Long burn thrust to weight

[Eric]

Does the minimum 5:1 T/W rule apply the same to all launches? In particular, regressive motors with long burn times.

For example, my 40lb 1/2 scale Phoenix with the M650. Its average thrust of 147lbs puts my rocket below 5:1. But for the first 4.5 seconds its thrust is much higher then 5:1.

Sims look like it will leave the rail at 92 ft/sec. At 16G of acceleration. Do some RSO's stick strictly to the 5:1 ratio?

 

[LithosphereRocketry]

At least some RSOs allow it - CL VII just attempted his L3 with that motor & a similar sized rocket. Unfortunately the main deployed early, so no cert, but the RSO was fine with the flight.

 

[FredA]

You can answer this yourself - then show the RSO:
What does your sim say the velocity is at the end of the rail guidance?
If you can better ~45ft/sec you should be fine.
Otherwise you need more push or a longer rail.

 

[llickteig1]

You can answer this yourself - then show the RSO:
What does your sim say the velocity is at the end of the rail guidance?
If you can better ~45ft/sec you should be fine.
Otherwise you need more push or a longer rail.

Fred is right: you want a safe amount of thrust until the rocket reaches the end of the rail. The entire burn profile average thrust is basically immaterial.

Look at the burn profile in Thrustcurve.org for your motor. You'll find most long-burns have a spike at the beginning which provides adequate initial thrust to have a safe velocity off the rail. If you go by the motor designator, which is average thrust, it is a deceptive and inadequate measure.

--Lance.

 

[djs]

Note that Phoenixes aren't known for being awesomely stable.. so even if the off the rail speed is good, you would want to limit this flight to low wind conditions.

 

[Bat-mite]

As others have indicated, no two RSOs are exactly alike. A good, well-educated RSO ought to be willing to trust your thrust curve. But some would rather say no than incur what they consider to be a risk.

 

[Eric]

Thank you for the responses. I can see how off the rail velocity is more important then average thrust.

Are sim print off's valuable to show the RSO when he is questioning your motor choice?

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[ChrisAttebery]

If you're doing something the RSO might find sketchy, YES. Bring a printout of the sim results showing the velocity off the launch rod.

Are sim print off's valuable to show the RSO when he is questioning your motor choice?

 

[rharshberger]

If you're doing something the RSO might find sketchy, YES. Bring a printout of the sim results showing the velocity off the launch rod.

Its also not a bad idea to have a copy of the motors actual thrustcurve with you as well, just so the RSO can see that too and not have to default to the average.

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[rharshberger]

If you're doing something the RSO might find sketchy, YES. Bring a printout of the sim results showing the velocity off the launch rod.

Its also not a bad idea to have a copy of the motors actual thrustcurve with you as well, just so the RSO can see that too and not have to default to the average.


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[markkoelsch]

Just an opinion here- a faster burn, higher thrust motor might be better. Get it up and going in a hurry.

 

[Eric]

Yes, I'm not limiting myself to any certain motor. Just gaining knowledge on the options.

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[Cl(VII)]

As was said, I flew a 35 lb V2 on an M650. It had an initial thrust to weight of about 8.5:1, and a thrust to weight off the rail of about 7.3:1. I had check offs for minimum rail length and maximum wind speed for the flight to be a go based on the sims also. All of these were determined by carefully running the sims, and paying attention to the details. If you do those things, and document well that it is going to be a safe flight, then most (I would say almost all) will let you fly it. I had all of that documentation with me when I went to RSO in case they wanted it.

My point is, if you are going to do this kind of flight make really sure it will be safe, and make really sure you can prove that it is safe to the RSO. It sounds crazy when you say 35 lb (or in your case 40 lb) rocket and M650, so the burden will likely be on you to PROVE it is safe.

All that said it is a spectacular motor, and would be super cool in a Phoenix on a calm day off a 12' 1515 rail. It would be 8 seconds of group terror on a windy day off an 8' 1010 rail.

 

[Eric]

Hi Chris, Sorry to hear about the early deployment. I will definitely do my homework when that time comes and make sure the t's are crossed.

I am planning on 1515 buttons. I'm not sure how long the clubs rail is though.



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[rharshberger]

Hi Chris, Sorry to hear about the early deployment. I will definitely do my homework when that time comes and make sure the t's are crossed.

I am planning on 1515 buttons. I'm not sure how long the clubs rail is though.

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The rail length is a definite need to know, as the effective length of the rail is determined by the distance from the top rail button to the tip of the rail.

 

[Eric]

I think I have always just input 8 feet. That is the standard length. But those were 1010 rails I was launching off of. I'll have to see if they have a longer 1515 rail. Or get my own.
Does slicing a rail really work. Seams like there could be interference.

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[rharshberger]

I think I have always just input 8 feet. That is the standard length. But those were 1010 rails I was launching off of. I'll have to see if they have a longer 1515 rail. Or get my own.
Does slicing a rail really work. Seams like there could be interference.

