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phil

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My two-stage "Einstein" rocket looks as it if will be ready for International Rocketry Week in the UK . It's my first attempt at HPR staging so I want to get it right.

The basic design is a 3 inch phenolic body, glassed from the base to the sustainer motor section. It weighs about 3.5 kg dry mass, and is about 2.5m (8ft 4in) tall.

On a 54mm J295 to 38mm I212 is simulates to around 7000 ft and burns out at around Mach 0.8, which allows the use of a barometric altimeter without having to faff around with Mach delays.

I'm using the motor delay grain to deploy the chute on the booster. I'm having difficulty calculating the correct delay for the booster deployment. As a first attempt I disabled the sustainer motor tube in Rocksim, and launched it on just the booster motor. I figured that would give me an idea of the altitude and delay at which the booster parachute needs to deploy. It's telling me 5000 ft and 17 seconds, which means the booster could drift for miles. This feels wrong, as staging takes place at seconds and 1500 ft. I find it difficult to believe that the booster will coast for 13 seconds and 3500 ft. I rather think it would tumble.

Can anyone offer any advice how to get a better estimate of the booster delay?

Phil
 
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Handeman

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I would suggest a separate sim on only the booster with a flat or no nosecone. Just disabling the sustainer still gives you the drag profile of the whole rocket which is very different then just the booster. You should be able to adjust motor types to get burnout at about the same time, altitude, and speed you expect during the actual flight. Then just watch coast profile.
 

highpowerrocket

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Rocksim typically under estimates Cd and so the overall simulation altitudes are probably high. However, on that size rocket and booster motor you can expect the booster apogee to be high. In those cases I use an altimeter for booster parachute deployments. Another option is to use a smaller motor in the booster and a larger one in the sustainer. A smaller quick burning motor (like Aerotech Blue Thunder) will get the rocket off the pad and to a safe staging velocity while keeping the booster altitude lower. Use a larger motor (even long burning) in the sustainer to achieve the maximum altitude that you are looking for. The spectators will also appreciate this approach since they will be able to see the staging event much easier. Good luck.
 

Adrian A

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You don't need to modify anything in the simulation; Rocksim will tell you the booster information you need, directly, when you do the 2-stage simulation. Just use the result graphing tool, and select the booster altitude as one of the plots. Or use the flight profile tool and click the "details" button to get the booster information.
 

DAllen

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FWIW...I always set my mach delay on my HiAlt45k no matter how fast it is going to go. That way I am guaranteed that there will not be an ejection event under thrust due to a funky cross-wind. So for example on my L2 flight where I was using a J350 the burn is roughly 1.5 seconds so I set the mach delay to 4 seconds. At the very worst, I figured I'll have 2.5 seconds before the altimeter decided to do anything.

One of our club members had an L2 project ruined because of an ejection at liftoff. The booster section just whipped around on the cord under thrust beating the crap out of the rest of the rocket.

-Dave
 

phil

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Thanks. I've been playing with Rocksim for years and never noticed that it could give separate booster data.

The argument for setting Mach delay makes sense. Thanks for the tip. There's a lot of time and money gone into this rocket so I want to get it right.

Phil
 
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JimJarvis50

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I've done quite a few two-stage flights, and my 3" has motor deploy on the booster. I simulate the booster altitude as you stated, by simply not lighting the sustainer motor. In practice, I have found that the actual delay time needs to be reduced a little from the simulation result. For what it's worth, I use 75% of the predicted delay time.

One factor to consider (maybe) is whether you use a separation charge. I do - 0.3 grams - and it seems to me that just after separation, the parts are further apart than one might expect. I wonder if this slows the booster just a bit.

Jim
 

phil

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Hi Jim,

Thanks for the good advice. For the first flight the plan is to use a modified PML interstage with ignition and separation being the samer as the PML Quantum Leap. I hadn't counted on the booster travelling to 4000 ft after separation. Finding it will be fun!

In retrospect I'd have designed the interstage differently and allowed space for an altimeter and a tracking beacon. I think that a new interstage will have to be built for the next flights and more electronics procured.

When I built the sustainer motor compartment I put an electronics bay at the top with two conduits running down between the motor tube and the rockets skin. One was for a separation charge, the other for an igniter. The plan was to blow the stages apart then ignite the sustainer after a couple of seconds.

Phil
 

JimJarvis50

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Hi Jim,

Thanks for the good advice. For the first flight the plan is to use a modified PML interstage with ignition and separation being the samer as the PML Quantum Leap. I hadn't counted on the booster travelling to 4000 ft after separation. Finding it will be fun!

In retrospect I'd have designed the interstage differently and allowed space for an altimeter and a tracking beacon. I think that a new interstage will have to be built for the next flights and more electronics procured.

When I built the sustainer motor compartment I put an electronics bay at the top with two conduits running down between the motor tube and the rockets skin. One was for a separation charge, the other for an igniter. The plan was to blow the stages apart then ignite the sustainer after a couple of seconds.

Phil

For whatever reason (maybe because they don't glide as far), boosters are usually easier to find.

On my booster, I tie the tracker to the harness. It's inside a 1/2" pvc pipe with end caps. Works fine.

I think you'll be happier lighting things from above.

Jim
 

highpowerrocket

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From the little information on your booster it sounds like it will be at least marginally stable and should not tumble. If you redesign the interstage coupler with electronics then there should be no doubt.
 

phil

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You're right. The only launch opportunity I'm likely to get for a while is only 2 weeks away so a newbuild is not possible in those timescales. I'll certainly be building a new interstage over the next few months to allow dual deploy and space for a tracker.

Phil
 

Daedalus

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This PML article says that your simulation method gave a pretty good approximation to the booster altitude and deployment time.

I won't be at IRW so best of luck with the flight - I'm sure it will be fine.
 

phil

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Thanks for all the advice. I finished painting the rocket on Friday and have attached a picture.

The final data: length 109 in, diameter 3.1 in, dry mass 4.16 kg. It sims to 7762 ft on Cesaroni J295 and I212. The J295 is the smallest 54mm motor that can exceed a 5:1 thrust/weight at lift off.

Phil

Einstein.JPG
 

JimJarvis50

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Thanks for all the advice. I finished painting the rocket on Friday and have attached a picture.

The final data: length 109 in, diameter 3.1 in, dry mass 4.16 kg. It sims to 7762 ft on Cesaroni J295 and I212. The J295 is the smallest 54mm motor that can exceed a 5:1 thrust/weight at lift off.

Phil
Looks great. I'd be wary of a J295 for it though. The CTI pro54 vmax motors are great. I used one at NSL, and it got the flight off to a great start.

Jim
 

phil

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I plan to launch it on a VMax when they're approved in Europe. Maybe next year....

Phil
 
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