nitrous oxide + Ethanol engine

[n8ores]

Hi first time posting, I am also looking to build a nitrous oxide + Ethanol engine.

Thrust: 100 Kg
Run Time: 30s
Chamber pressure: 1000 Psi
Propellent tank pressure: 1200 Psi
Propellents: Nitrous Oxide + Ethanol
Fuel Flow Rate: 5.7 L/minute
Oxidizer Flow Rate: 16.3 L/minute

Chamber Material: Aluminium 6061
Chamber Wall Thickness: 5mm




I have created a design spreadsheet (Attached) which anyone else is free to use as well, Is anyone able to sanity check my calculations used here?

Also for the convergent and divergent sections I used angles of 30 and 15 degrees respectivley however I only used these because they are common angles, does anyone have any equations I can use to calculate the optimal divergence angle.

Attachments:

[n8ores]

BTW If there are any Aussies here we are designing and building this engine in Sydney Aus, if you want to get involed drop me an email just go to www.nathanparrott.com

[racket]

Hi Nathan

Sent you an Email , I'm in Sydney and interested :-)

Cheers
John

[enginewhisperer]

I've already been talking to Geoff about this, so will be involved

[Deleted]

Will be following this with much interest from Brisbane..
Cheers Chris

[ernie wrenn]

That is on HELL of a crew you picked up.

ernie

[racket]

Hi Nathan

It would probably be better to have a bellmouth inlet into the nozzle throat , ..............area ratio between throat and exhaust seems about right for a 1,000 psi combustor - 70:1 PR , though might be at the upper limit depending on the specif heat ratio ...........divergence half angle of 15 degrees should be OK with a ~98% divergence coefficient , not much to be gained by making it shallower

Lookin' good :-)

Cheers
John

[n8ores]

Thanks John,

What sources did you use to verify my calculations, equations or past experience etc?

Yes I was planning on a bellmouth at the nozzle throat but was lacking the formulas to accurately calculate the radius, do you have any suggestions?

In case others are interested the main sources I used for my calculations were:

"How to design test and build small liquid fuelled rocket engines" - Rocket Lab
www.nathanparrott.com/rocketengine/rocketlab.pdf
Good high level overview and covers the basics, I cross referenced some of the calculations with this website www.braeunig.us/space/propuls.htm

"Design of liquid propellent rocket engines" - Rocketdyne NASA SP-125 (Declassified)
www.nathanparrott.com/rocketengine/NASA SP-125.pdf
In depth detailed design document, covers pretty much everything there is to know about liquid fueled rocket engines.

Nathan

[racket]

Hi Nathan

I used my Zucrow text ....................I don't know much about rockets ,....... LOL, I just look at the graphs in the book :-)

Interesting article on bellmouths www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

I like that "Rocket Lab" Link , heaps of great reading .

Cheers
John

[n8ores]

*Confirmed convergence angles fall between 20-45 degrees in NASA-SP125 page 89
*Confirmed Divergence angles fall between 12-18 degrees in NASA-SP125 page 90
*Confirmed Nozzle throat arc contour has a radius between 0.5 and 1.5 the radius of the throat

Typical divergence angle of 15 degrees is almost always used in Conical nozzles as we cant be assed with the complexity of a bell shaped nozle.

Increased wall thickness to 8mm instead of 5mm and increased combustion chamber diameter to 52 mm, which also allows us to use 60mm aluminium tube.

[n8ores]

3D Design thus far those that are interested.

My major concerns at this stage are:

1), Current chamber wall thickness is 8mm which may be too thick for sufficient cooling, I dont know. My second biggest concern is the size of the motor, the combustion chamber is only 55mm long and the entire motor is only 150mm long! This seems incredibly small to me, but I guess its only generating 100 Kg of thrust and Ive double and triple checked the math, as well as comparing it to similar motors and it seems about right, if the system was 100% effeicient which it wont be.. I'm predicting that it will only end up being at best 50% efficient (50 Kg thrust) due to unoptimized injector design and other inefficiencies (assumptions in calculations such as Coefficient of discharge, L* etc). But since 100 Kg thrust isnt a design requirement (this motor doesnt have to power anything) I have not sought to scale up the design etc. It will be interesting to see how accurate real world results are when compared to the calculations, then we can use those results for the next iteration.

[Johansson]

Very interesting posts, but please start a new thread for your engine build instead of posting in this one. Two builds in one thread gets rather messy...

[Richard OConnell]

Haha, I'll split the topic

[Richard OConnell]

haha stil have no idea why it makes me show up as the topic starter every time i split a topic.. something to look into later i guess Looks like its gonna be pretty powerful for its size. Any hints yet on what you are going to use it on?

[n8ores]

Hi there, bit on the tank design



FYI to design the tanks you simply take your required fuel and oxidizer flow rates and multiply them by your desired run time, in our case I wanted an engine run time of 30s and the required flow rates are Fuel: 5.76 L/min Oxidizer: 16.23 L/min I.e for 30s we require 2.5 L of fuel and 8.1 L of Oxidizer. The end peices are simply 150mm round plates that are welded into the tank. their thickness should be 2x the wall thickness.

To Design the required wall thickness you simply use the equation
T = (Safety Factor * Tank Pressure * Tank Diameter)/ (2 * Burst Pressure of Material)

T = (1.8 * 8273709 * 0.15)/(2*137540000) = ~ 8mm
*NOTE: 137540000 = 20,000 Psi = Burst Pressure of Mild Steel.

For simplicity both tanks will be made from the same 150mm ID tube, the oxidizer tank will have a height of approx 500mm (Volume = Area x Height = pi*0.075^2*0.5 = 0.008 m^3 ~= 8L) and the fuel tank height will be approx 200mm (Volume = Area x Height = pi*0.075^2*0.2) = 0.003 m^3 ~= 3L

I have to also add on 32mm to the height to account for the end pieces, but I might just make the tanks 600mm and 300mm in height that way if we want to have a bigger motor at some stage we can.


Image below shows the tanks with some of the associated control hardware (Ball valves and check valve).

Those pneumatic ball valves are about $600 each from memory.... hoping to get a low cost varient, OR simply attach a DC motor to the spindle of a manual ball valve....