HPR nose cone designs

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elihunter

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I'm just getting back into model rocketry again after about 20 years and am looking to use some of my engineering and physics education to do something big. Since I'm just starting out at the bottom I have a long way to go (hoping to get my level 1 cert next weekend on a PML 1/4 scale patriot) but I have a few questions about areas where it seems things could be improved.

My ultimate goal is to go for high altitude attempts. In reading quite a few threads on here I notice the N5800 motor seems to be a difficult motor to tame. Quite a few of these threads seem to focus on issues with the nose cone not being able to withstand the heat due to mach 3+ flights.

My question is why use the Von Karman nose cone with most of these? It's designed to minimize drag, why not go for something that would create a bow shock farther from the nose cone and avoid some of the heating effects? (See https://history.nasa.gov/SP-440/ch6-2.htm for a little more info) You wouldn't go as high or as fast, but would have a much better chance at the rocket making it I'd think.

If somebody has links to people that have tried this I'd love to see. Most of what I find for high power altitude and speed attempts seem to be doing the same thing and just trying to reinforce more and more so I'm wondering if I'm missing something obvious here.

Thanks in advance for any insights... hope this generates some good discussion.
 
Quite a few of these threads seem to focus on issues with the nose cone not being able to withstand the heat due to mach 3+ flights.
You raise an interesting point about nose cone shapes, but I don't think there's a lot of solid evidence that aeroheating is as much of a problem as some have suggested. Most sounding rockets use conventional ogive nose cones. Reentry is a much longer/higher-energy heating event than ascent.

You wouldn't go as high or as fast, but would have a much better chance at the rocket making it I'd think.
If you just want to survive a motor, you can make the rocket really heavy, but why use such a motor in the first place?
 
Bare Necessities uses a ~7.2:1 conical nose cone for reduced drag.

For CCotner and I, it's not about the rocket surviving. It's about surviving and going really, REALLY high.
 
I think you are misinterpreting the article. It is about reentry heating, not ascent heating. A von Karman nosecone is used because it has the lowest ascent drag. When you lower the drag you obtain less aerodynamic heating, not more, and more importantly higher altitude.

Nuclear RVs are typically low diameter conical shapes because they are designed for maximum unguided accuracy, to delay the onset of laminar to turbulent transion which reduces accuracy for unguided ballistic projectiles, and have high sectional density so not to slow down as they re-enter, but they suffer massive tip recession in the process which is exactly what hobby rocket folks are trying to avoid.

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

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

The maximum velocity that your rocket will reach is determined by the mass fraction of propellant in the vehicle, and the specific impuilse of the propellant. (Specific impulse in the simplest explanation is push per pound of fuel, and is equivalent to mpg in a car.) On ascent from earth through the atmosphere there are two forces that reduce this value: gravity loss and drag loss. You can't do much about gravity loss: either find a way to make the vehicle lighter and increase the propellant mass faction, or use a more efficient propellant with a larger specific impulse, both of which have limits. You can do a lot about drag: use a vehicle shape that provides the low drag, and design the thrust profile to reduce the velocity in the lower atmosphere which is denser, which greatly reduces drag. In model rocketry it is relatively easy to reduce vehicle drag by using the minimum diameter and drag shape possible, but the burn times are short so there is not much flexibility in changing the thrust profile dramaticly. There is however an optimum thrust for every rocket design that provides the highest apogee: more thrust generates a higher drag because drag losses are the time integral of 1/2*Cd*rho*A*V2 which lowers apogee while a lower thrust increase the gravity loss which is m*g*t which is the time integral of the weight of the rocket.
https://www.rocketmime.com/rockets/studies.html explains this.

Check out https://www.rocketmime.com/rockets/rckt_eqn.html for all the math involved in rocket flight.

Bob
 
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I don't think there's a lot of solid evidence that aeroheating is as much of a problem as some have suggested.

After a couple of weeks reading the forums and elsewhere it looked to me like the biggest problems seem to stem from mach 2+ flights destroying either the nose cone or the fins. That in turn seems to be because most motors available burn quick. So it seems a combination of building the rocket to survive the lower atmosphere at the expense of additional drag and using a 2 stage setup with a slow burning booster motor would get around a lot of these problems.

