Flexibility of "first launch" engine recommendations...

Hello,

Here's another newbie question. How stringent are the guidelines around "for first launch" engine recommendations. For example, I'm building a kit that I hope to launch in the near future that has Recommended Engines: A8-3 for first launch, but then lists engine types cascading up to C6-7. Could I launch it with a C engine on the first launch and what risks do I introduce by doing so?

Thanks!

Your main risks are losing the rocket and having the delay be wrong.

If the A8-3 or the recommended B gave you a little bit of a long delay, you might choose a C6-5 when you go to that impulse level. Extra weight, drag from misaligned fins, or drag from a rough surface finish can all reduce the altitude from the ideal number on the package. If the delay is off, you have a higher risk of zippering the tube or stripping the chute off, though that's not as bad if you have an elastic shock cord.

Obviously, more motor gives you more altitude. Depending on how big your field is, that can be a much higher risk of losing the rocket. Cutting a spill hole in the chute is a good way to get rockets back, as is only launching on a big enough field. A good rule of thumb is that the rocket should go no higher than twice the length of the shortest side of your field. If you're on a giant sod farm, that's not much of an issue. On a smaller field with trees and ponds, it's a much bigger issue.

In addition, the smaller engines will give you a better idea of the rocket's flight characteristics.
Does it fly to the estimated height, or does it exceed it? Does it have a tendency to fly straight, or does it exhibit an odd flight profile? Is it stable, or does it need additional nose weight? Most importantly, does everything work as it should, or do you need to tweek some things before sending it waaaay out there?

How much is your time worth?
Going with the recommended “first flight” motor (usually one of the lower power choices) risks at worst an “underwhelming” flight, but a very high probability of getting the rocket back to fly again.

Once you see how your specific built rocket performs on the recommended first flight motor, you are in a better position to predict what other motors, particularly more powerful ones, are going to do. Will the rocket stay in sight? How much does it drift on recovery? How does it handle wind? IS MY FIELD BIG ENOUGH FOR THIS MOTOR.

Going big on your first flight, especially when you have ANY wind and when you don’t have a huge open recovery area without trees and bushes and puddles,etc, has a very high probability of losing the rocket. Losing a rocket on its first flight, especially one that you put a lot of time and effort into, really bites.

For me, low power motors are cheap. My time (at least to me) isn’t.

Just a wild guess:
If you have built it excessively heavy the CG could be further back on the rocket than the manufacturer intended. Using a smaller motor actually moves the CG less to the rear than a when fitting larger motor, so it should be more stable on the smaller one. So the initial flight should be stable, then progress to larger motors which give smaller static stability margins if things are ok. If it is a bit squirrely on the small motor then check static stability.

You can find the CP using a simulation program or the cardboard cutout method. Mark that on the airframe with a small x. Once the motor and chute are loaded you can check the CG is in front of the CP, and the RSO might also ask you about that.

Other reasons above that other people have listed are also possibilities.

FWIW, I don't usually put a smaller motor in for "first flight" and have not had any problems. YMMV.

OverTheTop said:

FWIW, I don't usually put a smaller motor in for "first flight" and have not had any problems. YMMV.

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You are also a Level 3 experienced rocketeer, and therefore certainly far more adept at gauging rocket, wind, and field conditions than your average bear!

Your recommendation of running it by the RSO (if Ewomack happens to be at a club launch) is spot on. Any RSO should be able to steer a self-proclaimed "newbie" to the appropriate engine for a first flight.

If I recall correctly, aside from the new Estes Saturn V (which only HAS one recommended engine), the recommended first flight engine for most model kits is definitely NOT the highest recommended engine, but it also is not necessarily the lowest. For a Newbie, if he or she doesn't HAVE a club to fly with or any other experienced rocketeers around for further guidance, it seems reasonable to assume the manufacturer knows enough about the flight characteristics to recommend an appropriate engine for first flight. So I have a hard time telling a newbie to ignore the kit maker's advice and drive it up a notch or two. I definitely wouldn't jump from an A to a C.

Keep in mind that Estes only recommends their own product. You will not see recommendations for Aerotech, Cesaroni, Loki, etc. I put an AT E15-4 in my Saturn V. Unfortunately it did not get off the pad, so I don't know if it was the best choice. I'll let you know next month.

ewomack said:

Hello,

Here's another newbie question. How stringent are the guidelines around "for first launch" engine recommendations. For example, I'm building a kit that I hope to launch in the near future that has Recommended Engines: A8-3 for first launch, but then lists engine types cascading up to C6-7. Could I launch it with a C engine on the first launch and what risks do I introduce by doing so?

Thanks!

Click to expand...

