29mm and a 10lb rocket

[bill2654]

According to my calculations a 29/180 with a H238T-M will launch a 10.6lbs. Heres how I came up with the figures. You divide 238 (average thrust in newtons) by 4.45 and get 53.483146 average thrust in pounds. Then using the 5:1 thrust to weight ratio divide by 5 and get 10.696629lbs. Can a 29/180 really work on a 10 lb rocket?

[Peter Olivola]

I think if you run a flight sim on this combination you'll find it will clear the launch rod but isn't likely to reach 100 feet necessitating something approaching a negative delay.

[bill2654]

You are right. The sim said it only reached 179 ft. at 10 lbs. Reducing to 5 lbs and reached 490 ft.

[edwinshap1]

+ you'd need some gigantic initial boost thrust to get the rocket off the pad safely, regardless of how high it flys, the H128 just won't have enough boost to reach a safe speed by the time it leaves the rod/rail.

[ScrapDaddy]

+ you'd need some gigantic initial boost thrust to get the rocket off the pad safely, regardless of how high it flys, the H128 just won't have enough boost to reach a safe speed by the time it leaves the rod/rail.

He stated that it was an H238T.
And for deployment, couldn't he use an alltimiter?

[Pantherjon]

He stated that it was an H238T.
And for deployment, couldn't he use an alltimiter?

Yes, he could..BUT the tricky part would be to get it going fast enough off the rod/rail in order for the rocket to go in the vertical UP direction and not horizontal..

[edwinshap1]

Yes, he could..BUT the tricky part would be to get it going fast enough off the rod/rail in order for the rocket to go in the vertical UP direction and not horizontal..

thats what i was aiming for...no pun intended :P

but yea, getting 10 pounds off the pad fast enough on an H238 would be a considerable challange wouldn't it?

[ScrapDaddy]

Yes, he could..BUT the tricky part would be to get it going fast enough off the rod/rail in order for the rocket to go in the vertical UP direction and not horizontal..

Point taken, but in all truth, who would launch a 10 Lb rocket on what is really just a glorified G motor .(it's only 3% H)
So how long would the rail have to be?

[Zeus-cat]

I typically allow 100 feet for my parachute to deploy and open when using a dual deploy system when I fly "low and slow". I have done a number of flights where apogee is below 500 feet. I pop the drogue at apogee and I set the altimeter to deploy the main chute at 300 feet. The data from the altimeter shows the rocket does not slow down to its landing speed under the main chute until it reaches about 200 feet.

I don't think I would use altimeter apogee deployment on a rocket that you expect to only get to 100 feet.

[Pantherjon]

Your right Scrap...I ran a sim in RS for grins..Took my Thor 3" and said it weighed 10lbs empty..Loaded the H238 in it and launched..First it shows that it would need 82" of rail to achieve a safe velocity..Max altitude was 161 feet..Did show a successful recovery..But something I would not do..

Doable? Yes
Advisable? no

[bandman444]

This is purely theoretically isn't it?

I guess the real question is that is a 29mm in a 10 pound rocket really safe? and I think with the above posts we can say not.

Now after reread ing the first post thee question seems more along the lines of is the 5:1 thrust rule viable, just curious what you guys say to that specifically

I would think that in most cases it works but obviously in high thrust low total impulse motors it doesn't really fit.

Just my thoughts

[ScrapDaddy]

Your right Scrap...I ran a sim in RS for grins..Took my Thor 3" and said it weighed 10lbs empty..Loaded the H238 in it and launched..First it shows that it would need 82" of rail to achieve a safe velocity..Max altitude was 161 feet..Did show a successful recovery..But something I would not do..

Doable? Yes
Advisable? no

Possible to get 82 feet of rail, $700+ dollars
Getting a pad to hold that rail up, Priceless

[bandman444]

Inches of rail scrap....not feet...easy mistake

[cjl]

It's absolutely doable, but anytime you have a short-burn motor with close to the minimum safe T:W ratio, you'll have a very, very low apogee (and therefore not much time to deploy a chute). If you really wanted to launch a 29mm 10lb rocket, the H268 would be a better choice.

[bguffer]

Two stage rocket?

Bob

[bobkrech]

Yes, but why? It's technically doable to launch a 10 pound rocket on a 3 grain 29 mm H238T on a calm day, but you need a 3 second delay to deploy at apogee, so the recovery is really iffy. Reducing the minimum 6 second stock delay of a H238T to 3 seconds reliably is a real leap of faith.

A 38mm motor is a much better choice for a 10 pound rocket, and gives a number of options. It's not that a 29 mm motor is always a bad choice, it's simply that the H238T motor does not have enough total impulse to lift the rocket to an apogee that permits a reasonable delay error to still allow for a safe recovery. With an apogee of ~180', it going to be difficult to get an 8' parachute for a 10 pound rocket out and inflated properly if the ejection time delay is off. A 4G H180W or H250G would be better choices, but AT doesn't really have a good 29 mm motor for such a heavy rocket.

The new Pro29 282H399-12A White Thunder would be a great motor for this rocket. You would have an apogee of 530' and need to reduce the adjustable delay by -7 seconds so the ejection would occur at apogee at 5 seconds after burnout.

Bob

[sylvie369]

I don't think I would use altimeter apogee deployment on a rocket that you expect to only get to 100 feet.

It's also important to be aware that altimeters often have minimum arming altitudes. The MAWD, for example, doesn't go into flight mode until it has detected that it has reached 160 feet AGL. If you don't make 160 feet (for example, because you flew somewhat horizontally, a strong possibility in this case), your altimeter is just more expensive debris after you crash (and of course it also adds to the rocket's weight, making it even less likely to fly right).

