Low powered 75mm motor for testing

[Lt72884]

Just a few comments on your sim file after a quick review. As received, it requires a little modification to at least these parameters:

• Launch rail length of 100 cm? For a rocket this size you probably should be using a 3 m rail.
• Ground hit velocity of 8.77 m/s is a little high. Might need to increase your main chute size.
• Perhaps with a rocket this size and expected apogee, you could consider redundant altimeters?
• Your forward launch lug position could move forward a bit more, closer to your CoG.
• Your drogue deployment is set at 1st ejection charge. Set it at apogee.
• Your main deployment is set at 1st ejection charge. Set it at specific altitude during descent, say between 500'-1,000'.

There might be more issues. However, it's good that you are using a well known kit, rather than scratch building. Don't forget to account for additional masses and their positions, e.g., epoxy, etc.

thanks for this information. I left most launch prefs at default until we get all the correct weights. I THINK our launch rail is 8 feet that the filed is letting us use.
Ground hit is a little high, a bigger chute or less mass is a good idea. Ill bring that up to them tonight.
we have 2 or 3 eggtimer brand altimeters and GPS going in for redundancy
As for weights, would a total constructed weight be good? or do i need to get the weights of EACH component separately?
in OR, how do i add epoxy weight and posistion to the rocket?

thanks

 

[bobbyg23]

I am looking at your file and it is very concerning. 1st thing I notice right off the bat is that is a 40lb rocket with a 60" parachute!!!!!!!!!!!!

 

[bobbyg23]

103" rocket will take at least a 12' rail to launch it.

 

[tsmith1315]

i know literally nothing except the small L2 j270 i use

A J motor is not small.

Have a look at the statistics for this large launch (776 flights), and notice that ~86% of the flights at this very large, popular launch were H and under flights.

https://www.rocketryforum.com/threads/ldrs-40.156939/post-2218083

In fact out of 776 flights, only 7 were L-motors. This may help with your perspective and understanding of why so many folks are disagreeing with the time frame you've been handed.

This post from the next page has statistics from LDRS 39, with LDRS meaning Large, Dangerous Rocket Ships, and being the premier national launch for big rockets. Here again, out of 987 flights, H was the most common impulse flown. That L2200 is a big motor, 99.6% of the way to L3 impulse.

https://www.rocketryforum.com/threads/ldrs-40.156939/post-2218113

 

[bobbyg23]

7 cals of stability is waaaaaayyyyyyyy to much.

 

[Neutronium95]

For reference, AeroTech made a small tweak the the propellant of the L2200 and that made the M1500. It's mucj closer to an M motor than any 54mm L, with the possible exception of the Loki L2050.

 

[Lt72884]

7 cals of stability is waaaaaayyyyyyyy to much.

ok, so since this is a kit that we purchased, whats a good move to lower that calaber down? we are going to move the 9lb mass down a bit so its not in the nose cone.

what is a caliber we should aim for?

thanks

 

[Lt72884]

I am looking at your file and it is very concerning. 1st thing I notice right off the bat is that is a 40lb rocket with a 60" parachute!!!!!!!!!!!!

is there a equation used to find the parchute diameter for weight? thanks

 

[rharshberger]

Probably, but I bond all my 75mm anyway, just to be safe. It can't hurt and it could help.

If its a single grain bonding should not be necessary, of course any grains can be bonded.

Edit: the K1800ST is I just realized a...2 grain...highly recommend grain bonding.

 

[cls]

is there a equation used to find the parchute diameter for weight? thanks

the best of the best

https://fruitychutes.com/help_for_parachutes/parachute-descent-rate-calculator.htm

 

[Lt72884]

LOL, i just told my other teammates about the chute and caliber and their reply via text "Wow, so now we need to get a new chute and motor. We really suck at this game"

 

[Lt72884]

the best of the best

https://fruitychutes.com/help_for_parachutes/parachute-descent-rate-calculator.htm

THANK YOU!!!!!! will this work for a drouge chute as well?

 

[cls]

THANK YOU!!!!!! will this work for a drouge chute as well?

yes, pretty sure, simply choose the descent rate. I'm sure Gene will help if you have questions. tell us where you end up!

