Altitude control problem. Help!

SilicdyneTRF
Wildman Rocketry
The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
ArchitectOfSeven said:
Now seriously though if anyone has actually tried using skin friction to stay inside of a waiver or whatever please tell about it.
Click to expand...

Of all the ways to add drag to the rocket, surface roughness is probably the least confidently model-able, verifiable and implementable.

You are telling me you cannot think of easier and known quantifiable ways to easily add drag to a rocket? I can think of at least 5 better ways.
 
I'm calling BS on your "reasonably optimized" 25,000' simulation. The Contrail M1491 is 75mm diameter, 54" long, weighs 10.75 pounds and has 5675Ns impulse (all from thrustcurve.org). The RATTWorks M900 is 65mm diameter, 72" long, weights 7.25 pounds and has 6463 Ns impulse (again, from thrustcurve.org).

I've flown the M900 to 27,032' for my Level 3 flight.

Your motor is a larger diameter and less impulse, which means more drag and less umph AND you say you have added 5 pounds of water tank mass.

I did a quick OpenRocket simulation with just the motor and a nosecone with fins to make it stable and I'm coming up with just under 21,000'. Well, well shy of the 25,000' you claim. And it isn't a rocket you can expect to get back.

25kfeet.jpg


Edward
 
Last edited:
The simplest way to reduce altitude is to just add some draggy components such as flat pieces at the bottom of the fins and make the nosecone less aerodynamic.
If you want to get fancy you might lighten the rocket as much as possible to reduce momentum and use the highest thrust, shortest burn thrust profile so you achieve the highest velocity (drag increases proportional to velocity squared).
Surface effects are the least effective.

In OpenRocket put in thicker fins to see what happens. For instance a fin that’s a quarter inch long and three inches thick just aft of the fins you have now.
 
Last edited:
The class project is to build and fly a customized 75mm contrail motor. Professor made that decision

Cool -- just realize that is a different goal than 10k at ERSA.

Skip ESRA and do what the professor wants - don't mix the two.
 
FredA said:
The class project is to build and fly a customized 75mm contrail motor. Professor made that decision

Cool -- just realize that is a different goal than 10k at ERSA.

Skip ESRA and do what the professor wants - don't mix the two.
Click to expand...

The OP hasn't said if they are also entering the rocket into the Spaceport Cup/IREC. It appears that their primary goal was the FAR-1030 competition which comes up sooner (June 2nd) and is almost identical in goals of the IREC contest.
https://friendsofamateurrocketry.org/wp-content/uploads/2018/03/FAR-1030-5R-Competition.pdf

Apparently, the (nondisclosed) college team designed a 30K rocket around the Contrail M motor, ordered everything, and now realize they can't make 30K and have blown their monetary and time budgets. Then they blame their professor. And then "shoot the messengers" here after posting a ridiculous solution with incomplete background information.

Hitting reset, here's my suggestion to the college team:

1) Do some quick fundraising to find another ~$500 to build a 5" diameter rocket to hold the Contrail M. My suggestion is the Madcow https://www.madcowrocketry.com/5-fiberglass-blue-iguana/
I have that kit and it goes together in a weekend. It will go about 12K ft with a small M. Sims to 10Kft if I add 5lbs.

2) Work through the simulations with the nominal Contrail configuration. Adjust the ballast size and location(s) to get 10Kft at the FAR site elevation.

3) Do a test flight in May to further tweak the altitude performance.

4) Enter the contest and win.
 
jsdemar said:
Hitting reset, here's my suggestion to the college team:

1) Do some quick fundraising to find another ~$500 to build a 5" diameter rocket to hold the Contrail M. My suggestion is the Madcow https://www.madcowrocketry.com/5-fiberglass-blue-iguana/
Click to expand...

My suggestion. For $300 buy the 2050cc tank assembly (instead of the 3200cc tank) and that will put the motor impulse right where you need to get 10K. It might be less as Tom Sanders might take the 3200cc back for trade.
 
If a drag plate at the end or a similar air brake is considered, don't forget to use shear pins on every joint. I believe it was Dirk Gates, who tested a drag plate once and experienced drag separation.

Reinhard
 
jderimig said:
My suggestion. For $300 buy the 2050cc tank assembly (instead of the 3200cc tank) and that will put the motor impulse right where you need to get 10K. It might be less as Tom Sanders might take the 3200cc back for trade.
Click to expand...

For $500 they have another rocket that could be used for both 10K and 30K (if fins reinforced). For $300, they get a tank and less options for TTW ratio and CG/CP.

All of the drag options (plates, flaps, etc), will make it difficult to assure good static and dynamic stability margins.
 
jsdemar said:
For $500 they have another rocket that could be used for both 10K and 30K (if fins reinforced). For $300, they get a tank and less options for TTW ratio and CG/CP.

