear closure is slightly lessened by the thrust. Another way to look at this is to think about what happens when there is a failure. In a rear closure failure you never see the rear closure being forced up into the motor from the thrust, the pressure always wins and the closure gets blown into the ground. So rear closure only needs to withstand internal pressure.

Forward closure does not produce thrust, however if it buts up against a bulk head in the rocket then inertia from the rocket will in part push back against the closure and lessen the stress, but I would not count on it. I’m thinking this should be rated for pressure plus thrust although reality is it will be less than that.

The side walls are a bit different in that pressure acts radially where as thrust acts latterly (but only the proportion of the thrust acting on the rear closure not the forward closure).

Hope this makes some sense.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

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Re: Don't Debate This, Too

Postby OverTheTop » Wed May 02, 2018 3:23 am

However, in practice building, a composite pressure vessel that is also gas-tight is a totally different challenge. If the case only needs to withstand the forces and not be gas-tight at the motor pressure, that changes things drastically and for the better in terms of the construction.


If you are working with composites, especially at the performance edge, process consistency is king. Most reliable testing methods for composites rely on destructive testing and statistics for a reliable outcome. You don't want a nice CF panel with debonding or a significant void internally. Think like marine plywood vs class E ply. Voids are minimised and must not overlap in the marine. Of course they have the luxury of looking at the plys when they do the stackup. You have to rely on consistency and repeatability. Consistent materials, quantities, temperatures, times and sequences should be documented and repeated. Don't rely on memory, since based on my experience it is easy to forget the minute details.

Should be achievable :wink:
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Passaretti
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Re: Don't Debate This, Too

Postby Passaretti » Wed May 02, 2018 7:55 am

OverTheTop wrote:
If you are working with composites, especially at the performance edge, process consistency is king. Most reliable testing methods for composites rely on destructive testing and statistics for a reliable outcome.


100% in agreement here. I spent part of last year getting set-up to hydrostatically test tubes for this reason and, given all the recent trouble, I am also considering building split-ring test set-up per ASTM D2290 to test a lot of samples quickly for hoop strength. For those interested in D2290 - Check out this paper:

http://mtmcongress.com/proceedngs/2015/ ... 0PIPES.pdf

Getting ready to hydro test my first test tube was how I uncovered the first serious problem last fall. The first test tube (Test 1) didn't even hold a 12" tall column of static water pressure. It was a humbling, eye-opening experience, to say the least. This tube was constructed last summer using wet layup, roll-wrapped CF fabric 8 layers or so. The layup was not consolidated. The result was basically what a screen door on a submarine would look like ...

Yes, I'm using a grease gun with a check valve to create hydrostatic pressure. No, I was not going to put 1600 PSI into the tube in my sink. This was a test to make sure I could get a few PSI in before committing to the full test (not in my sink).

I had high hopes that my first tube would hold pressure. Turns out I wasted my time making and installing plugs which were meant to mimic the bond area for my rear closure (approx 1" axial length).... The water droplets on the outside of the tube are not the leaks.
20171022_083058.jpg


The leaks can be seen at the bottom of the tube. The water just pissed out ... The weight of the water in the tube created enough pressure to cause the leaks.
20171022_083254.jpg


Since then, I have had two more tubes fail in a similar manner (did not hold static water pressure). Both of the tubes were wet-layup roll wrapped CF fabric 5 layers for the first one and 10 layers for the second. They were both compressed using perforated, release coated heat shrink tape (I thought this would solve the problem from Test 1). The 5 layer (Test 2) failed "screen door" style; the 10 layer (Test 3) also failed the same. Both test tubes 2 & 3 were held at 120deg F or for 12 hours to encourage the epoxy to flow and initially cure more predictably. More layers didn't help. Shrink tape for consolidation also didn't seem to help.

This is post wet-layup, post heat shrink tape. You can see the excess epoxy bleeding through the perforations in the shink tape.
20180318_173444.jpg


So I said, OK, wet-layup is introducing too much air and I'm not consolidating the fabric enough to push out the air and close voids with the shrink tape. My next attempt was to try to try vacuum infusion. In theory, vacuum infusion should improve consolidation and drastically reduce voids.

The next tube (#4) was constructed by roll-wrapping about 15 layers of CF fabric and vacuum infusing the part. Immediately, after applying vacuum, I noticed the fabric creasing badly, which was disappointing, to say the least. I anticipated this problem to some extent and tacked the fabric in place every revolution as I was tightly wrapping it with infusion approved spray tack to hold it in place. I proceeded with the infusion, knowing I likely would not get a useable part out of it. The creases were so bad I did not bother pressure testing it as they caused long dry spots which inhibited fusion.

