More minimum diameter level 1 rocket questions

[Liam Steele]

Hello,

I am designing and building a 54mm G12 fiberglass minimum diameter rocket with an AeroTech I65W-PS Single Use DMS motor that will go around 8000ft and a max velocity of around Mach 0.80. I am trying to ask as many questions and learn as much as possible before I build and fly this rocket. I have a couple of questions regarding avionics and recovery:

1. This rocket will use a dual deploy recovery method, but what would be a good size or target descent rate for the main and drogue chutes?

2. I am planning on using a TeleMetrum v4.0 as the primary flight computer, along with a backup flight computer of some kind, and maybe a couple of onboard cameras. Any suggestions on alternate avionics or good backup flight computer choices?

I have already posted a similar question on a different thread that I started, but I felt that this should be on a separate thread.

Thank you,
Liam Steele

 

[NTP2]

I’d just ask there, we don’t have a strict topic adherence policy or anything, but to answer your questions

1 the dissent under drogue should be fast enough to stop drifting but slow enough to make the main not rip this is highly dependent on your set up.
2 a egg timer would work well (probably a quantum)

 

[David_Stack]

Liam;

You already asked these same questions as an addendum to your original thread in post #21. Then in Post #25 Boatgeek provided the following feedback to your question on descent rate:

"Typical descent speeds under drogue are 80-100 FPS, and 20-25 FPS under main. The latter depends heavily on what kind of terrain you're landing on. Sod is far more forgiving than concrete or dry lakebed. Remember that you need to keep the fins on in order to get your certification.".

If you search the forum you will see innumerable posts discussing descent rates, and as Boatgeek noted, the answers depend on a number of variables, some of which are established by the flier (how good is my construction, how much confidence do I have that the rocket can survive an impact of 'x'), and others by external factors like field size, wind speed, and the surface you are landing on.

 

[watheyak]

Have you checked your thrust to weight ratio? A dual deploy fiberglass rocket might be too heavy to achieve an acceptable velocity off the rail.

I love I65s, but even very light cardboard rockets struggle with the long and low thrust.

 

[Steve Shannon]

Have you checked your thrust to weight ratio? A dual deploy fiberglass rocket might be too heavy to achieve an acceptable velocity off the rail.

I love I65s, but even very light cardboard rockets struggle with the long and low thrust.

Liam,
To Scott’s point, ThrustCurve shows an immediate thrust of 144 Newtons which continues for half of a second and then drops off quickly, but the average is still just 80 Newtons. Our safety code says 5:1 thrust to weight should be used, so your entire rocket must weigh less than 16 Newtons (80/5). That’s 1631 grams of mass for the entire rocket including the motor, which is 752 grams. So your rocket, without motor, must be 879 grams or less. That’s less than 2 pounds. It can be done, but you need to be aware of this limit.



https://www.thrustcurve.org/motors/AeroTech/HP-I65W/thrustcurve.svg

 

[Titan II]

Our safety code says 5:1 thrust to weight should be used...

That statement out of context is somewhat deceptive. Here it is in context.

"The thrust-to-weight ratio of a rocket typically should be at least 5:1. However, the RSO may approve a thrust-to-weight as low as 3:1 ratio. Initial thrust-to-weight ratios lower than 3:1 may only be authorized by an RSO if an active stability system is included."

 

[bad_idea]

That statement out of context is somewhat deceptive. Here it is in context.

"The thrust-to-weight ratio of a rocket typically should be at least 5:1. However, the RSO may approve a thrust-to-weight as low as 3:1 ratio. Initial thrust-to-weight ratios lower than 3:1 may only be authorized by an RSO if an active stability system is included."

Since the last sentence mentions "initial thrust-to-weight," I've always assumed the first sentence meant initial thrust-to-weight too.

I'm not a huge fan of pushing the limits on TTW into that region between 3:1 and 5:1, but in this case I'd feel fine going with 5:1 using the 144N figure for initial thrust. If the intent of the rule is otherwise, perhaps its language should be revised for clarity.

 

[Neutronium95]

With any reasonable minimum diameter design, the TWR should be fine. If this wasn't a minimum diameter rocket, fiberglass would likely be too heavy, but this is a minimum diameter design.

@Liam Steele What do you want from this rocket? Building a 54mm minimum diameter rocket just for an I65 seems kind of odd. Do you want to fly longer motors in it in the future? It's one thing to build a rocket for an I65, but there are a ton of significantly punchier 54mm motors out there for after you get your L2 cert.

For recovery, I'd target roughly a 100 ft/s drogue descent rate, and a 20-25 ft/s main descent rate. At this size, you'll probably be able to fly it drogueless, just separating the rocket in half at apogee to tumble until the main deploys. I like Top Flight thin mill parachutes, they pack small and are affordable.

If you're looking for design inspiration, I'd strongly suggest looking at the Wildman Rocketry Mach 2 kit. It's a 54mm minimum diameter kit that's set up for head end dual deploy, which is just normal dual deploy, but the main is packed inside the nosecone.

 

[rcktnut]

Liam,
You must have a sim program, show us your intended design, that would help us help you.