All of the drag options (plates, flaps, etc), will make it difficult to assure good static and dynamic stability margins.
Click to expand...

For $0 they can fly the one rocket at both competitions and have fun.
 
Last edited:
I’m envisioning some sort of rocket version of “the shaggin wagon” from dumb and dumber
 
ArchitectOfSeven said:
Seriously some of you guys are acting like complete jerks. Calm down and maybe we can get something more productive out of this other than making me quit this hobby because the people act like assholes. If you would like a little context for the dilemma my class group started out designing a rocket for the FAR 1030 competition with the intended goal of 30k. We submitted our proposal, got it accepted, budgeted out the whole damn thing and made the material order. A few days later, the professor "informs" us that we are required to go for 10k instead which incited a bit of a panic as we literally already blew the entire budget and can't just spit out an additional $120 for 5.5" blue tube or whatever would make it go closer to target. At that point I started to look for ways to bleed kinetic energy quickly post burn and mathematically, boosting the skin friction can work, providing it doesn't just melt off (a potential concern because all that energy has to go somewhere). Anyway, we had another chat with the professor and we managed to change the order for the injector components so we can build the L369 instead which makes this much easier. We can also manipulate the impulse a bit when we design and make the nozzle as well. Crisis averted.

Now seriously though if anyone has actually tried using skin friction to stay inside of a waiver or whatever please tell about it. I'm genuinely interested in hearing how that went. Concerning the air brake method, I had thought about that one but didn't have space or budget left to implement it so it got tossed out pretty quickly.
Click to expand...

I think you did get your answer on skin friction. People have tried it and it isn't as effective as other methods. A drag plate could be cut from a 12" diameter piece of 1/2" plywood from Lowe's Depot. If the team can't afford $10 for that, you have no business being on this project. Space-wise, it can paste on the outside in the same diameter as the fins, so it doesn't really make the rocket bigger.

The big issue underlying the response you've gotten is that this forum sees a lot of college students breeze in thinking that they know everything. They ask for advice on a branch, but they're not seeing the forest or even the tree. Often they're on projects where they are well out of their depth technically but think that they're OK to fly an M because they built a bunch of Estes kits when they were kids. They then argue with the people making suggestions because the suggestions don't fit in with what the student expected. That's kinda galling for people with several dozen to hundreds of high power flights under their belts. I suspect that many people think that those students are a bigger danger to the continued existence of the hobby because of the high risk of accidents than people getting frustrated and leaving because they don't get good information.

You probably don't fit every one of those stereotypes. It's probably not fair that people started off assuming you were like the last annoying college student who came around. But the world isn't fair. There are lots of people on this forum who will give you good advice if you'll listen.
 
jsdemar said:
Apparently, the (nondisclosed) college team designed a 30K rocket around the Contrail M motor, ordered everything, and now realize they can't make 30K and have blown their monetary and time budgets. Then they blame their professor. And then "shoot the messengers" here after posting a ridiculous solution with incomplete background information.
Click to expand...

Hmm not quite. We were on track to at least compete in the 30k competition(maybe not competitively because I wasn't sure we would clear 25k) but then the professor arbitrarily decided that we were going to go for 10k anyway, despite what we had already committed to. It was probably mostly due to a misunderstanding early on but the part where he accepted the preliminary design review for the 30k rocket is what got us stuck in the parts order. That problem got dealt with so no need to actually do this anymore.

Anyway, a solution is only ridiculous if it doesn't work, is costly, or is overly complicated. Glueing abrasive grit to a body tube isn't costly or complicated so it comes down to whether it will actually work well. My main concern with it was if it would generate a thicker than normal boundary layer that would reduce the effectiveness of the fins. Hence, why I was asking if anyone had done that and what the results were.

@boatgeek, I know I don't know everything and I don't want to give the impression that I think I do. I know what my fluid mechanics textbooks say about fluid flow and roughness so I thought it might be worth the consideration at least considering how obnoxious the problem was to begin with and that in a bit of stubbornness I REALLY didn't want to actually change the basic design of the rocket. Tbh I was pretty irritated that the professor gave us such a backwards problem to solve at all. Normally the object is to figure out how to improve performance, not intentionally make it worse.

Another thought is my view of rocketry as a hobby. For the last two years I've personally built a few model rockets and helped build some liquid fueled monsters as well. Doing this I think my perspective is that I don't really want to build "model" rockets as much as I want to build something like a replica of Goddard's early sounding rockets in full scale and with the liquid motors. Of course this means that I can't really participate in the NAR and TRA launches much but my area has a large community of like-minded people and a well-equipped facility to launch from. I would like to be a part of this larger community for the long term because I enjoy watching things do a vanishing act on a pillar of fire and a lot of you have fantastic amounts of experience that I can hopefully learn from and contribute to. Hopefully my ridiculous questions in the future don't ruffle quite so many feathers and we can have some fun with this hobby instead. Seriously though, I'm gonna sand coat some rockets and put them in the wind tunnel I discovered I had access to yesterday. Results coming soon...

