Augmented Rocket Design

[Retro]

Augmented rocket based on hobby Aerotech motors for primary stream. 38mm reloadable solid fuel rocket motors I-285 and J825.
Plan to install diffuser aft of primary nozzle ahead of constant duct.

I need a formula to determine diffuser inlet diameter, constant duct diameter, and convergent nozzle diameter.

 

[GregLocock]

Is this for low speed work?

The equations for subsonic performance aren't wildly complicated, look in your thermo book for 1 dimensional flow, venturis and the like. What is the exhaust velocity of one of these rocket motors?



Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[GregLocock]

Incidentally the thing is called an eductor, they are commonly called jet pumps and are widely used.

http://science.blogdig.net/archives/articles/August2007/06/Design_of_Eductor.html

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[rb1957]

why ? what are you trying to achieve with a different diffusor ? there are plenty of references for designing nozzles/diffusors ... wiki, hobby sites,

 

[Retro]

"why ? what are you trying to achieve with a different diffusor ?"

I could just copy the diffuser shown in the old NACA report. However the 2.88" diameter just looks to small for the 1.5" dia motors I plan to use. I could just use a straight mixing duct and forgo the diffuser that would be simplier. However the NACA report I'm referencing indicated the diffuser increased the augmented thrust with shorter duct required.

I'm trying to move the rocket motor up into the body tube to achieve better CG/CP and reduce nose weight required for aft motor position. This is a finless conical rocket with no guidance. Venting the motor exhaust seems to reduce the
krushnik effect. And three vents or "bifurcated like" inlets seem to act as tube fins to assist guidance. I would like to achieve some thrust augmentation as an added bonus. Similar to the Sprint ABM missle and NASA GTX design without fins only inlets are directed inward toward a single central recessed motor location. The 2.88" diffuser inlet diameter does not yield very large side inlets. Increasing the duct inlet diameter would allow for larger inlets on the conical body.

Checking out your tips. Thanks!!!

 

[rb1957]

cool, didn't imply a "spanish inquisition", just letting us know more about the problem helps us help you !

"augmented thrust" sounds like something for nothing, possibly by increasing the area of the exhaust you're decreasing the velocity and maybe reducing the losses ... maybe

or are you entraining outside air and mixing it with the rocket exhaust ?

what's the exhaust velocity of the motor ? if it's supersonic there could be reasons to carefully design a diffusor (ie shocks). how much thrust is generated at altitude (remember the speed of sound decreases with altitude; but then i'd expect your exhaust velocity to fall as well, as the fuel gets consumed)

moving the motor up the tube will increase the installation losses, friction on the side of the tube. this will also be affected by diameter, bigger may not be better (more area, but lower velocities)

 

[btrueblood]

"or are you entraining outside air and mixing it with the rocket exhaust ?"

That's the general idea. I have seen somewhere a report by somebody using model rocket engines, and variously sized eductor tubes mounted concentric to the body tube. The testing was simple - they slotted the eductor to fit over the fins, and when the rocket launched, took videos of them. The higher the altitude that the eductor reached before falling off (i.e. when drag became greater than developed thrust), the better the design.

What the OP is describing worries me, though, with multiply-ducted exhausts. These engines put out a lot of condensed-phase species that will rapidly plug a cold duct. Be very careful when testing such schemes.

 

[Retro]

"or are you entraining outside air and mixing it with the rocket exhaust ?"

Exactly see attached photos of my original test rocket fabricated to test the concept. This was a standard spike inlet/diffuser/constant duct with a convergent subsonic nozzle. It was scaled down from 1948 NACA augmented rocket plans.
It had small altimeter to collect apogee (2,000' AGL range)data to compare against standard rocket of same size. Standard rocket achieved slightly higher (200-300 ft) reading indicating I came up short but still it worked quite well enough and was very cool model. As you can see the secondary subsonic nozzle is quite large I doubt if plugging would be a problem a supersonic nozzle might be more of a concern.

 

[btrueblood]

Retro, the idea of jettisoning the eductor I thought was a good one, if you could make it slip off reliably. The point being, a given inlet area/throat/rocket exhaust flow combination will work (i.e. give more thrust than the associated weight/drag penalty) only over a narrow range of flight speeds. I think. It's been awhile since I did the math...Greg is right, Dr. Oates does some nice 1-D treatments of various schemes, and I think augmented exhaust is one of them, in his Aerothermodynamics books.

http://www.amazon.com/gp/product/15..._m=ATVPDKIKX0DER&pf_rd_r=00CS8T3C9R0YR1XQSAB4

 

[GregLocock]

It's an impedance matching device. The ratio of the motor exhaust to the flight speed is critical in determining the efficiency of the rocket. By using an eductor to entrain a large mass of air into the exhaust the velocities are more nearly matched, the mass flow rate is substantially increased, so you get more thrust for a given rate of mass of fuel used.

It is a neat idea, oddly not used by the real rocket boys, who when push comes to shove favor simplicity over efficiency.

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[rb1957]

just of like a high by-pass jet engine (like on all airliners today).

i'd suggest that there's a real reason (other than they haven't read about it or think they're better than that) why the big rocket people don't use this ...

 

[btrueblood]

"It is a neat idea, oddly not used by the real rocket boys, who when push comes to shove favor simplicity over efficiency. "

Two issues, one already mentioned (drag/weight penalty once a certain flight speed is reached), the second is that at lift-off you have potential for reversed exhaust flow up the side of your rocket...you know, where the nasty volatile stuff you don't want to burn yet is stored...that potential exists because typical booster engines operate below stoichiometric oxidizer/fuel ratios (aka fuel rich), as these ratios maximize thrust. But, rich mixtures also re-ignite in the sea-level atmosphere...and if they re-ignite before any vehicle speed has developed (which they will), there is a good chance for the reversed flow. The above may or may not hold for solid propellants, as I'm not sure of the exhaust species for a typical solid grain, but I'm reasonably confident it would hold for typical liquid boosters.

At a certain rocket company, they did look at whether that low-altitude exhaust energy could be recovered by some type of eductor (with a rig to maintain pressure via external blown air until liftoff was achieved), or even a turbine-driven fan to maintain positive pressure, but the concept quickly got heavy and complex. And, the analysis carried the assumption that the eductor's weight could be jettisoned at an optimal point in the flight, not necessarily a simple thing to do with a running booster engine still dumping exhaust through the dang thing.

Rocket engine designs are horribly complex, and every effort is made to squeeze extra ergs of energy out of the fuel carried whenever possible...if the mission analysis says it's worth it. You're trading extra pounds of fuel to be carried vs. extra lbs. of engine weight. Fuel gets burned, so the weight penalty decays with flight duration, but engine weight stays until you stage/seperate.

File this under the "more than you wanted to know" column.

 

[GregLocock]

The Soviets did use a sort of air augmented booster, but turned it into a ram jet, which I suppose is the ultimate rich mixture you mention!

Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight