Auto-Extending Nozzle?

[S4qFBxkFFg]

So I had an idea for something that might be useful for 1st stage rocket engine nozzles - as (greatly simplified) background, while in atmosphere, nozzles should be smaller, and in vacuum, they should be a lot larger.

This means that it would be ideal if a nozzle could literally lengthen and widen during the ascent.  There are probably a variety of ways to actually do this, but tell me what you think of this idea:

First, look at this: https://www.google.com/search?q=collapsible+cup&tbm=isch

I'm thinking of the ones made of solid material - not the flexible silicone type.

Imagine the nozzle is like one of these collapsible cups, at launch, it's in the "collapsed" configuration, with only the innermost ring actually forming the nozzle.  During the ascent, the outer rings are gradually released (explosive bolts?) and due to the acceleration, fall into place around the previous nozzle ring so that the whole nozzle lengthens and widens during ascent.

I'm aware this probably makes things like regenerative cooling impossible (or a lot harder), but could this be useful, or has it already been done?

Edited July 14, 2017 by S4qFBxkFFg

[DDE]

Bam, here's a classic RL-10:

And bam, here's it in Nertea's cryogenic rockets pack:

They're generally used to reduce interstage length, not to change ISP dynamically, though.

[magnemoe]

Looks like the bottom part don't need cooling. 
That was something I looked at as an issue.

This would not work well with tightly clustered engines like falcon 9 but would be very nice for core stages like Ariane 5 

[S4qFBxkFFg]

Interesting - if slightly annoying that my idea was unoriginal.

[Steel]

The biggest issue with this sort of thing is always getting it to work efficiently. An idea de Laval nozzle has a smoothly varying internal surface. If you have nozzle segments that get added into place on the end, chances are you'll get a gap or a bump which will trip a whole load of shockwaves which will disrupt the flow at the exit. This means unless the extensions are extremely well designed you might not realise the ISP gains that you hope for, or worse you might lower the ISP and still be carrying more weight.

EDIT: I didn't know the RL10B-2 did that, thats's quite the feat of engineering, though greatly simplified vs the OP's suggestion by having the nozzle extension in place before ignition.

Edited July 14, 2017 by Steel

[softweir]

3 hours ago, S4qFBxkFFg said:

*snip*

During the ascent, the outer rings are gradually released (explosive bolts?) and due to the acceleration, fall into place around the previous nozzle ring so that the whole nozzle lengthens and widens during ascent.

(My emphasis.)

Unfortunately, the problem with the idea lies right there in the bolded phrase. If the nozzle extension improves ISP, then the upward acceleration it experiences due to the pressure of gasses inside it will prevent it dropping - would have to be forced down. If it is inert enough to drop, then it "weighs" more than the extra force it returns from the gas stream.

I like the idea of a "collapsible cup", but some other mechanism would be needed to extend the nozzle! I have no idea if there are any mechanisms that could do that and deal with the extremes of heat, vibration and pressure experienced by rocket nozzles.

[magnemoe]

9 minutes ago, softweir said:

(My emphasis.)

Unfortunately, the problem with the idea lies right there in the bolded phrase. If the nozzle extension improves ISP, then the upward acceleration it experiences due to the pressure of gasses inside it will prevent it dropping - would have to be forced down. If it is inert enough to drop, then it "weighs" more than the extra force it returns from the gas stream.

I like the idea of a "collapsible cup", but some other mechanism would be needed to extend the nozzle! I have no idea if there are any mechanisms that could do that and deal with the extremes of heat, vibration and pressure experienced by rocket nozzles.

The RL-10 uses an rotating stag who moves the segment down, it looks pretty lightweight however having multiple segments will make this much harder. 

[GoSlash27]

American ICBMs use this design for compact upper stages. I don't think Isp is a major design consideration for it. I imagine the weight and complexity would offset any efficiency improvements.

Best,
-Slashy

[Bill Phil]

Yeah. Not much real advantage except for compactness, which is useful, as it does reduce interstage mass as well (the question really becomes an issue of how much mass then gets added...).

For optimum ISP, aerospikes are generally considered a good solution.

[magnemoe]

30 minutes ago, Bill Phil said:

Yeah. Not much real advantage except for compactness, which is useful, as it does reduce interstage mass as well (the question really becomes an issue of how much mass then gets added...).

For optimum ISP, aerospikes are generally considered a good solution.

Interstage is part of first stage, adding 5 kg to first  stage if equal to adding 1 kg to second in an dual stage setup. 
For ICBM you want to reduce missile size, this is critical if you want to re-use siloes or launch from an sub. its not an major issue for orbital launches. 

[NoobTool]

It seems to me that it might be easier to employ a variable geometry rocket nozzle on an aerospike type engine instead of a more traditional bell-shaped nozzle. You could simply extend or retract the central spike. Although one of the primary benefits of aerospike engines is efficiency along a broader range of ambient atmospheric conditions, so it's hard to say how much of a benefit could be achieved with variable geometry.

Edited July 19, 2017 by NoobTool