OP said that his sims gave him 179 feet. Flying that with an altimeter would be a rather pointless gamble. The odds of costly destruction are very high.

[cjl]

Yes, but why? It's technically doable to launch a 10 pound rocket on a 3 grain 29 mm H238T on a calm day, but you need a 3 second delay to deploy at apogee, so the recovery is really iffy. Reducing the minimum 6 second stock delay of a H238T to 3 seconds reliably is a real leap of faith.

A 38mm motor is a much better choice for a 10 pound rocket, and gives a number of options. It's not that a 29 mm motor is always a bad choice, it's simply that the H238T motor does not have enough total impulse to lift the rocket to an apogee that permits a reasonable delay error to still allow for a safe recovery. With an apogee of ~180', it going to be difficult to get an 8' parachute for a 10 pound rocket out and inflated properly if the ejection time delay is off. A 4G H180W or H250G would be better choices, but AT doesn't really have a good 29 mm motor for such a heavy rocket.

The new Pro29 282H399-12A White Thunder would be a great motor for this rocket. You would have an apogee of 530' and need to reduce the adjustable delay by -7 seconds so the ejection would occur at apogee at 5 seconds after burnout.

Bob

Good point about the H399 - that would be an excellent choice for a rocket this heavy. As for AT not really having any good 29's for a rocket like this though, did you consider the H268R? It's a much larger H than any of the ones you mentioned, and has a 7:1 T:W ratio off the pad.

[bill2654]

It never ceases to amaze me how many replies you get with a hypothetical question.

[UPscaler]

let's not forget what happens when you don't get off the rail fast enough.

[MarkII]

It never ceases to amaze me how many replies you get with a hypothetical question.

Why not? With a hypothetical, you are asking people to speculate about something that has not happened. That's guaranteed to generate a whole slew of answers. Isn't the forum all about exchanging ideas?

[sylvie369]

It never ceases to amaze me how many replies you get with a hypothetical question.

I'm pretty sure that answering hypothetical questions is what made the space program happen in the first place.

In fact "we take hypothetical questions seriously" would be a pretty good slogan for all of rocketry.

[JDcluster]

Two of them might do the job, but not by itself.
I have just such a rocket; a 4" Thoy NightHawk that comes in around 10lbs ( maybe a tad more) & I need at least an H268 to lift it.

I've done an H 268 to 2 H220's later air-starting 4 G 55's & they all lit!


The H 268 only took up off the rod about 70ft or so then the H220's kicked in at the pre-programmed 1/2 sec after liftoff.


Is I remember correctly; it reached an altitude of around 3800 ft or so.... It's a little hard to remember it's been about 5-6 years ago & the payload / altimeter section was in a tree for a year or so.


How's that for not being Hypothetical; real world data!


JD

According to my calculations a 29/180 with a H238T-M will launch a 10.6lbs. Heres how I came up with the figures. You divide 238 (average thrust in newtons) by 4.45 and get 53.483146 average thrust in pounds. Then using the 5:1 thrust to weight ratio divide by 5 and get 10.696629lbs. Can a 29/180 really work on a 10 lb rocket?

[bobkrech]

Good point about the H399 - that would be an excellent choice for a rocket this heavy. As for AT not really having any good 29's for a rocket like this though, did you consider the H268R? It's a much larger H than any of the ones you mentioned, and has a 7:1 T:W ratio off the pad.

No I didn't. I thought the TI was lower, bur being nearly a full H, the apogee with a H268R is about 600'. The AT268 would work if the ejection charge can be shortened to 5 seconds, or better yet with electronic deployment.

While there are a few 29mm motors that can work, a 38mm motor mount is the minimum sixe I would use in a 10 pound rocket. You simply have a much larger selection of motors in 38 mm than 29 mm for such a heavy rocket.

Bob

[cjl]

Absolutely agreed - although the 29mm can be done, it limits your motor selection so much that you'd only be able to launch on a couple of motors. 38mm would be a much better choice.

[JDcluster]

A Kosdon I 385 will also work, that's if you have the head room for that motor;it's 15" long.




JD

Absolutely agreed - although the 29mm can be done, it limits your motor selection so much that you'd only be able to launch on a couple of motors. 38mm would be a much better choice.

[Adrian A]

When I'm flying mid-power with an altimeter on a small field, I've gone down to 250' for the main deployment altitude. I z-fold the chute and it pops open immediately, and it looks like it hits terminal velocity less than halfway from there. I bet an altimeter-controlled apogee chute deployment 150' off the deck would work, even for the larger chute needed for a 10' rocket. But skip the 30' shock cord.

[tomar]

I z-fold the chute and it pops open immediately

It amazes me how many rocketeers don't know about the Z-fold technique for parachutes. I can't tell you how many times I've shown this to experienced HPR fliers who have never seen it. I always share it with new fliers both model and HPR. You are correct, INSTANT open. Although, it's probably not the best technique for limited space applicaions or where a larger parachure is going into a small airframe. But then, what technique is?

BTW The same goes for streamers.

[rocket_hall]

A friends motor CATO'ed and blew the the rear closer. It was a perfect flight to 30-40' chute out at apogee safe recovery landed 10' from the pad.

Richard

[MarkII]

IAlthough, it's probably not the best technique for limited space applicaions or where a larger parachure is going into a small airframe. But then, what technique is?

Folding the spiked canopy by thirds and then rolling the shroud lines tightly around the outside (the Estes-style method) has always worked for me. Have you ever stuffed a regular-mill nylon parachute into an ST-8F (0.885" ID) airframe? And then see it deploy perfectly?

BTW The same goes for streamers.

Either fan folding or folding it in half repeatedly until it fits. I have had success with both methods. How about putting a 6"x60" drafting paper streamer into a BT-20?