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Splicing rails by adding sections works just fine as long as you use the plates and fasteners made by 80/20 for that purpose. Our club has a 96" 1515 and a 48" 1515 that we join with 3 joining plates (each with 4 fasteners).

 

[Cl(VII)]

The rail length is a definite need to know, as the effective length of the rail is determined by the distance from the top rail button to the tip of the rail.

Emphasis mine.

This is an important detail, and one that can be easily overlooked.

 

[Eric]

Okay. Does the top button really need to be at the CG like I have read? Or can it be placed back a little further?

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[OverTheTop]

Okay. Does the top button really need to be at the CG like I have read? Or can it be placed back a little further?

That is around where most are. I suspect it doesn't make a heap of difference. Quite a few rockets have them not in that location and fly well. Personally I wouldn't get too hung up on it (no pun intended).

 

[Bat-mite]

Okay. Does the top button really need to be at the CG like I have read? Or can it be placed back a little further?

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Some people say CG, others say CP. No one seems to have any science behind why. The only thing I can offer is to consider two points. The higher the top button, the straighter the rocket will be on the pad. However, as stated above, the lower the top button, the longer the rocket stays on the rail during initial thrust. Find the sweet spot. For most, the sweet spot ends up being somewhere around the top CR.

 

[Eric]

Thanks for the good advice.

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[boatgeek]

My experience with RSOs is that the more you can show them that you've thought about issues, the more likely they are to approve the flight. A sim packet with a thrust curve is a lot more likely to get the thumbs up than a "hey, here's my flight card and it'll be awesome!!!1!".

 

[SpaceManMat]

The rail length is a definite need to know, as the effective length of the rail is determined by the distance from the top rail button to the tip of the rail.

I belevie OR takes this into account actually. Not sure about RS.

 

[dhbarr]

I belevie OR takes this into account actually. Not sure about RS.

I don't think so, as I rarely put lugs on my sims but rail length is still effective.

 

[Eric]

Yes, I don't put buttons or lugs on my sims. Just detailed fin dimensions, rocket diameter and length. I always mass override and enter my actual weight, and then the measured CG.


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[Handeman]

Does the minimum 5:1 T/W rule apply the same to all launches? In particular, regressive motors with long burn times.

For example, my 40lb 1/2 scale Phoenix with the M650. Its average thrust of 147lbs puts my rocket below 5:1. But for the first 4.5 seconds its thrust is much higher then 5:1.

Sims look like it will leave the rail at 92 ft/sec. At 16G of acceleration. Do some RSO's stick strictly to the 5:1 ratio?

I think how much the RSOs stick to that 5:1 is going to vary from club to club. At BattlePark we have a laminated sheet for the RSOs as a quick reference of weight vs. average thrust. If your rocket falls below that 5:1 line that is a red flag for the RSOs. It doesn't mean the flight won't be allowed, but the RSO will take a much closer look at all the things folks have mentioned above. If the RSO disallows the flight, isn't comfortable allowing the flight, or is too busy with a line of waiting flyers, he will refer you to one or more of the Board of Directors (BOD) for further discussions. There are things that can be done to mitigate safety concerns with low ratio flights such as moving the flight to more distant pad or require using a pad with a longer rail. You might find yourself out at the 300' pads, or even the away cell with an L1 or L2 motor but you may still be able to fly.

In your case, it might be best to contact the Prefect or BOD before the launch and provide your sim, motor, rail button location info, etc. That way you can have a decision before you even get there and the whole RSO process my be expedited for you. Makes it easier and less stressful for everyone.

Good Luck with your flight

 

[Buckeye]

Don't forget wind in your simulation. Your rail speed should be 4X the wind speed as well (This rule of thumb has some legit math behind it). The stability margin as the rocket leaves the rail is also helpful and can change dramatically with wind in the sim.

 

[wighty44]

Don't forget wind in your simulation. Your rail speed should be 4X the wind speed as well (This rule of thumb has some legit math behind it)...

For those who might want to know, that ROT is based on limiting the CP shift to 1 caliber. More detail on this topic can be found in the document NAR Safety In Sport Rocketry (Training for RSO and Safety Check 2013), page 25

 

[markkoelsch]

For those who might want to know, that ROT is based on limiting the CP shift to 1 caliber. More detail on this topic can be found in the document NAR Safety In Sport Rocketry (Training for RSO and Safety Check 2013), page 25

Which, if my quick math is correct, is far faster than most rockets are traveling off of the rail. If my math is correct, that works out to about 117 ft/sec off the rail for a 20 mph wind. To give you an idea using a typical 6 foot rail, flying a Wildman Junior you need a pretty high thrust motor to accomplish this. An J570 will do it, an At I600 will get close, Loki J1000/K1127 will both do it too. The rocket needs to be moving to fly in this wind and meet the 4x wind velocity discussed above. The alternative would be a longer rail and still high thrust motors, but possibly a little lower thrust might get it.