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ChrisLentz

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I have been building rockets for about a year.
I have Rocksim.
My understanding....

Rocksim will calculate where my CP is based on the dimensions of my airframe.
The CG variable doesn't matter as much during design because you can always change the CG very easily with nose ballast.
As long as I keep my static stability margin at a stable distance as the end result, I should be able to reach the altitudes simulated.

Am I way off base here with the simplicity of the build?

Design a rocket, use the CP determination by Rocksim, maintain a SSM by adjusting your CG. Fly.

Could it really be that simple to DESIGN and complete a rocket?

My first scratch E30T-7 was built with this concept and it looked like it went exactly as the sim projected. Without my altimeter onboard it is hard to tell but the timing went according to sim.

I did adjust to the weight of the motor when designing taking in mind shift in CG as the propellant is exhausted.

Mass and CG override is my friend.

The point is I guess I am trying to make is...could it be this simple as to not really worry about the CG during the design phase.
 
ChrisLentz said:
I have been building rockets for about a year.
I have Rocksim.
My understanding....

Rocksim will calculate where my CP is based on the dimensions of my airframe.
The CG variable doesn't matter as much during design because you can always change the CG very easily with nose ballast.
As long as I keep my static stability margin at a stable distance as the end result, I should be able to reach the altitudes simulated.

Am I way off base here with the simplicity of the build?

Design a rocket, use the CP determination by Rocksim, maintain a SSM by adjusting your CG. Fly.

Could it really be that simple to DESIGN and complete a rocket?

My first scratch E30T-7 was built with this concept and it looked like it went exactly as the sim projected. Without my altimeter onboard it is hard to tell but the timing went according to sim.

I did adjust to the weight of the motor when designing taking in mind shift in CG as the propellant is exhausted.

Mass and CG override is my friend.

The point is I guess I am trying to make is...could it be this simple as to not really worry about the CG during the design phase.
Click to expand...

I'm far from an expert, but it would seem to me the danger to your approach is finding out you need so much nose weight you can't attain sufficient lift-off velocity. Related to that, but less dangerous, is the fact that without accounting for the mass required to reach sufficient stability during design, your sims will surely be off. It's interesting that your E30 appeared to work that way, but you didn't indicate how much mass you added- while somewhat counter intuitive, optimum mass increases altitude. If you added no mass, and the sim was pretty close with final build mass, then it makes sense what you saw, but I would venture a guess that that is the exception, not the rule.
 
one should be aware of the projected cg point and use that as a starting point during design. if the final build weight is fairly close to the simulation then so should the cg.(one should of course check the cg of the finished rocket and adjust as needed) reasonably sure that both open rocket and rocksim account for the cg shift when doing the flight simulations.
Rex
 
Obviously your real-life, measured CG balance point trumps the sim. And yes, if your real-life CG is at least one cal forward of your CP, then you are stable. But the usefulness of sims is calculating expected delays, determining Mach transition fluctuations, determining speed off the rail, determining performance in various wind conditions. The flight sim is only as accurate as the build sim. Other things like base drag affect stability, and some rockets are stable even if they appear not to meet the CG/CP rule (the MDRM, V2, and Fat Boy come to mind).

Get an altimeter and start comparing your sims to real life to make them better. It's a win/win.
 
to answer the original question, yes it really is that simple.
Rex
 
Here is my latest build
g125 screen shot.jpg

I have added a few extra hundred g to compensate for moving my SSM by a couple of inches.
 
if you were to make your fin's root edge an inch shorter and add an inch to the fin semi-span, you could probably remove some of your nose weight (which I presume is that tiny blob of Osmium at the tip of the nose :)).
Rex
 
When adding nose weight, I try and get the density to match the material I'll be using for the weight. That way, you know how big of a blob you'll need, e.g. 8g/cm3 for a steel bolt, 1.5g/cm3 for clay, and so on.
 
Sadly, the fins on the sim are slightly off with regards to the TTW. There are 2 tabs that go through the wall.
One of them locking in the motor mount behind the forward centering ring.
The fins are actually cut already so I can't really change them now. My fin stretcher is broken lol!

And I haven't added the nose weight either.
Here is a shot of without mass over ride.
without mass overide..jpg
its missing a couple of things. Motor retainer (wired in place with the shock cord attached at the other end)
This is also a reason why adding some weight and shifting the CG during the build process.
The final CG and weight is put to sim before flight to ensure stability.
Seems to work for me.

So far.
 
An issue is that long and narrow fins are not efficient. You're better off with a larger fin span and a smaller fin root.

Fins are wings, and should have a have a height equivalent to 1 to 1.5 times the airframe diameter. This makes the fin span 3 to 4.5 times the airframe diameter.

If you were to resize your fins to reduce the root by a 1/3 to 1/2, and increase the span by 1/3 you would find the CP moves aft a fair bit.

For your first rocket I suggest making your CG with the motor loaded forward of the CP by ~2 calibers (airframe diameters), preferably without adding significant nose weight.

Also a G125 is a high power motor because the average thrust is > 80 N, so you will need to be L1 certified to launch it.

Bob
 
Another thing to keep in mind is that if you add a lot of nose weight and you have a large payload volume you may need to augment the ejection charge of a smaller motor like a G. They typically only have 1gm of BP that may not be sufficient in larger, heavier rockets. I once witnessed a group of college students who were going for their L1 crash several rockets in a row because they just assumed the ejection charge of the motor was sufficient to move a heavy nosecone and associated laundry.

