Ducted Ejector Ram Rocket Solid Fuel

[airrocket]

Anyone ever experiment with this concept? Small fuel rich rocket motor entrains additional atmospheric air in larger Ramjet tube = zero speed ramjet. No turbine required= high ISP and thrust.

Ignition of a central solid fuel rich /oxidizer rocket fuel grain located in the intake diffuser section pitot burns (gas generator) a sparking device (igniter) reignites the fuel rich plus entrained atmospheric air (close to stoichiometric mix) in the combustion chamber and perhaps again in the nozzle (base burning/afterburning).

Need help formulating the slow burn fuel rich solid grain? Perhaps the rocket group could assist with this aspect. Also need a combustion chamber ignition source, perhaps RC glow igniter? Use metal spinning techinque and TIG welder to form stainless steel ramjet cylinder.

http://www.islandone.org/LEOBiblio/SPBI104.HTM


airrocket

[heada]

I have seen ideas similar to this before. Are you envisioning a standard rocket motor just aft of the diffuser(as shown in one of the drawings) or using the entire combustion chamber of the ramjet as the solid rocket motor (also shown in one of the drawings)?

If a "standard" solid rocket motor is placed just aft of the diffuser you get problems like Krushnic effect or suction lock(aka Bernoulli Lock).

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Krushnic Effect

A very dramatic phenomenon where your rocket makes a tremendous amount of noise and smoke but doesn't go anywhere! This happens when the motor is recessed into the body tube by more than one tube diameter. If so recessed, the cylindrical volume below the motor forms a secondary expansion chamber which allows the exhaust gasses to expand below atmospheric pressure before leaving the rocket. Surrounding air aspirated into the exhaust stream causes turbulence which negates much of the thrust, along with creating the characteristic roar. A multi-stage model that ejects its booster motor, but not the airframe, is a perfect example. Very damaging; it almost always destroys the lower body tube beyond use. Named for Richard Krushnic, the rocketeer who characterized the effect in the late '60s. Not to be confused with Suction Lock

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Bernoulli Lock

A phenomenon similar to the Krushnic Effect where the rocket seems to be "glued" to the pad at liftoff. This afflicts larger, flat-bottomed rockets launched too close to pads with flat blast deflectors. The exhaust gasses escape at great speed through the small annular space between the rocket and the pad creating a venturi which generates a low pressure region at the base. This pressure deficit can be significant, and if it is greater than the thrust being generated by the motor, the rocket won't go anywhere! This is quite possible as a 2" dia.rocket has, potentially, over 45 lbs (200 N) of "suction" available to hold it back, while a 3" rocket has over 100 lbs (460 N)! The old Centuri "Point" was an infamous Bernoulli locker when launched from an Estes Porta-Pad with its perfectly matching round blast deflector.

If on the other hand the entire combustion chamber of the ramjet is used as the combustion chamber of the rocket motor, you'll have a very inefficient rocket motor that more than likely will not get to the speeds needed to create positive thrust with the ramjet.(low CC pressures giving low thrust burn/incomplete burn, over expansion at the nozzle giving low thrust)

Can it be done, sure. Can it be done by hobbyists? I don't know but I doubt it. It would require significant knowledge of rocket motor design and propellant making.

-Aaron

[airrocket]

I have never experienced the Krusnic effect. I believe you and others make a valid point and do not doubt that it exists. However I have seen evidence to the contrary that suggest it may not be as pervasive or unavoidable as some suggest. Check out the construction zone article in the Sep-Oct 07 Extreme Rocketry magazine "The katana Ducted Cone Rocket, Part1".

My concept based on the Ejector Ram Scram Rocket RBCC concept utilizes the solid rocket motor only as an overly rich gas generator and air entrainment device. The rich gaseous exhaust mixture would be reignited in the larger diameter empty motor casing mixing with the entrained air and exhausting through the empty casing nozzle as a much larger ram rocket motor quickly transitioning to a solid fuel gas generator ramjet. I would probably have to mix my own solid fuel/oxidizer to achieve a slower burring fuel rich mixture. I feel compelled to at least give it a try and see what results. Admitting you could very well be correct "I told you so".

Really looking for a commercial gas generator source prior to attempting to mix my own. Any suggestions for sources or design information?

Appreciate the advice.

airrocket

[heada]

For a NASA study of just what you're suggesting:

http://gltrs.grc.nasa.gov/reports/1997/TM-107422.pdf

Note, the fuel was JP-10 (jet fuel, highly refined kerosene) and not a solid like you're describing.

