Why are university HPR teams having such difficulty with airstarts?

[AlphaHybrids]

The main drawback to my method of sealing is that it requires a lot of testing. I'm sure the adhesion properties of graphite are different after a nozzle is fired compared to when it is machined. Also, the ability to scale up is probably very difficult as well. I'm very confident that John will have a method that is robust and scale-able and less mad scientist in his lab.

Edward

 

[MClark]

I have sealed nozzles on motors with head end ignition, not for pressure but to keep dust out.
I use aluminum tape stuck to a phenolic disc. Phenolic is used as tape will not stick to graphite.
It is on top of nozzle, outside diameter to fit liner tube, inside diameter (hole) to match convergence, a smooth radius on hole so not to tear tape on edge. If you need more pressure use multiple layers of tape.

Rockets headed for space should be using head end ignition with mechanical safe and arm not an igniter stuffed up the nozzle.

 

[AlphaHybrids]

Mark - I never thought of placing the disk on top of the nozzle convergence section, I like that. LDPE sheet seems like it would be ideal in that location. Thanks for sharing.

Edward

 

[MClark]

Nozzle seal is less susceptible to damage on inside.
If you want seal on outside stick it to an aluminum ring and put between the nozzle and aft snap ring with seal up.

M

 

[heada]

What motor manufacturers have HEI as a standard/approved option? I agree it is an ideal solution along with a thin sealing membrane similar to the aluminum tape but HEI is another topic where the teams would have to devise their own solution since there isn't a COTS solution I'm aware of.

 

[AlphaHybrids]

Are you meaning commercially approved for TRA/NAR launches? I don't know of any motors that currently provide a head end option.

Edward

 

[MClark]

Nobody

 

[heada]

Many of the teams are making their own motors or are flying under TRA research rules and could modify a forward closure for HEI themselves but I think it'd help not only the Uni teams but the hobby in general if there were approved HEI closures for AT/CTI/Loki since it seems those are the most common COTS motors used for airstarts. Just 1 more thing that wouldn't need to be designed and created from scratch which should only help increase success. Does TRA/NAR have the ability to ask motor manufacturers for something like that?

 

[AlphaHybrids]

It is a small niche for products and for the manufacturer comes with a lot of increased liability if it isn't installed correctly. Flyers will also be installing igniters before the rocket is on the pad, which I see as presenting a larger safety concern than the one that it proposes to fix.

I'm also against TRA/NAR telling private manufacturers that they need to create a product based on a desire rather than an absolute need.

Edward

 

[MClark]

The demand is too low.
TRA/NAR does not make requests like that.
Perhaps you should ask a manufacture. Aerotech and Cesaroni designed and certified two motors each for a contest we ran,

Mark

 

[heada]

I was thinking more a request to them rather than a demand but I agree, the usage would be very low and would probably not offset increased liability and decrease in safety.

 

[tfish]

Not certified..but a start

https://www.rocketmotorparts.com/Low_Cost_Head_End_Initiator/p1577809_16501656.aspx

Tony

 

[AlphaHybrids]

Thank you for posting that Tony. I really like the integral bayonet connector for the wiring.

Edward

 

[MClark]

Looks like a NASA Standard Initiator with BYOB bang.
Thirty bucks is way better than the $800 NASA pays with the bang.

Firing current requirement may too much for some electronics.

 

[saadzmirza]

Thanks for the clarification Xrain

Also I disagree with the philosophy of everyone else trying to seal the motor, whether to seal it atmospheric pressure or keep it to 200 psi (which is probably overkill) - I think you're going to have a lot of trouble proving that it will function as intended in flight.

And even if your nozzle plug bursts at 200 psi, unless you can guarantee that the pressure in the core will be sea level (which is much harder), your propellant won't light if you're using a standard igniter. if your core is at ambient pressure at say 35,000 ft (which it probably will be with most burst disks), and your standard igniter won't pressurize the core, the propellant simply won't light. That's what happened to our Princeton flight and I don't want other people to experience that kinf of failure.

