Supesonic transition: how close is too close?

If I have a conventionally built mid to high power rocket (cardboard, light ply, epoxy, plastic cone), what is the practical limit for Vmax? How close can it get - in general terms - to the supersonic transition zone before I have to worry about it reverting to a pile parts? I'm just looking for a general guideline to reference when picking motors in the sim. 1100 fps? 1000? 950? I know that the sim won't really be accurate until I can collect some data for the design in question and adjust it accordingly, but I'm looking for a starting place.

You do not need fiberglass, carbon fiber or epoxy to go supersonic. Many fiberboard airframes with plywood fins glued with wood glue and/or epoxy have gone supersonic successfully. The issue is not strength, it's stiffness. Airframes that are not stiff enough buckle and fold over, fins that are not stiff enough flap and flutter tearing off the airframe. Plastics used at high speed, Mach 1.5 to Mach 2.5, depending on altitude, time at high velocity, and thickness, will soften due to mach heating and collapse. For example, stiffness depends non-linearly on thickness. Many rockets use 1/16" to 1/8" fiberglass fins, but you can usually get the same stiffness by using 1/8" to 1/4" Baltic birch fins. Although twice as thick, the plywood fins will have the same weight as the thinner fiberglass fins which is twice as dense as wood. These trade-offs are all part of the design process you will learn as you advance in the hobby.

Classic examples of fiberboard rockets that will handle whatever motor they are rated for are built by LOC Precision. (And there are many others from other manufacturers as well.) They can be built stock and will survive supersonic flight if they are flown on the recommended motors. One reason why folks tend to fiberglass larger rockets is for survivability during transportation as the trip to and from the launch field can be more dangerous to your rocket's health than the actual flight.

The principal advantage of fiberboard rockets is their lower cost, ease of assembly and light weight. You can frequently obtain the same flight performance with one full lower impulse class motor which cost 1/2 the price of the motor reload needed to get the same performance in a fiberglass rocket simply because it can weigh twice as much. The disadvantage is that larger fiberboard rockets are subject to transportation damage and damage on hard landings.

I don't want to sway you one way or the other, but to make you aware that you do not need fiberglass or carbon fiber to go supersonic.

Bob

That's great and all, and I appreciate the reply, Bob, but it sidesteps my question entirely. I'm not looking to go supersonic, I'm not asking what it takes to go supersonic. What I am looking for is a safe subsonic max limit to be a point of reference for selecting motors for rockets that are not built stiff/strong enough to make the transition. What is a safe Vmax for a typical basic rocket? That's all I'm asking.

Does this help?

Rick, I use 1100fps as the threshold velocity for supersonic transition. stay below that and you will be safe

EXPjawa said:

That's great and all...but it sidesteps my question entirely.

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Happens a lot around here...

Public Missiles used to have a prediction table of their rockets with various motors. Some motors were then highlighted in yellow color, indicating that extra construction techniques were needed. Not sure how they came about this, but you can sim their rockets and see the max velocity threshold where they felt that their design needed reinforcement. Much of this data is probably particular to their relatively weak plastic Quantum body tubes. Maybe they still have this table on their website, but given the bazillion motor choices these days, maintaining the table will be unwieldy.

Short answer: Any LOC or Madcow paper/wood kit is probably good to Mach 1+

BBS said:

Rick, I use 1100fps as the threshold velocity for supersonic transition. stay below that and you will be safe

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Not Exactly. Take some time researching the Transonic range. You can be sub-sonic and still damage your rocket. Jeffmhopkins' graph illustrates this point.

Buckeye said:

Public Missiles used to have a prediction table of their rockets with various motors. Some motors were then highlighted in yellow color, indicating that extra construction techniques were needed.

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Actually they still have it (usually a "Motor Recommendations" link after each rocket description): https://publicmissiles.com/images/pmlalti.pdf

EXPjawa said:

That's great and all, and I appreciate the reply, Bob, but it sidesteps my question entirely. I'm not looking to go supersonic, I'm not asking what it takes to go supersonic. What I am looking for is a safe subsonic max limit to be a point of reference for selecting motors for rockets that are not built stiff/strong enough to make the transition. What is a safe Vmax for a typical basic rocket? That's all I'm asking.

