Anyone know anything about engine Nozzles? - Ablative Nozzles

David Ellard · 2003-01-13 00:26:25

I?ve been reading heaps about rocket technology in the past few months, and while I was reading about engine performance and ablative cooling I came upon an idea.

I know that some engines have been designed to Ablate (Burn off) material from an engine nozzle to cool the engine as it fires. This saves money by not having to include expensive and complex cryogenic cooling.

I was also reading how a rocket engine?s efficiency changes with the air pressure outside the nozzle, (A large nozzle works best at a high altitude, and a smaller one is more efficient at sea level.) I wondered if you could take a high altitude nozzle and pack it with ablative material of a certain consistency, so that as it fired the nozzle?s ablative material would slowly burn away expanding the nozzle as the rocket climbed to higher altitudes, and giving the engine a high efficiency throughout the flight.

Are there any engineers (or just intelligent space buffs) out there who could tell me if this would work?

mauk2 · 2003-01-14 21:56:58

Ive been reading heaps about rocket technology in the past few months

Good for you!

I wondered if you could take a high altitude nozzle and pack it with ablative material of a certain consistency, so that as it fired the nozzles ablative material would slowly burn away expanding the nozzle as the rocket climbed to higher altitudes, and giving the engine a high efficiency throughout the flight.

Ok, this is an interesting idea.....

Now, as far as I can recall, the largest flight system using ablative nozzles these days are the Space Shuttle SRB's. (I don't think that has changed, although they tinker so much on that thing who knows what flight hardware is these days.)

You have to use ablative nozzles with most solid rockets, actually, since there is no liquid fuel to pump for regenerative cooling.

So, your idea seems like a good one, on the surface. However, in the SRB the only part of the nozzle that is truly ablative is the throat. The throat erodes quite a bit in each flight, but the mouth of the nozzle hardly erodes at all, if any.

Since the expansion ratio of a nozzle is mouth area over throat area, in practice the SRB is actually LOWERING its expansion during flight. You see, the throat gets larger while the mouth does not.

An example: The throat is 10 square feet in area at launch, while the mouth is 300 square feet in area. At launch, the expansion ratio is 30 to 1. At the end of the flight, the throat has eroded out to 12 square feet, while the mouth is still 300 square feet, an expansion ratio of 25 to 1.

To make your idea work, you'd have to coat the inside of the nozzle with some carefully weakened ablative material, so it would erode away smoothly and perfectly during flight. Plus, you'd have to ensure that the mouth, where stresses are least, erodes more than the throat, where stresses are greatest. This could be challenging, for only a minor gain in efficiency.

The way most rocket designers address this issue is by staging the vehicle. Build nozzles optimized for low altitudes, run them for a couple of minutes tops, then dump them off and light up engines optimized for high altitudes. With chemical fuels you have to stage anyway, might as well kill two birds with one stone.

Plus, aerospike nozzles are automatically pressure correcting, without having to fool with fancy ablative coatings. That's why the X-33 was going to use that kind of nozzle.

So, to sum up, yes it's a good idea, unfortunately there are simpler and easier ways to get the same performance.

Hope that answers the question!

David Ellard · 2003-01-15 02:37:41

Thanks for the response, I was actually thinking about using ablative expansion in a pressure-fed liquid design, with the emphasis on reducing costs.

I agree with you that the hardest part is finding a consistency that would ablate at the correct speed. But I imagine that after the initial design and testing, mass producing ablative cooled and expanding rocket nozzles shouldn?t be too much of a problem.

I would think that the costs would be far lower then any type of cryogenically cooled/expanding nozzle/aerospike designs.

orionblade · 2003-01-15 12:29:32

Hey, they already used your idea in a ramjet engine. Kaiser Marquardt, the big 1950's ramjet researcher built a solid rocket motor inside a ramjet, and there was an ablative nozzle on the engine. When the engine burned, it changed the shape of the nozzle, since the rocket would be going faster and faster, and eventually switch to burning kerosene as a ramjet. The nozzle went from a very large aspect ratio to a rather small one, since the flow through the engine would already be close to supersonic, the ramjet nozzle only had to restrict airflow a little to make the jump to M>1 flow, then expand it out to a mach number of M=2 or 3. The slope of the nozzle wall would be shallower, in other words, than the rocket's nozzle. Older designs used a small rocket motor completely ejected from the ramjet prior to mode change, while this engine used the remnants of the rocket motor to auto-ignite the kerosene fuel for the ramjet. I believe development continued into the '90s, resulting in an operational muniton for the RAF and several other european air forces. I'll see about a link for you later.

