Dual deployment first event: Ejection delay or altitude?

[MetricRocketeer]

Hi TRF colleagues,

I think that I know the answer to this question, but I would just like to make sure, please.

I was watching an Apogee Components RockSim Live video, and Tim was talking about dual deployment. He kept talking about making the first event — in other words when the main parachute deploys — occur at apogee. He did not mention making the first event occur as a function of the ejection charge.

However, you cannot specify the main parachute to deploy at apogee unless the rocket contains an altimeter, right? Otherwise, how would the rocket know that it has reached apogee? Without an onboard altimeter, you have to make the first event occur as a function of the ejection charge. I am correct here, right?

Thank you.

Incidentally, I trust that you all had a nice Thanksgiving.

Stanley

 

[Voyager1]

Dual deployment typically uses an altimeter that deploys a drogue chute at apogee and then a main chute at a predetermined altitude of, typically, between 500‘-700’ AGL. Alternatively, you can use motor ejection of a drogue at apogee and an electronic deployment of the main at a lower altitude, as above. This electronic deployment could be via an altimeter ejection or a chute release module.

The “first” event you mentioned should be the drogue deployment at apogee.

 

[Handeman]

By definition, Dual Deployment requires an altimeter. There are two events, an apogee event and another at some lower set altitude. Even if you expand the definition to include motor ejection at apogee and a chute release at some lower altitude, an altimeter is required for the lower altitude event. Even a JLCR has an altimeter to determine when to release.

 

[BDB]

If you plan to fly dual deploy and the motor contains an ejection charge, it's best practice to use the altimeter to deploy the drogue at apogee and the main at a lower altitude (e.g. 500 ft). You don't have to remove the motor's charge--just make sure that it fires after apogee (many of us don't adjust the delay at all). That way the motor's charge is a backup.

 

[tsmith1315]

Otherwise, how would the rocket know that it has reached apogee? Without an onboard altimeter, you have to make the first event occur as a function of the ejection charge. I am correct here, right?

For the rocket to know, yes.

Do keep in mind that some don't consider motor ejection + chute release as DD. Perhaps his definition of dual deployment requires the use of an altimeter.

Either that or his phrase "occur at apogee" isn't to be taken too literally, and really means "occur as close to apogee as you can manage with your method".

 

[Azamiryou]

By definition, Dual Deployment requires an altimeter.

Wouldn't it still be dual deployment if it was controlled by timers or even remote control instead of an altimeter?

 

[Titan II]

Wouldn't it still be dual deployment if it was controlled by timers or even remote control instead of an altimeter?

Yes.

 

[Handeman]

Wouldn't it still be dual deployment if it was controlled by timers or even remote control instead of an altimeter?

I would say no.

It depends on what authority is defining DD. You could argue a coaster wagon with a motor drive is an automobile too. I don't thing DOT will let you license it for street use though. I'll defer to the early altimeter manufactures that came up with the term DD to describe what their altimeters were doing.

My personal opinion any flight using motor eject is not a DD flight which is why I think you are stretching the definition when you use motor eject and a JLCR and call it DD because there is only one pyro event on the flight. Not that the end result isn't the same. Like taking a car from NYC to Chicago or a plane. Either way you get to Chicago, but most people would only describe one of the trips as a road trip.

I've never heard of anyone using a timer for the second event. In fact, if I was the RSO, I wouldn't approve a flight that used a timer for the second deployment. It's one thing to use a timer for apogee deployment when you can accurately predict apogee time like you do to set motor delay times. After the motor eject, the fall time is much harder to estimate. How do you know it won't fall at 100 ft/sec rather than 55 ft/sec.

As for RC, again, I wouldn't call that DD, its a RC flight, but that's another coaster wagon to my way of thinking. You are free to call it whatever you want.

If you want to include all those methods as DD, have at it. I won't agree, but I won't say you're wrong either. It's just a discussion of semantics.

 

[Steve Shannon]

The term Dual Deployment was actually trademarked by the inventor of one of the first “Dual Deployment Altimeters” that was programmed to ignite a charge at apogee and another at a lower level. Although he tried to defend his trademark, it became a generic term for flights that had an apogee drogue deployment and a lower main chute deployment. I wish I could think of his name. It might have been Adept; hopefully someone will help me out.
I cannot settle the argument of whether both events must be controlled by an altimeter to be called Dual Deployment, but I know that’s what the inventor meant. It has become a term that simply means two different chutes, one at apogee and one lower. My opinion isn’t any more valid than anyone else’s.

But I will say this: Using motor deployment for the apogee event other than as a backup to an altimeter doesn’t make sense. The apogee is where a delay grain is most apt to be out of spec. All delay grains are required by NFPA 1125 to be tested at sea level. Nobody intentionally has an apogee event at sea level and dual deployment flight are often those where the apogee is expected to be high. After all, that’s the entire reason for dual deployment, to prevent a high altitude flight from drifting far under a parachute ejected at apogee.
Tests have repeatedly shown that the delays of high power composite motors are inconsistent over time and quite sensitive to ambient pressure. So, using motor deployment at apogee stands a fair chance ejection before or after apogee by a couple seconds. Alan Whitmore, the chair of Tripoli Motor Testing, tested a bunch of motors in 2019 (I think) and found that their delays were pretty inconsistent even at sea level (he’s located at a relatively low elevation). It’s in the Tripoli Report for spring 2020 (I believe). So, it just doesn’t makes sense not to use an altimeter to detect the apogee event and control the drogue chute ejection.

 

[manixFan]

Hi TRF colleagues,

I think that I know the answer to this question, but I would just like to make sure, please.

I was watching an Apogee Components RockSim Live video, and Tim was talking about dual deployment. He kept talking about making the first event — in other words when the main parachute deploys — occur at apogee. He did not mention making the first event occur as a function of the ejection charge.

However, you cannot specify the main parachute to deploy at apogee unless the rocket contains an altimeter, right? Otherwise, how would the rocket know that it has reached apogee? Without an onboard altimeter, you have to make the first event occur as a function of the ejection charge. I am correct here, right?

Thank you.

Incidentally, I trust that you all had a nice Thanksgiving.

Stanley

I think it would be very helpful to this discussion if you linked the video you watched so it's easier to try and understand the point Tim was making.


Tony.

 

[cerving]

Regarding the OP's question, you do not configure the altimeter for either time delay or for altitude; the altimeter automatically senses apogee, and deploys the drogue chute at that time. Typically, the only configuration you ever need for a drogue is to delay it by one or two seconds if you're configuring an altimeter as a backup.