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You are assuming that there's a generic speed limit for a fiberboard airframe hobby rocket with plywood fins and a plastic nosecone, and you are assuming that there is something special about Mach 1. Neither assumption is correct, and there is no simple answer to your question.

I did not sidestep your question. I gave you the correct answer. The speed limit of your rocket depends on the actual construction of your rocket which included the materials of construction and the thickness of those materials. For the typical model or high power rocket, how you use the materials is far more important than what the materials are.

It's perhaps easier to illustrate what I mean by comparing 3 different commercial aircraft: a Cessna 172, a Boeing 737 and a F-4 Phantom. All three aircraft share identical construction: aluminum skin over an aluminum rib structure with a plastic canopy. A 172 has a never to exceed speed of 188 mph, a 737 has a never to exceed speed of Mach 0.82, and an F-4 has a never to exceed speed of M=2.23. It is the design of the aircraft, not the materials of construction, that limit the maximum speed. In these 3 examples, the reason why you can not exceed the specified aircraft speed is due to a structural or control surface failure due to flutter which is undamped vibrations that will eventually shake the aircraft or the control surfaces apart.

References:

https://en.wikipedia.org/wiki/Cessna_172

https://en.wikipedia.org/wiki/Boeing_737

https://en.wikipedia.org/wiki/McDonnell_Douglas_F-4_Phantom_II

For rocket kits, the manufacturer should supply you with a list or recommended engines or an impulse class of engines that are suitable for the rocket built according to the manufacturers directions. LOC does this by impulse class and PML does this by listing. PLM also has options with most of their kits such as thicker (stiffer) fins and stiffer airframes that permit safe operations above the flutter velocities of their stock kits.

Either ask the manufacturer if a certain rocket design is suitable for the motor you want to use, or do the calculations and simulations to determine if the rocket design can withstand the velocity of the motor you want to launch and modify the design accordingly.

Bob

Bob's absolutely right. I've abused my LOC Vulcanite which is made with carpenter's glue, with the J600. Well over mach without any issues.

It really does come down to design and construction - just try to get a Jayhawk up to mach...

cbrarick said:

Bob's absolutely right. I've abused my LOC Vulcanite which is made with carpenter's glue, with the J600. Well over mach without any issues....

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That would be ~1500 fps or Mach 1.38. According to the LOC datasheet, the Vulcanite is rated for 38 mm F thru J motors. The 3 most powerful 38 mm J motors, the CTI J530 and J600 and the AT J825 will push it to 1700 fps or Mach 1.56.

https://shop.locprecision.com/product.sc?productId=126&categoryId=12

Bob

Worsaer said:

Not Exactly. Take some time researching the Transonic range. You can be sub-sonic and still damage your rocket. Jeffmhopkins' graph illustrates this point.

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Yes exactly, I use 1100fps as my threshold if I want to stay under Mach 1, when doing my simulations. Rick asked what the practical maximum velocity is that he should consider in his simulations to not exceed mach. He sounds like he is already in the ballpark. Based on his original post, I would bet that he is aware of the transonic range. if you want to stay out of transonic velocity use <900ft/s, that's fine too, but it is not necessary if you use sound building techniques. I think that is what Bob is saying in his own way, as well.

Its how you build it . I have a Estes fat boy with a 29mm hole built with stock body tube and nose and cheap ply from Michaels for centering rings and fins. Ive hammered it with kosdon H-255 and Cti H-410 it holds up great .Got 10-12 flights on it.

Mach 1 is 757 MPH or approx. 1,110 FPS. Transonic buffeting starts at about Mach 0.8. Stresses start to increase dramatically at Mach 0.9. If you hover in the Mach 0.9 to Mach 1.1 range too long, you subject your rocket to a lot of stress. I try to keep my subsonic rockets from going faster than Mach 0.85 (640 MPH or 940 FPS).

For those that I want to break mach, I try to punch through the transonic zone as quickly as possible and shoot for top speeds above Mach 1.4 or so.

I do have a Mad Cow FG Arcas that only tops out at about Mach 1.2, but fortunately it is all FG and was built with FG cloth internal fillets so it can handle mach speeds. You can build a cardboard and plywood rocket that will break Mach 1, but if it hangs too long in transonic speeds, you risk it shredding.