You have an interesting idea in applying the ablative nozzle for altitude compensation, however. Try getting a book, Introduction to Space Technology. It's got everything from rocket motor design to orbital mechanics. Barnes and noble 30 buks I believe. Hasta Tarde,
Rion Motley

Mark S · 2003-01-15 15:24:46

NASA Marshall's Fastrac engine was supposed to use an ablative liner that could be replaced during flights so they could simplify the design of reusable engines. It's a rotten shame that the X-34 testbed program was never finished.

I don't think that using ablation to change the shape of the nozzle will be practical, for the reasons that Mauk stated. The ablation rates across the inner surface of the nozzle would have to be carefully controlled to achieve the optimum shape over all fight regimes and engine throttle settings. But ablative liners in liquid engines should still be pursued in the interests of reducing the cost and turnaround time for these powerplants.

orionblade · 2003-01-16 13:22:03

I don't know that it's all that impractical, but possibly unneccesary. CNC machining has brought an amazing amount of cost savings and simplification to the manufacturing process. Literally, I've designed a component on a computer, dragged it onto a floppy, walked down the hall, put the floppy in a CNC mill, and had a component made. Regenerative cooling isn't all that hard to deal with as long as the engine is large enough to permit you to machine the channels in the nozzle for coolant flow.
As far as the practicality of using ablation to adjust for altitude, it may simply be lighter to go with another stage, instead of adding all that ablative material which will actually reduce your exhaust velocity. That's another consideration that I didn't think of until just a few minutes ago. As the ablative material burns, it will increase your mass flow rate, but also reduce the temperature of the exhaust flow. You'd be better off to simply go with an extendable nozzle on an upper stage of a two or three stage rocket, and make the whole thing out of one of these new metal ceramic composites or other refractory material.
but still, get that book, and take a look. Fail or succeed you can always learn something by trying out your ideas. Get a blow torch and some durham water putty... have fun!
Rion

soph · 2003-01-16 14:06:04

CNCs are great. A single computer programmer can program 50 machines worth of work, and you cut down the people needed to do the machining. One guy can operate 5 machines, just checking each to make sure the material is being processed correctly, and typing in new programs.

orionblade · 2003-01-21 23:55:06

Talk about great, I'm taking a class now on CNC, fun stuff. Anyhow, one other point is a pressure fed design may actually wind up heavier than a turbopum fed design, so you may wish to go hybrid, using some form of piston pump with a pressure system to keep feed rates up.

Keep 'em coming,
Rion

SpaceNut · 2019-07-08 21:06:00

bump another to fix topic

GW Johnson · 2019-07-12 15:22:50

Using ablatives to change nozzle area ratio is an odd idea. I don't think it'll work the way the original poster wanted.

Max erosion effects will occur at the throat, enlarging it at a greater rate than anywhere else. That has the effect of lowering expansion ratio as the burn proceeds. That's the reverse of what the original poster wanted. Plus it's a small effect, not a large one.

GW

Tobin · 2020-11-07 05:48:00

I think this has some merit. There is the obvious issue that as the rocket exhaust expands down the nozzle it cools and expands. The ablation will decrease markedly. As with a hybrid rocket the ablation is very sensitive to both.
Also you would add mass to the nozzle that would to a small degree affect its responsiveness and power required to steer the rocket.
But with a 3d printer it should be quite easy to produce a design that is in effect a light weight foam with less density as you go down the nozzle. This should ablate in the desired way and shouldn't add much mass. It would add some mass, and would reduce initial exhaust velocity and temperature; but extra mass should increase low altitude thrust which is desirable. The benefits of a low cost altitude adjusting nozzle is not insignificant.

tahanson43206 · 2020-11-07 08:08:42

For Tobin re #11 .... there is an abundance of free 3D printer model design software.