Are there other common problems I should be focusing on as well?


If you just want to survive a motor, you can make the rocket really heavy, but why use such a motor in the first place?

I would think adding some drag is a bit different than adding mass when going for altitude attempts. Mass is with your rocket the whole flight, the aerodynamic drag lessens as you reach the upper atmosphere. So while I don't disagree with you I'm wondering if the compromise on drag is worth it so you can protect the rocket in the lower atmosphere.


Bare Necessities uses a ~7.2:1 conical nose cone for reduced drag.
Thanks for this, looking over their stuff.

Is there a list anywhere of some of the high altitude attempts? I've been reading over projects I could find from university attempts and some others but the ones I've found seem to focus less on aerodynamics and more on material strength to survive the flight. But maybe that is because they're going for altitude and just trying to reduce drag as much as possible.
 
After a couple of weeks reading the forums and elsewhere it looked to me like the biggest problems seem to stem from mach 2+ flights destroying either the nose cone or the fins...
Some nose cones (typical commercial fiberglass layups) are probably not capable of surviving high mach. I've seen a lot of speculation that FWFG cones have come apart from heat, but I don't think I've seen a photo or any solid analysis, only conjecture.

Fins are vulnerable to flutter and delamination, and those will be worse the faster you go in denser atmosphere.

https://www.aeropac.org/100k/ is a nice description of a two-stage long-burning altitude rocket, with plenty of construction details. They used a conventional aluminum-tipped FWFG nose cone.
 
Thanks for all the great responses and links, lots of stuff for me to go through now.


A von Karman nosecone is used because it has the lowest ascent drag. When you lower the drag you obtain less aerodynamic heating, not more, and more importantly higher altitude.
Reentry or ascent - a more spherical shaped nose cone would help the heat issue by both moving the shock wave farther out from the nose as well as spread the heating over a larger surface area rather than concentrating at a small point. I've seen several references to using aluminum tipped nose cones to withstand the heat where it's suspected the junction of the fiberglass and aluminum tip is a failure point.
Ideally I'd just go with the lowest drag design but if the materials don't stand up to that reliably it seems the next best thing is to make some compromises.

https://www.rocketmime.com/rockets/studies.html has some really good info. I was planning on making up some excel spreadsheets to do basically what he's done.


Aeropac's was one of the first documents I'd read over on high altitude rockets. They did it with an aluminum tipped FWFG Von Karman nose cone so maybe I'm focusing too much on trying to "fix" something that's not broken.

I guess it's hard with model rocketry to get good data on failures unless the rocket survives mostly intact. The last thread I was reading was Manny's attempt where he was guessing nose cone heating contributed to the failure. Does anybody know if people have taken temperature measurements while in flight? Maybe it's easier just to use CFD modeling.

I'll do some of my own calculations over the next few days mostly focusing on the nose cone. I just wanted to see if I was headed down a path that had already been looked at or was completely wrong. I still think there may be something to it, I'll update this with some real numbers in a few days.

Thanks again for everybody's input.
 
Reentry or ascent - a more spherical shaped nose cone would help the heat issue by both moving the shock wave farther out from the nose as well as spread the heating over a larger surface area rather than concentrating at a small point. I've seen several references to using aluminum tipped nose cones to withstand the heat where it's suspected the junction of the fiberglass and aluminum tip is a failure point.
Ideally I'd just go with the lowest drag design but if the materials don't stand up to that reliably it seems the next best thing is to make some compromises.

https://www.rocketmime.com/rockets/studies.html has some really good info. I was planning on making up some excel spreadsheets to do basically what he's done.


Aeropac's was one of the first documents I'd read over on high altitude rockets. They did it with an aluminum tipped FWFG Von Karman nose cone so maybe I'm focusing too much on trying to "fix" something that's not broken.

I guess it's hard with model rocketry to get good data on failures unless the rocket survives mostly intact. The last thread I was reading was Manny's attempt where he was guessing nose cone heating contributed to the failure. Does anybody know if people have taken temperature measurements while in flight? Maybe it's easier just to use CFD modeling.

