Sentinel upscale 2019

[neil_w]

Thanks mbeels.
Using descent rate calculator (such as this one.) in mbeels post.
I entered 36 ounces rocket weight, (rocket parachute and motor, although the motor will not weigh as much after it burns out the propellant, delay and ejection charges, but I thought I saw some of those motors weighing in at 16 ounces.

Good heavens, no you didn't.

In the E and F range your motors will weigh in at less than 2 oz on the pad. The casings (AT single use) are about 1.5 oz, based on a quick data perusal. Go run those numbers again!

Also note: OR will give you descent rate calculations as well. Once you have a model in OR it'll tell you just about everything you need to know.

 

[mbeels]

Yeah, the total weights of the F44W and E30 are 48 g and 43.3 g respectively! And half of that is propellant, so you'll have about a single oz of empty casing weight.

 

[Event Horizon]

Thanks guys, I thought something was fishy there, but I've never held a motor larger that a C6-7 so I didn't know.
Anyway here's the results at 20 ounces of overall weight. A 36" dia. chute has a 13.73 ft/sec descent rate.
I find the results pretty functional.
Of course there's always more questions. Should I reduce the size of the chute diameter if it has a spill hole for the descent rate calculation ?

You entered:

• Rocket mass: 20 ounces
• Parachute shape/type: circular
• Parachute size: 36 inches measured by diameter

Calculation Results

Descent rate:

• 13.73 ft/sec
• 4.18 meters/sec
• 15.06 km/hr
• 9.36 mph
  
  

 

[neil_w]

Unless you make the spill hole gigantic it won't have much of an effect on descent rate. Normally you're not fine-tuning descent rate to that extent anyway.

 

[Event Horizon]

Thanks neil_w

 

[mbeels]

It will probably come down nice and gently. And with those bottom fins hanging down, it may not be a bad thing. You could always change out the parachute later if you find it has a tendency to drift too far.

 

[Event Horizon]

Those fins do hang down well below the bottom of the body tube.

They will be subject to abuse. I hope they can take the abuse.

They are made from 3mm plywood.

They are TTW with a bulkhead in front and a bulkhead behind the fin tabs.

The fin tabs are glued to the body tube, to the motor mount and to both the bulkheads front and back of the tab.

That's why the thing is so dang heavy.....along with all the paint......the motor tube is 12 inches long with a bulkhead at the end and an ejection baffle in there too.

The next one will have a 29 mm motor mount ...... All the parts are here except for the 29mm motor mount.

 

[Bruiser]

May I suggest you draw it up in OpenRocket and see if a 29mm will help.

This rocket sounds just like my pseudo scale Lockheed Defender. There's only like two 24mm motors that it will fly with and they are the Aerotech single use motors. I had planned on Estes motors but they just won't do. I was ready to hack out the 24mm mount and install a 29mm mount when Neil pointed out that the rocket would not benefit from that. The extra weight of the motor would require even more nose weight and the combined extra weight would negate the extra thrust. So I'll buy a couple of motors made of gold and launch it once or twice then it will go on the shelf. Next time I will be more mindful of weight

-Bob

 

[neil_w]

A 29mm will definitely help. This rocket has a more traditional design, and likely plenty of stability margin. A BT-80 rocket is right in the middle of mid-power range, so it was inevitably going to be pushing its luck with a 24mm mount. But it should be able to fly well on those F motors, and look great in the process.

Bigger rockets -> bigger motors -> emptier wallet, can't fight the math....

 

[Event Horizon]

Aerotech E30T-7 has 48.3 newtons of thrust or 10.85 lbs of thrust for 1.2 seconds. The delay is 7 seconds.

The rocket will weigh in at a pound +/-

I think this is a Lawn Dart enabling motor for this rocket??

The F32T-6 produces 61.3 newtons of thrust or 13.78 lbs for 1.7 seconds. The delay is 6 seconds.

I think this motor may actually get the rocket to an altitude that the time delay will blow off the nose cone before turf impact.

Yikes.....the fear of lawn dart CATO

maybe I should retire this rocket to static display status before that occurs.

 

[Event Horizon]

"Quote"
This rocket sounds just like my pseudo scale Lockheed Defender. There's only like two 24mm motors that it will fly with and they are the Aerotech single use motors. I had planned on Estes motors but they just won't do. I was ready to hack out the 24mm mount and install a 29mm mount when Neil pointed out that the rocket would not benefit from that. The extra weight of the motor would require even more nose weight and the combined extra weight would negate the extra thrust. So I'll buy a couple of motors made of gold and launch it once or twice then it will go on the shelf. Next time I will be more mindful of weight

-Bob
"unquote"

Bruiser,

Please tell me which two motors those are.

Thanks

 

[Bruiser]

They are the same motors that I think Neil was writing about for this rocket. That's what caught my eye.

I just went back to my thread to verify and they are the Aerotech E30 and F44 motors.

-Bob

 

[neil_w]

Aerotech E30T-7 has 48.3 newtons of thrust or 10.85 lbs of thrust for 1.2 seconds. The delay is 7 seconds.

The rocket will weigh in at a pound +/-

I think this is a Lawn Dart enabling motor for this rocket??

The F32T-6 produces 61.3 newtons of thrust or 13.78 lbs for 1.7 seconds. The delay is 6 seconds.

I think this motor may actually get the rocket to an altitude that the time delay will blow off the nose cone before turf impact.

