Duplex motors

[Sooner Boomer]

I really don't know where these ideas come from, but here goes;

The idea is to have two motors, or more properly, two seperate grain assemblies in series. They would be standard reloadable-type cases, joined by a coupler that holds a small (time-wise) delay element. The benefit being that you could have two "types" of propellant, for example, a goes-like-heck booster, followed by a long burn sustainer. Something like the Aerotech "glider" motors that are a slot-burner that transitions to an end-burner, but with seperate motors. Almost like staged motors, except they never seperate.

Is this easily doable? Is it practical?

I realize that you probably couldn't utilize off-the-shelf propellant loads. The upper propellant will have to pressurize the empty lower engine casing, so the pressure/burn rate curve would be a bit off, but the result would be an underpressured design rather than a self-disassembly load.

 

[manixFan]

There was a guy who was trying to develop a sea launched weather sounding rocket that used a dual compartment motor. He spent quite a bit of time and energy on it as I recall. It believe it was back in the early 2000's. The idea was that the rockets would be stored on buoys and launched autonomously. I recall one design had a delay element that burned through and allowed upper grains to ignite. Another used grains that were basically two different propellents cast into one grain - a fast along the core and a slow outside. He lost funding and moved on to other projects.

I can't remember where he posted his progress but it was very interesting to follow along. Maybe someone here will have more info.


Tony

PS: One of the biggest issues is the fact that once the faster grains burn out that area of the motor case is now exposed to the full heat of the burning propellant. So you need extra thick liners to protect the case which reduces efficiency. In a single use motor the case just has to survive the burn, but in a reloadable case you want to protect the case for reuse.

 

[Rocket501]

I really don't know where these ideas come from, but here goes;

The idea is to have two motors, or more properly, two seperate grain assemblies in series. They would be standard reloadable-type cases, joined by a coupler that holds a small (time-wise) delay element. The benefit being that you could have two "types" of propellant, for example, a goes-like-heck booster, followed by a long burn sustainer. Something like the Aerotech "glider" motors that are a slot-burner that transitions to an end-burner, but with seperate motors. Almost like staged motors, except they never seperate.

Is this easily doable? Is it practical?

I realize that you probably couldn't utilize off-the-shelf propellant loads. The upper propellant will have to pressurize the empty lower engine casing, so the pressure/burn rate curve would be a bit off, but the result would be an underpressured design rather than a self-disassembly load.

People actually have tried this. It tends to not work out well. The closest option is doing the same sort of thing as Aerotech's new K76. It has a boost and sustain grain all in the same case.

There are two sticking issues: the delay separation element and liner integrity.

As for the first, a rocket is under a high level of internal pressure. Many delay grain options would likely break early from the pressure which would result in a motor Cato or take too long to burn through/be at a high risk of self-extinguishment. There are ways of trying to deal with this such as dual electronic ignition, but all have issues.

As for the rest, after the first section burns out, the raw liner is exposed to the flame. The thickest commonly used liners, aerotech's 98mm phenolic, can withstand direct contact with hot gases from a fairly gentle propellant for a max of about 16 seconds. Ideally speaking, this concept would have a quick boost grain before a long sustain, and you'd probably end up wanting more than just 20 seconds or so of burn time.

This could be done, but it is complex and it is usually just better to do both in a single case with either two separate grains or a dual-thrust profile or use two stages. These other options are also much simpler to develop, probably higher performance, and more reliable.

 

[Alan15578]

Take a look at the SRAM two pulse motor.

 

[dhbarr]

https://en.m.wikipedia.org/wiki/Dual-thrust

 

[cbrarick]

In summary: No, that doesn't work
Yes, Dual-thrust motors are commercially available
If you don't like that option, do a 2 stage rocket.

 

[cbrarick]

or go look up sugar shot 2 space.

they are a been there - couldn't do that - with a endless budget compared to us mere mortals.

 

[Rob702Martinez]

https://randysrocketry.blogspot.com/2007/09/early-dual-burn-motor-test.html?m=1

 

[JohnCoker]

There are commercial motors that use multiple types and even grain geometries in the same case. AeroTech has done this for example by combining Warp 9 and White Lightning propellant grains.

 

[FredA]

It's all about your insulation.
Start by taking a "regular" motor and putting a grain spacer in the aft grain position and see if you can keep the motor intact.
Once you get that working, you can think up how to use the capability.
Until then, you're wasting time on fancy drawings, IMHO.

 

[Sooner Boomer]

OK, so it's not a totally crazy idea, just one that is hard to make work. Rocket science is not easy, otherwise everybody would be doing it!

 

[M Dive]

Don’t forget the AT I59.
Karl B signature series.

 

[Banzai88]

View attachment 437605




View attachment 437606

https://randysrocketry.blogspot.com/2007/09/early-dual-burn-motor-test.html?m=1

That RaD motor burn was wicked cool! Whatever happened to the concept? More trouble that it's worth?

 

[JohnCoker]

Don’t forget the AT I59.

I updated the propellant for the I59; so now there are three.

 

[3stoogesrocketry]

I updated the propellant for the I59; so now there are three.

K375 NW?

 

[JohnCoker]

K375 NW?

Fixed as well.

 

[DeeRoc29]

That RaD motor burn was wicked cool! Whatever happened to the concept? More trouble that it's worth?

I made the grains for that motor. Rick M gave me the idea, but im pretty sure it's been done by others. Each grain consists of different propellants cast in layers with varying web thicknesses. It's labor intensive, but a dual thrust effect can be achieved this way, too. Check post 109 here to see another one in flight. I used a hot blue formula to boost and then a slower white to sustain.