I'll do some of my own calculations over the next few days mostly focusing on the nose cone. I just wanted to see if I was headed down a path that had already been looked at or was completely wrong. I still think there may be something to it, I'll update this with some real numbers in a few days.

Thanks again for everybody's input.

As much as detaching the shock from the nose reduces the heat load, though, it also hugely increases drag. Most of the time on these minimum-diameter N5800 attempts the goal is for super-high velocity which isn't comaptible with a high-drag nose.

The Aeropac flight used very low-thrust motors. Their first-stage N1100 has almost four times the burn duration of the N5800, and a wonderful burn profile for such things - it's nice and regressive, kept the speed low even though it put out 14,000 Ns. Their sustainer was also a slow-burner. By keeping the burns slow they kept speed down until they were high enough to get air density down. This really doesn't translate well to the N5800, which is more of a 3.5s kerblammo which gets you going fast RIGHT NOW.

Temperature telemetry on an N5800 flight would rock out loud - but the dynamics of model rockets doesn't lend itself well to that. First off, most of these flights end badly. Electronics tend to get turned into chunky salsa when the airframe they're involved in disintegrates at greater than the speed of sound - to say nothing of their 'meeting' with the desert floor. If you want to guarantee good telemetry, you'll need to send it to the ground in real-time. This will require a lot more than simply a data logger - you'll need the transmitter, a decent microcontroller (or small computer) to get the raw data stream from the DAQ unit into something you can transmit reliably, and then you'll need all the equipment on the ground. All this equipment is expensive and will add significant weight - the other big enemy of high-mach flights.

Personally I'd love to see, say, ten thermocouples along the nose cone. There would definitely need to be one at or as near as possible to the tip, then spread the others relatively evenly. There's no need for more-accurate numbers than a thermocouple can give you here - +/- 1 degree isn't that big a deal when you're dealing with thousand-degree airflows. I would worry about how to mount them, though. They need to be as close to the surface as possible to minimize heat transmission errors (which are enormous in highly dynamic situations like a high-mach model rocket, when encased in highly-insulating materials like fiberglass). You can't just drill wells into the surface of a FWFG cone without horribly compromising the structure, so it would either need to be a custom cone or a commercial cone with a thick coating of something heat resistant (and it can't be ablative - if the coating goes away you lose the thermocouple!). You could then add more T/Cs along the body, at the fin leading edges, and wherever else. You just have to keep them isolated from the 'rip the probe off the surface' airflow while close enough that they give you numbers that are useful. Better measurements of surface temperature are optical - which means they're going to be HARD on something moving at mach 3, much less something small and over a mile away.

CFD on something like this would be pretty awesome, but remember it's limitations. Unless you know the boundary conditions, and have a test case where you know what to shoot for, the simulation will basically say whatever you want it to. A combined fluid-thermal model with composite materials is going to be pretty finicky, too, without some sort of experimental validation. It's great for rough estimates before building, though, so you can use it to get an idea where to start. Frankly, I'd love for someone with the resources to do some wind tunnel testing of potential composite cones and fins.

EDIT: And if you know how to work the supersonic flow solver in MAYA/TMG, I'd love to know more - I use it all the time for flow/thermal models of heat exchangers, fluid heaters, and related as part of my day job, but I haven't seen much on how to use the supersonic flow functions.
 
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Alex,

My first thought when I started looking into all this was the thrust profile of the available motors doesn't seem to be a great fit for high altitude flights. I decided I didn't want to get into making my own motors so the goal became optimizing the rocket's design to work with what's available.

I was already planning on doing real time telemetry so that one's no surprise. I expect to lose a few rockets and I'd want the data from those failures to help figure out what went wrong. I'm thinking of using an arduino on board and am planning on getting my ham license in another month when the test is available in my area.

I was actually thinking about trying to take temperature measurements from inside the nose cone, of course the data would be significantly better on the outside skin but something is better than nothing. If you just have a fiberglass nose cone recording temperature points on the inside you should be able to back out some useful data for at least the total heat energy absorbed by the nose cone.