Anyone can download one (or more) of this design systems, so hopefully you can as well. With such a tool, you can design an ablative nozzle in accordance with your vision, and offer it to the forum for evaluation. There are many examples of how this is done in the archive, and there is a well developed 3D printer topic.

While thinking about things (like this) is good, it is better to try to build a model to see if the concept will hold up to study.

Edit#1: Since the operation of a rocket can be modeled with CFD software, it is also possible for you to run a free CFD program, to see if your mental image of what would happen can match up with the physics rendering of the software. You might find the model matches up well with your mental picture of what would happen.

(th)

SpaceNut · 2020-11-07 10:09:25

Welcome to NewMars Tobin. Sure length and mass changes are critical to increasing payload performance but the nozzle bell needs to be able to retain its shape against the pressue difference from inside to the outside as its shape is used to control the exhaust forces.

GW Johnson · 2020-11-09 09:13:11

I've got a decent article posted on "exrocketman" about how nozzles work. It covers both conventional bell nozzles and a couple of free-epansion designs. The article is "How Propulsion Nozzles Work", dated 12 November 2018. The site is http://exrocketman.blogspot.com. There is a navigation tool on the left of the page: click on the year, then on the month, and finally on the title if necessary, to go right to it. There are also some search keywords associated with these articles. This one has Mars, aerothermo, launch, ramjet, and space program as keywords. If you use "aerothermo", you will see only the articles that deal with aerodynamics/fluid dynamics, thermodynamics, and heat transfer.

GW

SpaceNut · 2020-11-09 19:47:06

Rocket Thrust Equations

GW Johnson · 2020-11-10 13:13:58

Nice to see NASA uses the same fundamentals that I do. But what they show is very incomplete.

The R and T and function-of-gamma stuff in the mass flow equation is characteristic velocity c*. The exit Mach equation is the compressible streamtube function evaluated specifically at the exit plane. The exit temperature and pressure equations are standard compressible flow functions evaluated specifically at exit Mach. The exit velocity equation is the definition of speed of sound (the square root factor), velocity, and Mach number evaluated specifically at exit conditions. That thrust definition has been standard for over a century now.

But all the real-world component efficiency stuff has been left out:

c* not only has an efficiency factor to it, it is also a weak function of chamber pressure empirically in experiments. The mass flow equation lacks a discharge efficiency on the throat area. And the thrust equation lacks a nozzle kinetic energy efficiency applied to the momentum term, but not to the pressure difference term.

My nozzle article on "exrocketman" tells how to incorporate all that real-world stuff. In detail. The way we engineers really did it, in real rocket development work. And I applied it to the free-expansion designs as well.

And if you think that's complicated, solid propellant rocket interior ballistics is far more complicated. This nozzle analysis stuff is but a small part of that. I do have a solid propellant rocket ballistics article posted on "exrocketman", too. Real-world ballistics, not that oversimplified crap in the textbooks.

My most recent posting on "exrocketman" documents the fundamentals of inlets, for ramjets and for gas turbine engines. Same components get used (and sized) very differently for those two types of engines. Scope includes subsonic and supersonic technologies.

GW

Last edited by GW Johnson (2020-11-10 13:16:38)

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#1 2003-01-13 00:26:25

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#2 2003-01-14 21:56:58

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#3 2003-01-15 02:37:41

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#4 2003-01-15 12:29:32

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#5 2003-01-15 15:24:46

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#6 2003-01-16 13:22:03

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#7 2003-01-16 14:06:04

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#8 2003-01-21 23:55:06

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#9 2019-07-08 21:06:00

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#10 2019-07-12 15:22:50

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#11 2020-11-07 05:48:00

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#12 2020-11-07 08:08:42

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#13 2020-11-07 10:09:25

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#14 2020-11-09 09:13:11

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

#15 2020-11-09 19:47:06

Re: Anyone know anything about engine Nozzles? - Ablative Nozzles

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