Certainly you could use a C on the first launch. As long as you built the kit according to the instructions, and used the materials they supplied, the chances of any issues are very minimal.

Good luck and be sure to let us know how everything works out.

If you go straight for the C, you were probably programmed at an early age:

lakeroadster said:

Certainly you could use a C on the first launch. As long as you built the kit according to the instructions, and used the materials they supplied, the chances of any issues are very minimal.

Good luck and be sure to let us know how everything works out.

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Of course you can.

The real benefit of first flight engine recommendations is to give a new rocketeer (that knows very little about recovery space and drift time) one last warning before they punch it and get sad they lost the rocket they just spend x-hours building. There was a thread by such an individual not too long ago asking whether the hobby was worth it if he was going to just keep losing rockets/.

Nytrunner said:

Of course you can.

The real benefit of first flight engine recommendations is to give a new rocketeer (that knows very little about recovery space and drift time) one last warning before they punch it and get sad they lost the rocket they just spend x-hours building. There was a thread by such an individual not too long ago asking whether the hobby was worth it if he was going to just keep losing rockets/.

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I'd agree with your comment if ewomack was a new rocketeer just starting out. He's a BAR

More here: https://www.rocketryforum.com/threads/new-member-and-returning-to-rocketry.153721/#post-1900610

Thank you everyone for those very helpful responses! I especially enjoyed the operatic cookie piece.

I decided to try a B engine as a sort-of-a-compromise. Assuming I make it to the launch coming up, I'll let everyone know how it all goes.

For those who still have something to say, please say it! I've learned boatloads from this forum!

By using a low altitude motor, you verify that the rocket flies straight. If not, you still have a chance of getting it back and making repairs.

High and non-vertical? That’s no good.

Estes is selling an experience. And that experience is much more enjoyable if we get at least 2 flights. Otherwise, newcomers might abandon the hobby and then, no one would be happy.

Wildman’s First Law: if you get your rocket back, the motor wasn’t big enough.

Wildman’s Second Law: No Engine Blocks! (Thrust rings transfer force and don’t keep you from putting a CTI 24mm 6 grain G motor into a rocket Estes lists D and E motors for.)

BABAR said:

You are also a Level 3 experienced rocketeer, and therefore certainly far more adept at gauging rocket, wind, and field conditions than your average bear!

Click to expand...

Don't take my L3 as a sign of competence!

OverTheTop said:

Don't take my L3 as a sign of competence!

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I might be stealing that quote!

In all seriousness, assuming the rocket is built well, there really isn't much difference in risk from flying on a C6-7 vs an A8-3. The differences are that you're going to go a LOT higher on the C than on the A, so you might not get it back. You'll also be going a lot faster, which might enhance any weird quirks (spins, wobbles, etc) that your rocket might have, same with the accelerations. While the C6-7 has lower average thrust (6N vs 8N), it has a higher peak (14N vs 10N), so will probably result in more acceleration off the pad. If you've got a big enough field, the rocket is painted a bright color, and you've got good eyes (or someone else with you with good eyes), you shouldn't have a problem using the C for the first flight.

OverTheTop said:

Don't take my L3 as a sign of competence!

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Amen, L3 doesn't not confer competence beyond at least one L3 flight!

OverTheTop said:

Don't take my L3 as a sign of competence!

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It means you have authorization to make bigger mistakes

I am changing the order of your quote, but hoping to keep it in context. I keep the original quote at the end. Italicized stuff is my addition.

You have to click on the "quote box" to see the modification.

jjwb22101 said:

difference in risk from flying on a C6-7 vs an A8-3

1. you're going to go a LOT higher
2. You'll also be going a lot faster
3. might enhance any weird quirks (spins, wobbles, etc) that your rocket might have
4. will probably result in more acceleration off the pad
5. you might not get it back

how this equates to "there really isn't much difference " throws me off a bit.

Especially #5, which to me is kind of like saying "Other than that, Mrs. Lincoln, how was the play?"

here is the original unchanged quote

I might be stealing that quote!

In all seriousness, assuming the rocket is built well, . The differences are that you're going to go a LOT higher on the C than on the A, so you might not get it back. You'll also be going a lot faster, which might enhance any weird quirks (spins, wobbles, etc) that your rocket might have, same with the accelerations. While the C6-7 has lower average thrust (6N vs 8N), it has a higher peak (14N vs 10N), so will probably result in more acceleration off the pad. If you've got a big enough field, the rocket is painted a bright color, and you've got good eyes (or someone else with you with good eyes), you shouldn't have a problem using the C for the first flight.