Another option is an aluminum tipped nose cone with thermocouples in the tip, I would imagine this would give at least some useful data for what's happening at the tip.

Something like paint that changes color at different temperatures could also be used, that actually could give some very useful data for the entire rocket. A quick search found https://www.racerpartswholesale.com/category/Genesis_Brake_Temperature_Paint

Sorry, no experience with MAYA here. I just took a CFD class last semester so it's not like I'm awesome at it, but I love challenges like that and am certainly up for modeling some of this, I'd just be slow at it.
 
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As much as detaching the shock from the nose reduces the heat load, though, it also hugely increases drag. Most of the time on these minimum-diameter N5800 attempts the goal is for super-high velocity which isn't comaptible with a high-drag nose.

The Aeropac flight used very low-thrust motors. Their first-stage N1100 has almost four times the burn duration of the N5800, and a wonderful burn profile for such things - it's nice and regressive, kept the speed low even though it put out 14,000 Ns. Their sustainer was also a slow-burner. By keeping the burns slow they kept speed down until they were high enough to get air density down. This really doesn't translate well to the N5800, which is more of a 3.5s kerblammo which gets you going fast RIGHT NOW.

Temperature telemetry on an N5800 flight would rock out loud - but the dynamics of model rockets doesn't lend itself well to that. First off, most of these flights end badly. Electronics tend to get turned into chunky salsa when the airframe they're involved in disintegrates at greater than the speed of sound - to say nothing of their 'meeting' with the desert floor. If you want to guarantee good telemetry, you'll need to send it to the ground in real-time. This will require a lot more than simply a data logger - you'll need the transmitter, a decent microcontroller (or small computer) to get the raw data stream from the DAQ unit into something you can transmit reliably, and then you'll need all the equipment on the ground. All this equipment is expensive and will add significant weight - the other big enemy of high-mach flights.

Personally I'd love to see, say, ten thermocouples along the nose cone. There would definitely need to be one at or as near as possible to the tip, then spread the others relatively evenly. There's no need for more-accurate numbers than a thermocouple can give you here - +/- 1 degree isn't that big a deal when you're dealing with thousand-degree airflows. I would worry about how to mount them, though. They need to be as close to the surface as possible to minimize heat transmission errors (which are enormous in highly dynamic situations like a high-mach model rocket, when encased in highly-insulating materials like fiberglass). You can't just drill wells into the surface of a FWFG cone without horribly compromising the structure, so it would either need to be a custom cone or a commercial cone with a thick coating of something heat resistant (and it can't be ablative - if the coating goes away you lose the thermocouple!). You could then add more T/Cs along the body, at the fin leading edges, and wherever else. You just have to keep them isolated from the 'rip the probe off the surface' airflow while close enough that they give you numbers that are useful. Better measurements of surface temperature are optical - which means they're going to be HARD on something moving at mach 3, much less something small and over a mile away.

CFD on something like this would be pretty awesome, but remember it's limitations. Unless you know the boundary conditions, and have a test case where you know what to shoot for, the simulation will basically say whatever you want it to. A combined fluid-thermal model with composite materials is going to be pretty finicky, too, without some sort of experimental validation. It's great for rough estimates before building, though, so you can use it to get an idea where to start. Frankly, I'd love for someone with the resources to do some wind tunnel testing of potential composite cones and fins.

EDIT: And if you know how to work the supersonic flow solver in MAYA/TMG, I'd love to know more - I use it all the time for flow/thermal models of heat exchangers, fluid heaters, and related as part of my day job, but I haven't seen much on how to use the supersonic flow functions.

Alex

All the nosecone temperature measurements you describe were conducted in-flight in the 1950's by NACA, the predecessor of NASA.

https://naca.central.cranfield.ac.uk/report.php?NID=6324

Here's one from NASA published in 1961 on the heating of a von Karman nosecon to velocities to Mach 10 https://archive.org/details/nasa_techdoc_19980228066

Bob
 
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Elihunter

We are not discussing model rockets here. Model rockets have no more than 125 grams of propellant (< 240 Ns total impulse) and weigh not more than 1500 grams. Model rockets do not go fast enough to have damage from aerothermal effects.