Since you really need to use an aggressive igniter that pressuirzes the core, you might as well design is so that you don't need a nozzle plug.

Desiging the igniter to pressurize a vented motor makes more sense imo.

 

[jsdemar]

Thanks for the clarification Xrain

Also I disagree with the philosophy of everyone else trying to seal the motor, whether to seal it atmospheric pressure or keep it to 200 psi (which is probably overkill) - I think you're going to have a lot of trouble proving that it will function as intended in flight.

And even if your nozzle plug bursts at 200 psi, unless you can guarantee that the pressure in the core will be sea level (which is much harder), your propellant won't light if you're using a standard igniter. if your core is at ambient pressure at say 35,000 ft (which it probably will be with most burst disks), and your standard igniter won't pressurize the core, the propellant simply won't light. That's what happened to our Princeton flight and I don't want other people to experience that kinf of failure.

Since you really need to use an aggressive igniter that pressuirzes the core, you might as well design is so that you don't need a nozzle plug.

Desiging the igniter to pressurize a vented motor makes more sense imo.

You must know something that professional SRM designers have missed for decades. ;-)

 

[DavidMcCann]

Many of the teams are making their own motors or are flying under TRA research rules and could modify a forward closure for HEI themselves but I think it'd help not only the Uni teams but the hobby in general if there were approved HEI closures for AT/CTI/Loki since it seems those are the most common COTS motors used for airstarts. Just 1 more thing that wouldn't need to be designed and created from scratch which should only help increase success. Does TRA/NAR have the ability to ask motor manufacturers for something like that?

Loki makes one.... not certified...
https://lokiresearch.com/secure/storeDetail.asp?id=52120032164096

 

[plugger]

Carolina Composite Rocketry makes HEI hardware for both CTI and Aerotech motors in 29mm, 38mm, 54mm, and 75mm. And maybe even 98mm, but I'm unsure of that. I've got a set of the CTI HEI forward closures in 38mm, 54mm, and 75mm. Here's a pic.

They're reusable and I'm very excited to use them as I've not had the best luck in the past bottom lighting sustainer motors via the booster ISC. Well, I have had luck in lighting, but not in recovering the booster as the electronics were fried by the sustainer motor coming up to pressure and filling the booster AV bay with exhaust gasses.

I'm planning on using the 54mm in April and the 38mm at our higher altitude launch site next season once the fire bans finish. The 75mm is for a couple years from now when I feel more confident with HEI staging from using the other two to actually have a crack at a 100k ft two stage flight.

 

[MClark]

Is there a way to Safe these units or do the motors need to be assembled at the pad?

M

 

[plugger]

Is there a way to Safe these units or do the motors need to be assembled at the pad?

M

I am going to use a combination of motor assembly on the pad plus a physical switch in between the e-match/BKNO3 charge and staging electronics which will only be "turned on" once the rocket is vertical on the rail.

EDIT: Mark, when you say "safe these units", what does that entail exactly?

 

[MClark]

The match/igniter is not installed
Or if igniter does fire there is a mechanical break in the ignition train (safe and arm) to prevent the ignition booster charge from igniting.

M

 

[plugger]

The match/igniter is not installed

Is it really possible to accomplish that with HEI systems? I can't quite wrap my head around how you'd assemble a motor using HEI without having the igniter/ignition system in place. For a second I thought the unit Tony linked to earlier would be install-able in the field after motor assembly but after looking at the pictures I'm not quite sure.

 

[saadzmirza]

From all of the links I posted professional motors of this size usually don't attempt to seal the motor in order to achieve ignition. The Black brant example was not pressure sealed, and the NASA document mentions calculating pressure as a function of time for vented chambers.

Do you have any evidence to back up your claim that professional solid motors usually use a nozzle plug in order to achieve ignition pressure? From what I've read it's only for moisture sealing and keeping insects/animals out etc., not a pressure seal

 

[Steve Shannon]

Is it really possible to accomplish that with HEI systems? I can't quite wrap my head around how you'd assemble a motor using HEI without having the igniter/ignition system in place. For a second I thought the unit Tony linked to earlier would be install-able in the field after motor assembly but after looking at the pictures I'm not quite sure.