-Chris
 
Then why can't you guys man up and drive that cool rocket to a 25k-50k ft waiver. You get the rocket to waiver, the RSO will fly it under his certificate if it's airworthy let him know it all upfront. You might send him two pages on flight sims and details. You'll really get to know teammates after being in a car non stop except for gas or eat or sh*t. Tell the prof you designed it for 25k ft and it's too late to turn back. And go for it budget allowing. Tell him with or without you blah university is launching to X altitude. Find a waiver and do it. I went for 22k ft. We had to get from TN to Utah in 3 days by a gas card and personal vehicles. Our prof didn't show up on second launch of a new rocket of lower performance in Virginia. We got an A. School got an award. I've survived worse crap in my life. Embrace the suck-age. You've put in far too much effort to destroy performance. Tell the guy changing performance requires a new motor or airframe and that its flying as is that he approved the prelim and it's too late now to say make it go 10k ft. The RSO will give you a maximum allowable altitude under the specified waiver so you need to find a waiver a few thousand above your design Alt. As long as your in compliance of NAR or TRA rules your good for an RSO to fly it if it passes inspection. If you want to be on the safe side find a TRA research launch date if you've got motor mods or more metal fin bits etc..

And to make you feel better. None of the senior projects outside of rocket or Baja finished anything worth squat. Tell the competition the prof is sick, they should clear an RSO to fly it instead. No biggie. Just get there and make it happen.
 
Andrew_ASC said:
Then why can't you guys man up and drive that cool rocket to a 25k-50k ft waiver. You get the rocket to waiver, the RSO will fly it under his certificate if it's airworthy let him know it all upfront. You might send him two pages on flight sims and details. You'll really get to know teammates after being in a car non stop except for gas or eat or sh*t. Tell the prof you designed it for 25k ft and it's too late to turn back. And go for it budget allowing. Tell him with or without you blah university is launching to X altitude. Find a waiver and do it. I went for 22k ft. We had to get from TN to Utah in 3 days by a gas card and personal vehicles. Our prof didn't show up on second launch of a new rocket of lower performance in Virginia. We got an A. School got an award. I've survived worse crap in my life. Embrace the suck-age. You've put in far too much effort to destroy performance. Tell the guy changing performance requires a new motor or airframe and that its flying as is that he approved the prelim and it's too late now to say make it go 10k ft. The RSO will give you a maximum allowable altitude under the specified waiver so you need to find a waiver a few thousand above your design Alt.
Click to expand...

Bruh I have a waiver for 40,960N-s and unlimited altitude 365 days a year at the competition site. It's kinda ridiculous tbh. Prof told us we can keep the same motor case but switch to L369 injectors and nozzle so the altitude isn't an issue issue. It's actually kinda cool because its such a long burning motor.
 
Seven, it's not the question. It's the lack of accepting the responses provided by people with a multitude of experience. I have seen fur covered rockets fly much better than I ever though they would, i.e. with what appears to be a lack of penalty. You are not asking for a slight reduction in height, you are asking for large reduction in performance. Plate drags, cone fins and the like will give you what you are looking for but reject it every time they are mentioned. Reducing the power of the motor is a no brained but again you don't even seem to reject it. Once the pros weigh in and are ignore, you are left with idiots like me responding or worse.
 
Yeah idiots like me that passed senior design. One rocket imploded. The other won a national comp. We had to sign forms and release medical info the day we packed bags. School didn't give a **** if we were dead. And our prof had our backs. He signed the same forms we did then showed up. The other prof as an assistant team advisor didn't want to even be at the launch site we were at. Just to please lawyers basically. The upper level execs and lawyers are rather fierce over liability with rocketry. But if your nutty enough to sign forms, well you can go. In sue happy world it's hard to do anything without taking full responsibility of all consequences. University wants all the publicity and none of the reality that it simply is a higher risk activity. He wants to fly competition rocket. Accept the fact your already dead. Move on and fly it. It's horrid advice but legally that's what it took in modern sue happy world to just compete. The execs had to deal with it. We signed enough forms to isolate UTC from us. And ESRA has even higher risks than what I endured. Though knowledge, experience, sense, and oversight of rocketry will keep it safe. I think NAR and TRA offered more legal protections than what the college offered. The biggest problem with student comps is the edge of performance and the lack of experience. That's what is so dangerous about it. When you sign a form willing to die for it, they let you launch. We did everything we could to ensure it was safe to what we knew. I hate to get all grim blunt and stupid. Hopefully all competitions aren't that bad. It's horrid advice but that's the route we took to make it work. Horrid advice. The other option was fail the course and repeat a year of college. Lol.