When you are discussing Mach 3+ flights you are launching high power and amateur rockets. Mach 3+ is beyond hobby rocket territory when it comes to materials of construction. Unfortunately some folks do not appreciate that "marine grade" epoxy is not suitable for stagnated Mach 3+ airflows. Using model rocket techniques and materials for Mach 3+ will result in a failure 99+% of the time.

Bob
 
Alex

All the nosecone temperature measurements you describe were conducted in-flight in the 1950's by NACA, the predecessor of NASA.

https://naca.central.cranfield.ac.uk/report.php?NID=6324

Here's one from NASA published in 1961 on the heating of a von Karman nosecon to velocities to Mach 10 https://archive.org/details/nasa_techdoc_19980228066

Bob

That is very, very cool (warm?)... and a bit of a fun read, since they use BG units everywhere I have no idea the relative magnitudes of what they're talking about half the time unless I pull out my calculator. :wink:

The temperature of 1660 R is interesting though - it's not that ludicrously hot, and is a nice little illustration of just how much cooler the air stream is away from a stagnation point (ie the very tip of the cone) - at stagnation it's what, 8-10,000 R?. It's also interesting to note it hit that temperature pretty much exactly the same time as it hit mach 10.
 
That is very, very cool (warm?)... and a bit of a fun read, since they use BG units everywhere I have no idea the relative magnitudes of what they're talking about half the time unless I pull out my calculator. :wink:

The temperature of 1660 R is interesting though - it's not that ludicrously hot, and is a nice little illustration of just how much cooler the air stream is away from a stagnation point (ie the very tip of the cone) - at stagnation it's what, 8-10,000 R?. It's also interesting to note it hit that temperature pretty much exactly the same time as it hit mach 10.
Alex

Read the papers again. The temperatures recorded are the surface temperatures, not the gas temperatures. The parameter being measure is the heat being transfered from the gas to the NC surface, not the gas temperature. To a large extent the gas temperature is irrelevant, it simply must be higher than the surface.

You can have a 6000 K plasma out in front of a surface and the surface can be near room temperature. (The example is the sun irradiating a plastic NC on a model rocket sitting on a launch pad waiting for someone to push the button.) While the sun is a ball of hot plasma, the amount of energy irrading the NC is fleetingly small so little heating occurs. Similarly in Low Earth Orbit, oxygen atoms and nitrogen moles are impacting the forward facing surfaces with 5ev and 10ev energy respectively, but the temperature rise is only 0.1C - 0.2C, the reason being the the densisty is so low, while the collision energy is high, a relatively few collisions actually occur so the total amount of energy transfered is low.

Conversely consider cooking vegatables in boiling water and steam. Both can be at 100 C, but steam also contains the energy of vaporization in it. A steam molecules transfers far more energy per collision with he food than boilling water does. While the density of the liquid water is much higher than that of steam, steam contains much more energy, pound per pound than water, so steam cooks food faster than boiling water, even at the same temperature.

It's the transfer of energy, not just the temperature, that gets object hot.

Lastly, the heat transfer coefficient of a turbulent flow is typically about a factor of 2 to 4 higher than a laminar flow. The gas temperatures are the same, but the gas mass (boundary layer thickness) (resevoir) that is involved in the heat tranfer is several times larger in a turbulent boundary layer than in a laminar one, so more energy gets transfer into the surface from a turbulent boundary layer per unit time than from a laminar boundary layer.

When it comes to surface transfer in a aerothermal heating situation, it's not the temperature, but the heat transfer, that matters. At first this is a difficult concept to grasp, but once you understand the mechanism of heat transfer, you will know how to live with it.

Bob
 
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Bob:

Nice explanation - you really nailed a good variety of heat transfer cases which illustrate how temperature is far from the only important factor.