Yikes.....the fear of lawn dart CATO

Dude: Do not do this by feel. Use simulations. This rocket will fly fine with the appropriate motor and delay, no need to shelve it.

I noticed that Thrustcurve was assuming a 3 ft rod, which explains why it couldn't find a good motor. I re-entered the rocket with dry weight of 18 oz (allowed an ounce for the parachute), 6 foot rod/rail, and got this:


I highlighted the single-use motors. The E30 or F44 will give similar flights; the F32 will get you up considerably higher, but it costs a bit more (hey, more N-s = more $). This table also gives approximate delays. For the E30, ideal delay is around 5s so you'd probably choose a -4 (it comes in -4 or -7). For the F44, which only comes in -4 or -8, you'll again choose a -4. The F32 comes in -4, -6, or -8, so you'll go with a -4 or a -6, depending on your preferences, and whether the rocket is actually more or less draggy than thrustcurve is assuming.

An OR sim might give you slightly more accurate results, since it uses a more detailed model of the rocket, but I'd guess the thrustcurve numbers are pretty close.

[edit: fixed delay discussion]

 

[Nytrunner]

There's always reloadables B)

Word on Thrustcurve sims, they'll show "red" if the launch velocity is under 50 ft/s, but above ~45 is fine (Heck, my L1 flight was "red" by thrustcurve's judgment)

 

[neil_w]

Word on Thrustcurve sims, they'll show "red" if the launch velocity is under 50 ft/s, but above ~45 is fine (Heck, my L1 flight was "red" by thrustcurve's judgment)

Good point. Here's the "failed" list:


Remember that the E15s they're selling today are exactly the same as the E20s. So you could fly an E15 or E20 (I buy E15s because they're marginally cheaper for whatever reason), definitely with 4 second delay. That'll get you a slower liftoff if you're into that sort of thing.

 

[mbeels]

The E30 comes in 4 or 7 second delays, and the F44 comes with 4 or 8 second delays. Either the E30-4 or F44-4 would give you good flights. As neil_w said, no need to shelve it! It'll fly fine.

 

[Event Horizon]

Thanks Bruiser, neil_w, Nytrunner and mbeels.
I appreciate the good advice.

 

[Bruiser]

A 29mm will definitely help. This rocket has a more traditional design, and likely plenty of stability margin.

Bigger rockets -> bigger motors -> emptier wallet, can't fight the math....

Well, that's true enough. If changing to a 29mm won't require extra weight then there is no question what I would do next...

Head to Hobby Lobby and buy every 29mm motor they have on clearance

Bob

 

[neil_w]

Keep in mind though that the 29mm BP motors have comparatively low thrust. An 18 oz rocket is on the edge for them. My friend flew her TARC rocket on an Estes E16, at about 13-14 oz dry weight. Let me tell you, that was a slow-motion flight. Straight and true, but we were a bit taken aback by it. Thrustcurve shows 31 ft/sec off the rod for an F15, that is really pretty slow.

A 1 lb rocket is really too heavy for the BP motors.

 

[Nytrunner]

Word of caution:

Hobby Lobby has Estes blackpowder 29mm motors. THe E16 and F15 have lower thrust than most composite motors, and lower initial thrust than even the Estes D12.

Go to thrustcurve.org and look at the thrustcurve for a motor, specifically the first 1/4 second. The average thrust (number after the letter in motor names) can be deceptive.

edit: Dangit Neil! You're quick

 

[Event Horizon]

neil w

Where did you find the option of changing the length of the launch rail in thrustcurve's motor program??

Thanks

 

[neil_w]

When you're logged in and creating or editing a rocket it looks like this:

 

[Event Horizon]

Thanks neil w.
That would do it.
I didn't bother to create an account with thrustcurve, I just used their site as a guest

 

[LithosphereRocketry]

First of all- good looking build so far!

I think thrustcurve is pretty spot on with what this can be trusted to fly on. My 3" saturn 1B in my profile pic is flying on an E15-4w and weighs about 13oz. It flies fine on those but that's about the lower limit. 29mm would be ideal for this rocket but certainly not necessary, the E30/F44/F32 range would all do fine.

You said your dry weight was around 20 oz pre-motor? For that I think a 36" chute is overkill unless you're really concerned about those fins- 24" should be ok. That said it doesn't hurt to be extra safe.

 

[neil_w]

...

Welcome back!

 

[Event Horizon]

Thanks for the encouraging words Litho.

 

[Event Horizon]

Well, am I ever happy that I didn't clear coat the upscale Sentinel with Krylon Gloss Clear paint.
I sprayed the exact scale Sentinel with Krylon clear and it chicken tracked in different areas, not all over, which makes no sense that it was only some of the rocket.
The nose cone didn't chicken track at all.
Krylon has a real problem with their clear not being compatible with their color paints.
I let the paint on that rocket cure for at least two months. I did not contaminate it by touching it.
Now I'm tempted to try Future floor wax or just let it go with out a clear coat.
Ugh, dreading trying to refinish the exact scale because of Krylon.....

 

[Event Horizon]

I did a search regarding chicken tracking or wrinkling.
The result is it's my fault for applying the clear paint to thickly.
Well that's nice to know.
Still don't trust it, cuz it only happens with the clear.

 

[neil_w]

Which clear are you using, exactly?

 

[Event Horizon]

Krylon Gloss Clear, I'm not at home right now so I don't have the part number.
I'll look at the can and post it up.