Discussion in 'High Power Rocketry (HPR)' started by David Schwantz, Jul 2, 2019.
Other than "bigger" motor. What all can be done to gain altitude?
min dia or sub min dia
proper nose cone shape
proper fin design
no launch lugs or buttons
glass like finish
Minimise drag: https://en.wikipedia.org/wiki/Drag_(physics)
Fly from a high-altitude site, on a hot day.
Fill rocket with Helium......
Hey Tony, thanks. What is sub min dia ?? I have 1/16th fins,beveled all around including TE, fly away guides, rocsim has 12 oz in the nose. Just sprayed the clear last night, looks pretty smooth to me. No boat tail, rocket is a 29mm Go Devil that I modded the fins and made nose to add or remove weight as needed. With the thrust ring on the I205 I do not see how I could add a boat tail.
Sub min diameter is basically fins attached directly to motor case. No airframe. Usually just a fin can.
If I made a molding that fit on the motor nozzle and glued in place, would that be considered an illegal motor?
And Jim, I did try that, went down to the local C Store and got one of them mylar ballons filled with Helium, thought that since it was shiny it would make a good streamer for tracking, but I just could not get the damn thing stuffed in the tube....
Not sure about attaching to nozzle. I know there was a thread on here a few years ago where someone did his L3 on a sub-min diameter rocket.
here is a build thread on Chris Attebery's sub-min:
Fly on a cold day with a barometric altimeter. It will report higher altitude than reality.
In the range of small rockets such as we fly, the total energy lost to drag can be quite a large percentage of the total energy provided by the rocket motor. The remainder is what is exchanged for altitude (potential energy).
Use a longer burn motor of the highest total impulse you can put in the rocket. That decreases the velocity. Aerodynamic effects tend to scale as the square of the velocity. So taking the path a little slower generally means wasting less energy to drag.
There are some caveats to this though - you don't want to be trying to climb stuck in the transonic region!
Additionally, while a motor is burning, the base drag of the rocket is minimized. That is one of the major contributors to drag. And the longer the burn, typically the greater the altitude where it shuts down... and the lower the air pressure hence lower the drag.
Making the rocket as small in diameter as you can get away with helps reduce base drag particularly once the motor has shut down. It also helps get the nozzle exit diameter as close to rocket diameter as you are going to get.
Commercial motors run at low pressures (mostly) with low expansion ratios. At least having a proper nozzle will increase the nozzle efficiency and reduce the base drag. Such a nozzle has to be computed for the flight profile and launch altitude... and moves into the EX category.
In our small range, it is sometimes the case that adding mass to the rocket increases the altitude. Compute out the optimum mass. If it is greater than what you have, add some.
Use a tower not a rail for launch.
Hi Gerald, thanks for the input. I am using fly away guides as I have no tower, yet. AT I205, I cannot change nozzle without it being an X motor, as you said. Roc sim has me putting 12oz in the NC and it is just short of 10,000 agl. I will look for a long burn 29mm, I do have a 6 sec burn 38mm I am saving for my 38mmxl Black Hole from Mach 1 Rocketry.
On a given motor? Minimize drag and optimize weight. For a given total impulse? Minimize drag (so minimum diameter, drag efficient fins, efficient nosecone, and a good paint finish) and weight, then go with the minimum thrust motor that has you drag limited (increasing mass increases altitude), and optimize your mass.
Piston launcher? I know they are often used for LPR competitions. Has anyone tried it for HPR?
You can't add a boat tail, but what you can do is polish the clear coat. No matter how smooth it goes down it will always have what is known as "orange peel", the small amount of bumpiness that the finish is left with. Wet sand with 600 grit lightly, then go to 1000, then 1500 then 2000. Then go to the automotive aisle of the hardware store and buy some cutting compound and polish and a couple microfiber cloths. You can get an electric polisher for around $30 and use that with the compounds to make that rocket slipperier than a politician on roller skates. The more reflective it is the better. This will cut down on the parasitic drag and surface drag and makes a massive change.
Here is the 54mm MD I just finished to try and break the L altitude record. The last post shows just how glossy this rocket is.