Also, since you're in Iowa and one of the best groups for AM/EX rocketry is based in Quad Cities:

http://p209.ezboard.com/btqc

As for a fuel rich system that you could case, if you're going to be entraining outside air, it could be anything from HTPB (binder in most APCP motors - http://www.aeroconsystems.com/misc/htpb.htm ) to paraffin wax. A good example of this is most of the hybrid rocket motors. They take things like PVC, vaporize them (via a pre-heater) add decomposing liquid nitrous oxide and you get rocket thrust. (N2O decomposes into (2) N2 + O2 at about 450°F)

The thing that I think will be your biggest hurdle will be to create a rocket motor within the ramjet that will produce enough thrust to get the ramjet up to speed to operate as a ramjet without fouling the diffuser/combustion chamber/nozzle of the actual ramjet. If you look at the width of most rockets and compare that to the width of the nozzle opening, you'll see that the nozzle opening will be too small to use for the ramjet. Here are a few examples from commercially available motors:

Aerotech I284W http://www.aerotech-rocketry.com/custom ... m_assy.pdf
It has a grain width of 1.308" (33.2mm) with a nozzle opening of 0.266" (6.75mm)

Aerotech J1999N (800 pounds of thrust for 0.7 seconds)
http://www.aerotech-rocketry.com/custom ... p_assy.pdf
It has a grain width of 1.87" (47.5mm) and a nozzle opening of 0.713" (18.1mm)

In order to produce thrust to need to build pressure within the combustion chamber of the rocket motor. The only really way to do this is to restrict the nozzle exit (without restricting it too much) If you only wanted a fuel rich gas generator, an over-expanded nozzle on a APCP rocket motor would work as you'd never build up enough pressure to get a complete burn but it will not produce thrust.

The most common ramjet that I have seen is Decker's design. If you follow that design and have a diffuser entrance area of about 8 square inches you end up with a combustion chamber width of 6 inches. A combustion chamber width of 6 inches would call for a nozzle exit width of 4.5 inches and a length of about 3 inches and doesn't use a de Laval nozzle like is required for a rocket motor. Converting the nozzle to a de Laval nozzle is very doable and might even add to performance in some situations but the overall nozzle geometry for a ramjet is very different than a rocket motor.

Back to the biggest (and only really) hurdle is how to create a rocket motor within a ramjet that can generate enough thrust to move the ramjet to a speed high enough that the ramjet could then produce enough thrust to continue.

I don't really have any suggestions as to how to do that. Getting a liquid fueled ramjet that produces the desired thrust at the desired airspeed is hard. Solid fueled ramjets are harder. What you're suggesting is even harder yet.

-Aaron

[airrocket]

Great suggestions, links and advice, really appreciated. Yes I agree getting the initial thrust well be a challenge. I plan to experiment with small motors combinations first. I like the nozzle exchange idea I will have to research that further. I will attempt to fabricate and integrate a mass flow MAF oxygen sensor into the design to gather some data. From what I'm told India and South Africa both developed very successful ducted rocket programs back in mid nineties up to mach 5.

airrocket

[vhautaka]

This concept is also known as the ramrocket, and there's even a comprehensive Wikipedia article about it. Seems it works, and more, it could be more efficient even than hydrogen/oxygen engines.

http://en.wikipedia.org/wiki/Ramjet#Int ... ted_rocket
http://en.wikipedia.org/wiki/Ramrocket#Effectiveness

Design for a narrow speed range, etc.

[Ghrey]

I have done some tinkering with an after burner like assembly for a Flammable exhaust producing rocket for some time. Seems similar to your concept in some ways.

For me it is not getting the High speed gas stream that is vexing, it is getting the mixture with the entrained atmosphere correct, and keeping it that way as both velocity and your SRB are changing.

I went on to other projects...

[airrocket]

Decided to eat this elephant one bite at a time.

First project will be an air augmented rocket utilizing fanno flow (flow through a constant duct). Similar to the old Estes Augie II only scaled up and tweak some. The larger diameter duct tube will be longer with only a few inches of a conical cone center spike protruding. And the lower boost motor will be optional.

http://www.dars.org/JimZ/eirp_28.htm

Once I have gathered some data with that design I will progress to a solid fuel (ejector) ramjet or ram rocket design.