 

[rharshberger]

If this is a college team you guys are in bad need of a mentor, if you have a mentor what the hell are they doing? If this is a personal quest, then there is lot you need to learn before even attempting what you are asking about.

 

[Voyager1]

ok, so since this is a kit that we purchased, whats a good move to lower that calaber down? we are going to move the 9lb mass down a bit so its not in the nose cone.

what is a caliber we should aim for?

thanks

Typically, you would aim for a cal of 1.5 - 3. However, it depends on the rocket, motor and the launch conditions. With high cal values, say 5 and above, then you really need to watch your launch conditions, e.g., rail length, velocity off the rail, cross wind component, etc. High cal values will produce more wind cocking if the wind component is high. If you can't reduce the stability margin, then don't launch into a strong breeze!

Most of my HPR launches are with a hybrid motor. This typically makes for a longish body and stability margins of 8-10 cal, depending on the specific configuration! I will only launch into a near-windless sky - which usually means I don't get too many launches on launch day. Mind you, my velocity off the rail is typically around 30 m/s, so I do have some magin for safety, even into a breeze.

 

[bobbyg23]

Just an example. I use this brand of chute. According to them, you need 168" chute!

 

[Lt72884]

If this is a college team you guys are in bad need of a mentor, if you have a mentor what the hell are they doing? If this is a personal quest, then there is lot you need to learn before even attempting what you are asking about.

college team no mentor because they all said no haha

 

[Lt72884]

yes, pretty sure, simply choose the descent rate. I'm sure Gene will help if you have questions. tell us where you end up!

end up in last place is whats happening haha The committee is kind of upset now. Not my fault i told them. They expected way to much out of us in 4 months

 

[bobbyg23]

This manufacturer says 120". Each one will be different depending on how the chute is constructed.

 

[Lt72884]

Just an example. I use this brand of chute. According to them, you need 168" chute!

dang, i dont htink we can stuff that in a 4inch airframe haha. thats a big chute.
thanks for the links and pics.

Now how do i figure out drouge chute as well? does that website have charts for that too?

 

[Voyager1]

thanks for this information. I left most launch prefs at default until we get all the correct weights. I THINK our launch rail is 8 feet that the filed is letting us use.
Ground hit is a little high, a bigger chute or less mass is a good idea. Ill bring that up to them tonight.
we have 2 or 3 eggtimer brand altimeters and GPS going in for redundancy
As for weights, would a total constructed weight be good? or do i need to get the weights of EACH component separately?
in OR, how do i add epoxy weight and posistion to the rocket?

thanks

Individual weights are best, preferably with axial and radial positions included is preferred. However, you can include mass objects at any position to account for added components. For example, you could include an allowance for the epoxy used to install your fins as a distributed mass along the fin can, or along the fin roots. Do the same for the other areas where epoxy is applied, e.g., fitted couplers on the body and nosecone.

 

[Voyager1]

dang, i dont htink we can stuff that in a 4inch airframe haha. thats a big chute.
thanks for the links and pics.

Now how do i figure out drouge chute as well? does that website have charts for that too?

Look at chutes with high CD, say 2. Rocketman has these, and probably other vendors too. You can reduce the size (volume), but still have high drag.

 

[Lt72884]

Look at chutes with high CD, say 2. Rocketman has these, and probably other vendors too. You can reduce the size (volume), but still have high drag.

ok, thanks. i will look into those. Its all about the drag amount im guessing in order to have a smaller chute to fill a smaller volume.
I have looked up some information and i have seen that a drouge chute that reduces velocity to 50MPH before main deployment is ideal. So far im at 67MPH but hta is with the main opening 10 seconds after the drouge. I might just pop the main based on altitude such as 500 ft AGL

 

[mrwalsh85]

college team no mentor because they all said no haha

Not really a laughing matter. Let me ask you this. What club are you planning on flying this rocket with, since no one else has really asked...?

yup, we have had no support, no lectures on rocketry, no books, no nothing. I actually went out and got the model rocektry handbook and gave it to the professor at my own cost.
It almost makes me feel stupid in a way because i know literally nothing except the small L2 j270 i used

READ. THE. FREAKING. BOOK.

You'll learn more from the book about basic rocket principles than you will pissing folks off on TRF. You and your classmates are in college. You should be able to open a book and read it and be able to apply the basic concept of what you've read to the topics at hand. If you aren't able to draw conclusions from the literature you are reading, time to seek out another book. Same goes for websites... There are more than a few that explain basic rocket principles.