Hi Falcon, I have been watching your thread with interest. Thanks for your reply. Although I do agree with it being as polished as it can be, I do think you can spray without there always being orange peel. I very seldom wet sand, only to get out a piece of lint or something. These are the polish that I like to use, I also prefer an air polisher. The first pic is of the 29mm Go Devil that I just sprayed the clear on Sunday night, no sanding or polish, the others are rockets that I have done over winter. Yours look great, did I mention, I really like red Dave.
Was just about to pipe in about the orange peel bit.
No, orange peel is not a given. A proper 'color sanding' can eliminate all orange peel. What is left are any tiny flaws left from lack of skill, or missing scratches while going through the grits, and some other types of flaws. Also, some finishes will shrink over time, leaving some texture, but not orange peel. Nitro cellulose lacquer shrinks for a good long time, months. Automotive urethanes, less. This is why we use a non-shrinking grain fill before the finish coats go on.
A finish that is not color sanded will almost always have some sort of texture, be it orange peel, 'fat lips', or too dry.
For color sanding, there are lots of opinions, some better than others. I like to get the finish as good as can be out of the gun, which is generally very good, and then hit it hard with 1000 grit, wet, until 'uniform dull grey'. If it is not taken to 'UDG', there will be orange peel left, or something similar. After the 1000, jumping to 3000 grit Trizact will leave the finish ready for compounds, liquid or dry. The first 'cut' with the abrasives needs to be done with a sanding block, whether rubber, foam, or whatever. It needs to have enough of a 'plane' to cut the high spots down to UDG, and not just follow the highs and lows. For a urethane finish that is sprayed over a surface that has previously been cut flat, sometimes I will begin with a block with 2000 on it, then to the Trizact 3000. 600 is too coarse for a first cut, except with very thick coatings that will still be hit hard with 1000.
Photos of gloss finishes mostly lie. I am a photographer, and long time professional finisher. Capturing an image that actually shows the exact level of finish, or orange peel, is not easy. Most often, some blown out highlights give the impression of 'glossy', while much of the photo shows the subject matter. At the margins of the highlights that are not blown out, that is where the true quality of a finish can be seen in a photo, and this quality of lighting in photos is often absent, or minimal. I have been trying to capture really nice glossy pics for years in a studio environment, it is not easy.
The only time I have not gotten orange peel when spraying is when I use an airbrush with model paints and paint retarder on plastic models. I need to get a much better compressor (I use an Iwata airbrush compressor with a tank, and the tank is only .5 liters) I just can't afford it. I would also want to get a better water separator than the one that came with the compressor. I think I get away with the cheap one because I live in Colorado and it is so ridiculously dry here. Spraying in a purpose built booth is also an improvement I could make (Stop landing on the wet paint, damn bugs!!!!!!!!). For my L3 I want to put the effort in for a show finish, not just a 10 foot paint job like I give most rockets.
I used my cheap orbital sander with microfiber cloth stuck to the hook and loop fabric to cut and polish the Falcon 54, it worked really well, as you can see from the build pictures. I also just skipped from 600 to 1200 grit, then to cutting and polishing. I then just spent an hour or two with the orbital sander and microfiber cloth with the compounds and it knocked out all the roughness.
I am glad you like the build, I can't wait to fly it this weekend!
Something I haven't noticed being mentioned yet:
If you're wagging, you're dragging
If you're coning, you're dragging
If you're rolling, you're dragging
See the video here for an example of perfect alignment.
And by 'fin alignment' I include:
identical edge tapers
Deeper dive. In that video you can still see the small 'corrections' being made during the ascent. Some degree of this is unavoidable; a fin doesn't 'work' without some angle-of-attack. Minimizing the AOA and necessary corrections is the subject of dynamic stability. You can design the lift coefficients and mass distribution to minimize drag. Suggested reading is "Fundamentals of Dynamic Stability"; last I looked was available from the NAR bookstore.
And BTW, launching in still air will minimize the 'work' the fins have to do.
vcp, all very good pointers. I have done my best at alignment, I used fin guides and a long aluminum angle laying against the tube butted up to the fin to align straight with tube. All airfoils are flat, very small fins. Fillets were pulled with a pvc pipe, should be damn close. Tapers are 1/8th" either side, can't count how many times I cut my knuckles on them while sanding. My first 29mm Go Devil came down on just a streamer, so this one will too. Found some sparkly stuff to make it from.