You’ve hit the nail on the head. It’s simply not possible with any existing amateur head end ignition system to install the initiator after the motor is in the rocket and the rocket is on the pad and pointed in a safe direction. The RCS/Aerotech head end system (the one Tony referenced) at least allows the motor to be fully assembled before inserting the head end initiator.
That’s also the problem with all traditional multiple stage amateur rocket vehicles. The booster is the only stage which allows the initiator to be installed after the entire rocket is assembled, on the pad, and pointing in a safe direction.
That’s the problem that should be solved before working on how to make air starts more reliable.
This obviously doesn’t apply to parallel staged rockets where all of the initiators can be installed on the pad.

 

[MClark]

But the HEI igniter can be installed after the motor is in rocket.
People usually think of the igniter (a screw in type similar to the one Tony references) going in vertically from the top. If it screws in horizontal from the side it can be installed last through an access panel in the assembled rocket.
I did a 98mm motor doing this, used Holex igniter. It was single stage but had it been multiple stage or cluster the only difference would be the wires going to the onboard electronics rather than the launch control.

M

 

[FredA]

"not possible with any existing amateur head end ignition system to install the initiator after the motor is in the rocket and the rocket is on the pad and pointed in a safe direction."

Are there any "Pro" systems that let you install something in the middle of the sustainer while pointed up on the pad?
You're wanting something that is not practical.

There is a way to add an HEI initiator as the last part of the motor build, but unless we're talking trap-doors, etc. I don't see how you can install anything after the rocket is in launch position.

 

[Dave A]

Would make sense to have a switch (shunt) across the upper stage igniter leads, closed, until the vehicle is vertical, armed and ready to fly? Obviously the most vulnerable period is from when the igniter leads are untwisted and connected to the ignition electronics. That step should always be done when:
1) It is pointed away from the flight line
2) No persons should be inside a 90° wide path in front of the vehicle until it is vertical.
3) Probably best if only one person is making the last connections.
4) Always raise the vehicle before any electronics are armed.
5) I would only turn on the staging electronics after ALL altimeters are on and ready.

 

[MClark]

Would make sense to have a switch (shunt) across the upper stage igniter leads, closed, until the vehicle is vertical, armed and ready to fly? Obviously the most vulnerable period is from when the igniter leads are untwisted and connected to the ignition electronics. That step should always be done when:
1) It is pointed away from the flight line
2) No persons should be inside a 90° wide path in front of the vehicle until it is vertical.
3) Probably best if only one person is making the last connections.
4) Always raise the vehicle before any electronics are armed.
5) I would only turn on the staging electronics after ALL altimeters are on and ready.

This is where a mechanical safe and arm comes in.
If the igniter fires the path to the motor is blocked.
Basically a valve to stop the fire.

 

[FredA]

A switch that disconnects and shunts the twisted wires is the minimum.

I'd like to see a "reasonable" mechanical system that works for our scale projects (as in, it can't add pounds, be larger than 54mm, etc.) especially when "intimate contact" of the initiator to propellant is the best advice for reliable ignition.

I suppose we could develop the "TRA Standard Initiator" - but I'm not sure we have the will to do so.

 

[Steve Shannon]

A switch that disconnects and shunts the twisted wires is the minimum.

I'd like to see a "reasonable" mechanical system that works for our scale projects (as in, it can't add pounds, be larger than 54mm, etc.) especially when "intimate contact" of the initiator to propellant is the best advice for reliable ignition.

I suppose we could develop the "TRA Standard Initiator" - but I'm not sure we have the will to do so.

I agree; that should be the minimum for now. I also would like to see a mechanical system like Mark has mentioned developed as a TRA Standard Initiator. Once developed, that should become the new minimum.
What is needed? Why do you doubt the will to do so?