The university was okay with funding project mostly. They weren't okay with legally protecting us. And I'm not trying to bad mouth the college. They did great. But today you have to sign forms basically. So I hope liability wise your not in that same boat. It can be a lot unexpected weight to carry around suddenly. I'm thankful I just finished all my presentations in a separate course this semester and am finally done with all that crap.
 
ArchitectOfSeven said:
Seriously some of you guys are acting like complete jerks. Calm down and maybe we can get something more productive out of this other than making me quit this hobby because the people act like assholes.
Click to expand...

You can tell what a fun, enjoyable hobby this is by how wrapped around the axle people get.
 
ArchitectOfSeven said:
Hmm not quite. We were on track to at least compete in the 30k competition(maybe not competitively because I wasn't sure we would clear 25k) but then the professor arbitrarily decided that we were going to go for 10k anyway, despite what we had already committed to.
Click to expand...

So a director or customer made a last-minute change to the scope of the project which made nonsense of your budget and obviated your work-to-date? Welcome to the whole rest of your career as an engineer.

ArchitectOfSeven said:
Anyway, a solution is only ridiculous if it doesn't work, is costly, or is overly complicated. Glueing abrasive grit to a body tube isn't costly or complicated so it comes down to whether it will actually work well. My main concern with it was if it would generate a thicker than normal boundary layer that would reduce the effectiveness of the fins. Hence, why I was asking if anyone had done that and what the results were.
Click to expand...

ArchitectOfSeven said:
Seriously though, I'm gonna sand coat some rockets and put them in the wind tunnel I discovered I had access to yesterday. Results coming soon...
Click to expand...

Do this. I'd guess that you are going to find out why your question generated so many "try something else first" responses. How will you measure surface roughness? By the screen size for the sand gains? By perthometry or optical profilometry of the sand adhered to the rocket body?

ArchitectOfSeven said:
... I don't really want to build "model" rockets as much as I want to build something like a replica of Goddard's early sounding rockets in full scale and with the liquid motors.
Click to expand...

Speaking of Goddard -- von Kármán's assessment

nodirectline.png

FWIW, Frank Malina's work is probably more worthy of duplication -- his example more worthy of emulation -- but Goddard's rockets would be fun to watch go up too.
 
If this kid listens to the L-3's he might pass. They have the most flight experience. I don't care what the theoretical numbers say kid. Those hobbyists have experimentally flight tested and verified methods that actually work. They have crashed more rockets than you kid. Ignoring their advice is Goddard 2.0.
 
So-o-o,

How did this all work out? What were the results?
 
jderimig said:
So-o-o,

How did this all work out? What were the results?
Click to expand...

Well, as of right now I have a sort of Frankenstein's monster variant of the 75mm contrail that has test fired once for the class. It didn't generate enough thrust to fly because of a fuel grain/nozzle mis-match issue. With the burden of the class now out of the way I'm free to do whatever the hell I want to so it's back to setting it up for a proper M class configuration and getting 20-something thousand feet out of it. Besides for making some new nozzles I'm working over the recovery system to make sure it works properly and building a better nose cone.
 
ArchitectOfSeven said:
Besides for making some new nozzles I'm working over the recovery system to make sure it works properly
Click to expand...
Good to see you are thinking about reliable recovery. My advice is to not underestimate howdifficult it is. Talk to lots of people experienced in the L3 domain. Design for off-nominal recovery. Up is easy, down is hard.
 
OverTheTop said:
Good to see you are thinking about reliable recovery. My advice is to not underestimate howdifficult it is. Talk to lots of people experienced in the L3 domain. Design for off-nominal recovery. Up is easy, down is hard.
Click to expand...
I hear you. Going with a fully redundant system from batteries to charges and even doing different brands. It makes the pre-launch wiring a little complicated but practice and wiring diagrams makes perfect. ☺
 
The electronics tech is not the whole story. Don't underestimate how you pack the chutes and harnesses. My HPR recovery has been fairly reliable, but mainly because I worked out the wrinkles in MPR ;)

Also important on the day is your launch checklist. It can save your bacon by making sure something is not forgotten.
 

Similar threads

bandman444
Bryce's Handicap Record Quest: NSL 2024
2
Replies
59
Views
2K
MClark
M
K
My Level 2 Attempt - a Diurnal Flight of "Nocturnal Missions"
Replies
5
Views
603
kjhambrick
K
K
Rough Accelerometer Data from a Blue Raven in a 38mm Av-Bay -- Is the noise for real ?
Replies
8
Views
277
Spacedog49Krell
Spacedog49Krell
M
Papering Fins and Bubbling
2
Replies
39
Views
1K
aerostadt
aerostadt
cbrarick
Help me bring back the American Elm!
Replies
10
Views
478
cbrarick
cbrarick

Latest posts

Back
Top