I apologize for the confusion - I'm very familiar with heat transfer mechanisms (my official day job is 'thermal analyist',) I was commenting on 1: the fact that the air stream that 'close' to the tip of the nose was 1/4 the stagnation temperature, and that 2: the transfer is so ludicrously fast that the surface responded almost immediately to the air temperature changes. In my daily work I mostly deal with refrigerants (I've worked with the thermal properties of fluoropentane so much now I can just about recite the saturation curve by memory), cryogenics (multi-kilowatt liquid nitrogen cryostats), and rather a lot of work in plasma physics (really, all my thermal work has been, up to the last year, secondary to being a plasma guy).

Plasmas are great for illustrating how much density matters in heat transfer. Particularly good examples would be fluorescent lamps, which have a gas temperature somewhere around 5000K yet an outer wall temperature less than 100C. But take gas at about the same temperature, increase the current density so you can dump a hundred amps or so through an eighth-inch wide column, and you have an arc welder. Also good is the beam energy out of a plasma thruster - the gas is moving like it 'burned' at 2-3 million degrees then was accelerated through an isentropic nozzle (300ish ev directed energy - low-Isp thruster), yet the beam dump doesn't get all that hot, because it's still just not that much power.

I never had to deal with aerothermal heating much before - I've spent most of my professional life mucking around in rarefied flows or in liquids - so I'm still wrapping my head around how staggeringly high the convective heat transfer values are. Of course, I don't have the good sense to leave well enough alone, all I'm really getting is a stronger desire to (eventually) build my own minimum-diameter N5800 project, or even participate on something like the Aerocon project.
 
I never had to deal with aerothermal heating much before - I've spent most of my professional life mucking around in rarefied flows or in liquids - so I'm still wrapping my head around how staggeringly high the convective heat transfer values are. Of course, I don't have the good sense to leave well enough alone, all I'm really getting is a stronger desire to (eventually) build my own minimum-diameter N5800 project, or even participate on something like the Aerocon project.

It sounds like your experience and interests would be a fantastic addition to an existing high-altitude project. Most of those are by "rocket guys", not necessarily scientists (despite what the T-shirts say). Group projects can be a lot of fun as well.
 
It sounds like your experience and interests would be a fantastic addition to an existing high-altitude project. Most of those are by "rocket guys", not necessarily scientists (despite what the T-shirts say). Group projects can be a lot of fun as well.

Funny, I've been thinking just that - many of the toys I want to play with long-term are pretty far outside the range of projects a single person can design, test, and fly (much less afford). I'm still 'drinking from the firehose' in the realms of high-speed aero and combustion, but hey, if you know of any big team projects that need an overly silly number cruncher/test engineer/nerd who is given to Fronk-en-Steen-ian bouts of laughter at successful projects, then I'm sure I can fill such a role. :grin:
 
A von Karman nosecone is used because it has the lowest ascent drag. When you lower the drag you obtain less aerodynamic heating, not more, and more importantly higher altitude.

Depending on how you look at the problem, this can be wrong. The dissipated power is drag times speed. If one compares two rockets at the same speed, with different levels of drag, then the rocket with the lower drag experience less heating. If, on the other hand, one compares two rockets with the same thrust (at or near terminal velocity), then the rocket with the lower specific drag (cd*A) will experience more heating because it gets faster. Somewhat absurd example: Build a huge styrofoam rocket and launch it on a N5800. There will less aerothermal heating compared to minimum diameter rockets of the same weight.
Real rockets, will fall somewhere in between these two theoretical scenarios (constant speed vs. constant drag/thrust), so it depends on a case by case analysis to determine the thermal effect of a lower drag coefficient.

Reinhard
 
Just found this thread, but as the designer of the AeroPac rocket that won the Carmack Prize I am a firm believer that nose cone drag is the single key component of high altitude ( after picking the right motor!). Von Karman rules and the NACA reports are quite clear on this.

K
 
Just found this thread, but as the designer of the AeroPac rocket that won the Carmack Prize I am a firm believer that nose cone drag is the single key component of high altitude ( after picking the right motor!). Von Karman rules and the NACA reports are quite clear on this.

K

Ken, congrats on that achievement.

If you don't mind me asking, what was the L/D ratio of it and did you do anything special at the tangent area where it joins the airframe?

Greg
 
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