At this point, you're just winding everyone up on the forums. This is why the student groups have been a sensitive topic as of late. No one wants to come in and get their basic L1 and learning concepts before moving onto their big rocket, then they vanish. It's just a resume builder for them.

end up in last place is whats happening haha The committee is kind of upset now. Not my fault i told them. They expected way to much out of us in 4 months

No... Don't shift the blame elsewhere. Be more vocal about it. You're just rolling over playing dead and trying to get pity points out of the rocketryforum folks. Do your due diligence and read up on everything you possibly can. There are other sites than TRF where you can learn focused information on topics that you've been discussing. Use them. The basic questions you are asking are not going to be covered in an L2 exam.

Get crackin'. If you ask the right questions you may get some guidance to these sites, though at least one has been mentioned in this thread already...

 

[user 30299]

Those of you with university team & competition experience know this is an accident just waiting to happen.

We know this group has no mentor, and they have "implied" that no one locally will work with
them, but some club had to allow them to put in launches for their L1 and L2 flights.

Forum support is helpful, but it does not replace the importance of a mentor.

And putting the last-minute responsibility at a launch on the local RSO, without a heads-up
on the situation, would be poor form (or irresponsible?) on our part.

 

[TheTank]

Not really a laughing matter. Let me ask you this. What club are you planning on flying this rocket with, since no one else has really asked...?


READ. THE. FREAKING. BOOK.

You'll learn more from the book about basic rocket principles than you will pissing folks off on TRF. You and your classmates are in college. You should be able to open a book and read it and be able to apply the basic concept of what you've read to the topics at hand. If you aren't able to draw conclusions from the literature you are reading, time to seek out another book. Same goes for websites... There are more than a few that explain basic rocket principles.

At this point, you're just winding everyone up on the forums. This is why the student groups have been a sensitive topic as of late. No one wants to come in and get their basic L1 and learning concepts before moving onto their big rocket, then they vanish. It's just a resume builder for them.


No... Don't shift the blame elsewhere. Be more vocal about it. You're just rolling over playing dead and trying to get pity points out of the rocketryforum folks. Do your due diligence and read up on everything you possibly can. There are other sites than TRF where you can learn focused information on topics that you've been discussing. Use them. The basic questions you are asking are not going to be covered in an L2 exam.

Get crackin'. If you ask the right questions you may get some guidance to these sites, though at least one has been mentioned in this thread already...

It’s funny you mention this about the resume builder piece… I had an entire group show up from a school for L1 certs… their motor mounts were not glued in… at all. They got aggravated with me when I said they couldnt fly until they found some glue. I asked why they just didnt buy a cheap commercial kit and follow the instructions… the response: “it looks better on my resume if I designed and built it.” Their faces were priceless when I said organizations like Space X couldnt care less about you getting an L1 cert. While I had s good laugh at that comment, someone obviously told them that - I assume someone at the University.

 

[Banzai88]

Edit: the K1800ST is I just realized a...2 grain...highly recommend grain bonding.

https://www.rocketryforum.com/threads/low-powered-75mm-motor-for-testing.178643/page-2#post-2405961

 

[kramer714]

LOL, i just told my other teammates about the chute and caliber and their reply via text "Wow, so now we need to get a new chute and motor. We really suck at this game"

yes (new motor) and yes (parachute)....

For the parachute, in your Internal Requirements document you should figure out the speed under droge and the speed under main chute, and yes, for this heavy redundant altimeter. BTW, for this heavy think about the size and attachment of the shock cord.

 

[rfjustin]

This manufacturer says 120". Each one will be different depending on how the chute is constructed.View attachment 568734View attachment 568735

Can we talk about Phil McCracken in print screens from your phone please? LOL

 

[Antares JS]

Their faces were priceless when I said organizations like Space X couldnt care less about you getting an L1 cert.

Not entirely true unless you specifically mean just the level 1, which is kind of a gimme compared to 2 and 3... I did have my level 2 cert on my resume and the last three places I worked at (all in aerospace, not SpaceX, but the third was Orbital ATK) brought it up in the interview and seemed impressed by what I told them about HPR in response to their questions.