For a reinforcement of what has already been said, look at this Google search:

https://www.google.com/search?q=sounding+rockets&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjqzcGNxMTMAhVFYiYKHe6PBbQQ_AUIBygB&biw=1630&bih=939

You'll notice the vast majority of fins are short and wide rather than long and narrow.

So yes, it is that easy but it may not lead to very good results. May as well follow simple guidelines that will produce good results without a lot of post design effort.


Tony
 
Making them fly is the simplest part.
Building them in a fashion that pleases your senses is the hard part.
If all they had to do was fly safely, I'de be a flying fool!!!
 
You guys are awesome. I appreciate all of the information provided to me. I am completely self taught with extremely limited resources.-
I am of the thought that true rocket science doesn`t come from a kit build. Someone else designed rocket and a bit of glue doesn`t make you a rocket scientist in my opinion.

I will play with the fin design on Rocksim and see what your recommendations do for my CP. This will hopefully be my L1 Cert build.

According to CAR HP rules. `My high power rocket is equipped with authorized motor(s) with a total impulse between 160 and 40,960 Newton seconds.` It says nothing about greater than 80.
 
In the US, a G125 is HP because it's average impulse is over 80 - US rules
In Canada a G125 is MP because it's total impulse is under 160 - Canadian rules
 
TopRamen said:
Making them fly is the simplest part.
Building them in a fashion that pleases your senses is the hard part.
If all they had to do was fly safely, I'de be a flying fool!!!
Click to expand...

All I do is fly them safely. Notice my Newton count... Lots of flights. Not all my rockets are pretty...
Although, I'm considering to a very detailed MPR build just to slow myself down and challenge my craftsman side
 
ChrisLentz said:
You guys are awesome. I appreciate all of the information provided to me. I am completely self taught with extremely limited resources.-
I am of the thought that true rocket science doesn`t come from a kit build. Someone else designed rocket and a bit of glue doesn`t make you a rocket scientist in my opinion.

I will play with the fin design on Rocksim and see what your recommendations do for my CP. This will hopefully be my L1 Cert build.

According to CAR HP rules. `My high power rocket is equipped with authorized motor(s) with a total impulse between 160 and 40,960 Newton seconds.` It says nothing about greater than 80.
Click to expand...

Scratch-building is craftsmanship. Rocket science is understanding things like thrust, stability, and aerodynamics. The two may go together, but they don't have to. I build kits. Why? Because I don't have the time, money or space to truly scratch-build. But that doesn't mean I don't understand the physics behind what is going on, even when building a kit.

While it is true that in order to scratch-build a rocket and fly and recover it successfully, you must understand the science, the converse is not true (unless you scratch-build, you don't understand the science).
 
ChrisLentz said:
I am of the thought that true rocket science doesn`t come from a kit build. Someone else designed rocket and a bit of glue doesn`t make you a rocket scientist in my opinion.
Click to expand...

Scratch building doesn't make you a rocket scientist, either.

<soapbox>

This is a hobby, for enjoyment. Scratch building is fun. Building kits is also fun. Both can be very easy or extraordinarily challenging. Do what you enjoy, and don't worry about who is or is not a "rocket scientist".

</soapbox>
 
I am not so worried about who is or isn't a scientist. I am just saying that there is more science involved with scratch building.
I am a terrible craftsman, but I love the science. I do think that my craftsmanship is improving as the builds come along.
There was no intention on my part to insult anyone.

I love this hobby and am proud to be a part of something more cranial than stamp collecting. Although I am sure the stamp enthusiasts will slaughter me for it.
 
Also remember that RS doesn't take adhesives into account. The aft end of the rocket is usually heavier than RS's estimate due to whatever adhesives you use to attach your fins, especially if they are through-the-wall and you use larger fillets along with all the motor tube joints. So your CG is likely closer to the aft end of the rocket than what RS estimates.
 
Pat_B said:
Also remember that RS doesn't take adhesives into account. The aft end of the rocket is usually heavier than RS's estimate due to whatever adhesives you use to attach your fins, especially if they are through-the-wall and you use larger fillets along with all the motor tube joints. So your CG is likely closer to the aft end of the rocket than what RS estimates.
Click to expand...

You are expected to compensate for that by overriding the mass of the rocket and adjusting the location of the Cg.
 
You are expected to compensate for that by overriding the mass of the rocket and adjusting the location of the Cg.
Click to expand...

That works fine once it's built. But for people who are new to RS, they often don't realize that shift in CG during the design phase.
 
For scratch, build around the CG so that you don't have to add weight. Too much length is over stable and not optimal. Too little length requires added weight which unless you are building scale models and need to fudge CG vs Cp to keep scale, is not optimal.
 
ChrisLentz said:
I am not so worried about who is or isn't a scientist. I am just saying that there is more science involved with scratch building.
I am a terrible craftsman, but I love the science. I do think that my craftsmanship is improving as the builds come along.
There was no intention on my part to insult anyone.

I love this hobby and am proud to be a part of something more cranial than stamp collecting. Although I am sure the stamp enthusiasts will slaughter me for it.
Click to expand...

I haven't built a kit in ages.

Once you get a few kits under your belt and modified them for things like dual deployment, you are one foot closer to design/scratchbuilding.
For me, there is something uniquely satisfying about seeing something on paper become a reality, particularly if it is a subject, size and design not available as a kit.

That said, Do what ever makes you happy.
 
Last edited:

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