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I couldn't find a recent post from a new BAR who spent a whole lot of time and effort putting a fine finish on a featherweight rocket (I think it was a Swift), and was disappointed to the degree he was thinking of dropping the hobby when he lost it on its first flight. I don't think he used the recommended first motor (and he might have lost it even if he had!) As a kid, I got really disappointed when I lost rockets that I had put a lot of time into.

NAR safety code suggests that the MINIMUM field size for an A motor is 100 feet diameter, for a C is 400 feet diameter. (and yes, you do note this in your quote, but perhaps not emphasized enough is how much bigger the field needs to be for reliable recovery.) Aside from club launch locations, lots of places it is much harder to find a 400 foot diameter field than 100 foot. https://www.nar.org/safety-information/model-rocket-safety-code/

I'm not trying to be a stick in the mud. Just that when a self proclaimed newbie (even when he or she is a BAR and has flown rockets before) comes to US on this forum asking about motor choices, it seems a bit glib to say, "Just stick a C in it, 'there really isn't much difference in risk' "

Just semantics, but "could" you launch on a C motor/engine/wtf-ever we're calling them now, as you asked, vs "should" you launch it on a C impulse device are completely different questions.

"Could" you? Sure, as long as you can get fire into the motogine thing. I "could" light up a J in my living room. "Should" you? Since you're asking the question, I'll say probably not.

Give it a go on some smaller impulse generators, then go for the C.

BABAR said:

I am changing the order of your quote, but hoping to keep it in context. I keep the original quote at the end. Italicized stuff is my addition.

You have to click on the "quote box" to see the modification.





I couldn't find a recent post from a new BAR who spent a whole lot of time and effort putting a fine finish on a featherweight rocket (I think it was a Swift), and was disappointed to the degree he was thinking of dropping the hobby when he lost it on its first flight. I don't think he used the recommended first motor (and he might have lost it even if he had!) As a kid, I got really disappointed when I lost rockets that I had put a lot of time into.

NAR safety code suggests that the MINIMUM field size for an A motor is 100 feet diameter, for a C is 400 feet diameter. (and yes, you do note this in your quote, but perhaps not emphasized enough is how much bigger the field needs to be for reliable recovery.) Aside from club launch locations, lots of places it is much harder to find a 400 foot diameter field than 100 foot. https://www.nar.org/safety-information/model-rocket-safety-code/

I'm not trying to be a stick in the mud. Just that when a self proclaimed newbie (even when he or she is a BAR and has flown rockets before) comes to US on this forum asking about motor choices, it seems a bit glib to say, "Just stick a C in it, 'there really isn't much difference in risk' "

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I agree that from an enjoyability standpoint, it definitely makes the most sense to stick with an A or B motor to start with. However, what I interpreted OP's question as (and how I replied) was as a safety-related question. While it's true that a C motor in a rocket will make it harder to recover, it doesn't really increase the inherent safety risk of the launch by that much. A rocket that is safe on an A motor (assuming it's not super marginal in terms of stability, which any kit won't be) will almost certainly be safe on a C motor.

jjwb22101 said:

I agree that from an enjoyability standpoint, it definitely makes the most sense to stick with an A or B motor to start with. However, what I interpreted OP's question as (and how I replied) was as a safety-related question. While it's true that a C motor in a rocket will make it harder to recover, it doesn't really increase the inherent safety risk of the launch by that much. A rocket that is safe on an A motor (assuming it's not super marginal in terms of stability, which any kit won't be) will almost certainly be safe on a C motor.

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Agree that losing a rocket isn’t a safety issue, but to those that are somewhat attached to their rockets (spent a lot of time finishing it, put a lot of money into it, BORROWED the motor casing or electronics...) losing the rocket is still to them a potentially significant risk, and a very avoidable one by making appropriate motor choices.

One safety issue that seems rarely to be addressed: what happens when the rocket lands outside the anticipated field? People posting on this forum asking for advice are not always flying at club launches with big fields, and are often inexperienced (which is WHY they are asking our advice in the first place.)

This is where I think park flyers can get in trouble, when a rocket with (or without!) a functioning recovery device comes down on a public street or someone’s backyard. Coming down under chute on a public street will at least startle anybody driving by, and even if it doesn’t cause an accident (you are right, probably won’t) still will not endear the local community toward our hobby. A lawn dart is bad enough when it happens on your field IN the safety zone, reeeeaaaallllly bad if it happens in someone’s backyard during the barbecue.


So I still say it is wrong to encourage fliers to push untested rockets with motors larger than required for first trial flights. An exception would be a club launch where a qualified RSO can tell the flier, “Hey, our field is big enough for you to do this, but understand you may not get this one back if you launch with this motor. Do you want to do this ?”

Edited, I removed comments on multistage.