EeebeeE said:

Mach 1 is 757 MPH or approx. 1,110 FPS. Transonic buffeting starts at about Mach 0.8. Stresses start to increase dramatically at Mach 0.9. If you hover in the Mach 0.9 to Mach 1.1 range too long, you subject your rocket to a lot of stress. I try to keep my subsonic rockets from going faster than Mach 0.85 (640 MPH or 940 FPS).

For those that I want to break mach, I try to punch through the transonic zone as fast as possible and shoot for top speeds above Mach 1.4 or so.

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Zactly. Well said.

Thank you Evan. That is what I was trying to get at - where in the transition does the stress begin to be an issue. That confirms what I had been thinking.

Bob, I understand what you're saying - and I do appreciate the information - but again, considerations for how to survive mach isn't really what I was getting at. I'm well aware that structure and build make a big difference. FWIW, I'm mostly not taking about airliners or jet fighters here, most of my fleet could characterized as Cessnas. Make some of them Cardinals, Skymasters and an occasional 195 as well for variety. I was perhaps not clear on that initially, my apologies. Everything I've got in the mid-power range are either PSII kits or scratch build designs with similar structure. I haven't built anything from LOC, Madcow, etc.

But my intent is essentially to put a practical cap on max velocity for rocket when I don't know how well will handle the transition. My thinking at this time is that if I don't have enough experience with pushing mach to know (yet) if a given model is built well enough to survive it, then I want to play it safe. As I learn more, I'll do more. Perhaps my Leviathan will be OK, but I bet my modified (extended 9", TTW 3-fin, 29mm) Mean Machine would not. Until I better understand where that demon lives on the mach meter, I wanted a rule of thumb to act as a guide. That's all I was really asking for in this thread.

Gee, guess i need to dust off my rocket, now 8 years old and give it a J828. Can't be a mildman......

Rick:
You really need to scrap the estes tubing. It's really not a HPR material. It survives the baby H's but that's it. I think that's the demon near the mach meter.

See you at URRF!

EXPjawa said:

Thank you Evan. That is what I was trying to get at - where in the transition does the stress begin to be an issue. That confirms what I had been thinking.

Bob, I understand what you're saying - and I do appreciate the information - but again, considerations for how to survive mach isn't really what I was getting at. I'm well aware that structure and build make a big difference. FWIW, I'm mostly not taking about airliners or jet fighters here, most of my fleet could characterized as Cessnas. Make some of them Cardinals, Skymasters and an occasional 195 as well for variety. I was perhaps not clear on that initially, my apologies. Everything I've got in the mid-power range are either PSII kits or scratch build designs with similar structure. I haven't built anything from LOC, Madcow, etc.

But my intent is essentially to put a practical cap on max velocity for rocket when I don't know how well will handle the transition. My thinking at this time is that if I don't have enough experience with pushing mach to know (yet) if a given model is built well enough to survive it, then I want to play it safe. As I learn more, I'll do more. Perhaps my Leviathan will be OK, but I bet my modified (extended 9", TTW 3-fin, 29mm) Mean Machine would not. Until I better understand where that demon lives on the mach meter, I wanted a rule of thumb to act as a guide. That's all I was really asking for in this thread.

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Careull with a mean machine its easy to fold that thing up. I would not go much past a F-40

The Mean Machine flies well on F23-7FJ SU motors. I haven't tried to overpower it, since that's enough for around 1000' and the associated walk to retrieve. That was just given as an example.

The stuff I've built with standard BT tubing isn't intended to go that fast or be high powered. I do think that the PSII tubing is a big improvement, though the stuff I've done (or am planning) for proper high-power primarily uses LOC, or in a couple cases AT, tubing. But still, I'm just trying to feel out the line between mid and high power. I have no plans to abandon Estes tubing, but keep it in its place. To be fair with regards to that, I still scratch build a lot of stuff that is safely low or mid powered and don't really have a place in this discussion, they're more intended to be scale or unique designs, not really high/fast fliers. As I do more designs that are high-power, I'll keep Bob's advice in mind and build accordingly.

For what it is worth, I read the question the same as Bob and had a similar answer sitting in my brain. What is the subsonic limit of cardboard and plywood, well, there is none because it is supersonic.