Getting ready to do build the test tube using infusion.
20180407_131532.jpg


Dry wrapping the CF on a section of motor liner with two layers of PVC barrier layer/mold surface treated with release wax.
20180407_132758.jpg


Almost ready to apply vacuum.
20180407_142848.jpg


This was taken during infusion, the resin is moving left to right. It may be hard to see, but the creases are there... bummer.
20180407_150725.jpg


This is post grinding after I took off all of the nasty creases to get a semi-round OD.
20180409_145850.jpg


Dry spots line up with creases ... no surprise.
20180409_145910.jpg


Losing confidence in using CF fabric, I decided to start doing some tests with manually wrapped (90 degrees) CF tow and vacuum infusion. So far, I've been able to get one very thin wall 12K tow wound tube to hold static water pressure. It's a start! My next step is to wind a longer tube current thinking is to use one or two layers of tow cured, followed by 1-3 layers of CF also cured, followed by 1-3 layers more, etc. until the desired wall thickness is achieved (0.12") or so which will be ground down to the final thickness.

Hand wound 12K tow around a piece of 98mm liner tube with a PVC barrier layer/mold surface treated with release wax.
20180422_092629.jpg


Infusion in process. The tow wets out too slow as there aren't many voids like in fabric for the resin to penetrate. I don't think it's a good idea to infuse it and will likely try wet-layup next.
20180422_104729.jpg


The entire bag while still under vacuum was wrapped with flashing tape to mitigate the bag leaking and releasing during the overnight cure at 120F
20180422_113614.jpg


It's not pretty, but it's a start.
20180501_173641.jpg
20180501_173641.jpg (29.52 KiB) Viewed 1754 times


There are a few dry spots in the two even with infusion. I think that doing wet-layup tow followed by vacum bagging or shrink tape is the way to go.
20180501_173649.jpg




Should be achievable :wink:


The question still remains OTT, does the motor case need to hold gas pressure? I'm hedging my bets here but continuing with these tests, but I'm hopeful the answer is no. Time and testing will tell!
Mike Passaretti, TRA 5369

Passaretti
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Re: Don't Debate This

Postby Passaretti » Wed May 02, 2018 8:07 am

SpaceManMat wrote:
Passaretti wrote:Q: Does the rocket motor casing need to be gas-tight AND withstand the forces exerted by the motor, OR does it only need to withstand the latter?


Ok here’s my thoughts on this. The gas inside the the motor is being expelled at a rapid rate. So at least in part the thrust being applied to the motor is being physically supported by the pressure inside the motor. As the gas exits the motor it undergoes expansion and this expansion results in thrust being applied to the nozzle as well. However the internal pressure trying to push the rear closure down greatly exceeds the external pressure of the thrust being applied to the nozzle, so the net stress on the rear closure is slightly lessened by the thrust. Another way to look at this is to think about what happens when there is a failure. In a rear closure failure you never see the rear closure being forced up into the motor from the thrust, the pressure always wins and the closure gets blown into the ground. So rear closure only needs to withstand internal pressure.

Forward closure does not produce thrust, however if it buts up against a bulk head in the rocket then inertia from the rocket will in part push back against the closure and lessen the stress, but I would not count on it. I’m thinking this should be rated for pressure plus thrust although reality is it will be less than that.

The side walls are a bit different in that pressure acts radially whereas thrust acts latterly (but only the proportion of the thrust acting on the rear closure not the forward closure).

Hope this makes some sense.


Thanks for the input Matt. While I haven't tried it yet, I'm more concerned about whether the side wall of the case has to hold gas pressure. The closures should be easier to seal, especially given that I am taking a single-use approach and can bond them however I see fit to get a structural and gas-tight bond.

Here's something else to think about - Look at a 98mm CTI reload assembly. The forward seal disc and the nozzle are not sealed gas-tight to the liner. While forward seal discs are pre-bonded on, it doesn't appear to be done with much care for gas tightness. Most, if not all reloads are like this and the N5800 is definitely like this. Which tells me that the liner/nozzle or seal disc don't have to be absolutely gas-tight. However, the case keeps everything in compression and keeps it sort of tight? If it needed to be gas-tight, wouldn't there be an o-ring or other type of seal in place?
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Re: Don't Debate This, Too

Postby SpaceManMat » Wed May 02, 2018 8:35 am

The liners job is just to insulate the case isn’t it and keep the grains in place? Case and closures must keep gasses must keep gasses in otherwise you have 3000 degree air acting like a cutting torch. Thinking of smaller loads that I’m more farmilar with, while those do appear to use the liner as a seal I don’t think this is really what is happening / critical except at the ends where they combine with the closure to seal against the case. It’s normal to see melting of the liner but never at the closure where it’s protected.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing

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Re: Don't Debate This, Too

Postby SpaceManMat » Wed May 02, 2018 8:56 am

If you’re thinking of winding then it wonuld not be too difficult to build your own machine for this.
QRS: 124
AMRS: 32 L2 RSO
Highest Altitude: 13,647 feet
Fastest Flight: Mach 1.55
Largest Motor: CTI 1115J530 IM
Current Project: X Wing


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