Falcon, a little trick I learned a bazillon years ago it to use lacquer thinner, it makes paint flow and level great. mixes with just about anything, other than water base. I am lucky enough to have been able to put a paint booth in my basement. My last Craftsman compressor was the 33 gal one, lasted me 12 years till parts flew out of it this spring. it was like 300 bucks, so they are not to bad to buy. I do have a very good water trap though. Fresh air supply for my mask. Good lighting, but I think I will change it to new LED stuff. And a really big fan in the ceiling with all incoming air filtered. I can spray Imron and you cannot smell it in the house. Good luck with the first flight, send pics. Dave.
I did a couple Sims with Thrustcurve on a 29mm rocket, the CTI I 224 would provide a 6.8% increase in altitude over the AT I 205w. I played with rocket weight a little and 13oz no motor, 26.1oz pad weight had the highest altitude prediction.
CTI has the tail cone for the 29mm reloads and reducing base drag can help with altitude.
I flew a couple rockets on a small 29mm G40w and the 2.25" diameter ARCAS with a boat tail down to the size of the 29mm thread on retainer had a 17.8% increase in altitude vs the 2" AT Barracuda, with a similar build weight and finish (2,328' vs 1,976').
I have seen some days with higher humidity and cooler temps cause a noticable reduction in altitude. Motors can have no more then 6.7% in total impulse and 20% average thrust. Launch angle and how close to 90° the flight is can reduce altitude. I did a quick drawing with a 6" line and checked with a 26" line the AGL altitude can be reduced by 2.1% with a 22-25° launch angle.
Basically reducing drag is the main thing as OverTheTop said. Keep in mind that drag is proportional to velocity², so your highest thrust motors are not necessarily going to give you the most altitude (speed yes, altitude no).
Here's my description of the basics of what's going on:
HI John, thanks, for whatever reason I cannot get CTI info to load on rocsim. Planning on the flights to be on the 13th, will you make it? Also sent my alt 1 to John and the sensor port was blocked. I did pick up an alt 2 also. Dave.
Thanks John, will look it over after I unpack my rocket motor box. Dave.
Yes I plan on being at the launch the 13th.
You’ll want to use Open Rocket and RasAero II not RS. And use the optimiser functions.
Space man, cannot use OR, it will not load in my OS. All I have is Rocksim, so gotta use what I got. I do have it simmed to just over 9000' on the I205, with an added 10oz in the nose. At 11oz I started losing, also have not been able to load CTI 29mm files, I have all the 38mm and up loaded. Going to call Apogee this morning and have them see if they can help with the CTI stuff. I have ordered a 29mm 6gnxl case and a I224 reload to use. Flew last month to,guess, about 7000' with her. My JL alt1 didn't sense the flight. Sent it in and John had it back to me ASAP, the port was blocked. Did also pick up an alt 2, so if I can fit both, along with tracker in it, I will fly with 2 alts.
Hey, John we'll see you there. Dave is working on his level 1 rocket. A Mach1 "Red Shift". Hoping he will have it done for then. I have told, it can fly without paint for the first flight. But he is kind of like me, go figure. Dave.
Does rocsim have a motor size limiter? I know OR has the capability to only show motors of certain diameters.
Also, in the electronics and software section of the forum, there's s sticky by Neil_w with prepackaged installers for Openrocket for pc and Mac. They've saved folks much heartache
You designate one tube (or multiple) as the MMT and then you load motors into those tubes. When building a rocket, don't forget to use a standard tube ID for the MMT and mark it as such.
The only real annoyance I've had with RockSim is that some 3" motors are listed as 75mm and some as 76mm, so it's best to make the MMT ID 76mm and choose "matching diameter or smaller." For the ThrustCurve.org motor guide, I use a 1mm tolerance for larger motor sizes to avoid this problem.
If you want to get a quick idea of what motors might work for your rocket, try it out:
Then you can download the data file (if needed) and do a more complete simulation on the motor you choose.
Rocksim has that also, tried those installers year ago or so. My system to old, kind of like me
John, I have downloaded the thrust curve file for the CTI I224, but I can not get it to add to the Rocksim engine import. Thought I'd call Apogee in a few minutes. Thanks.
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