So we've outlined approximate basic details of our rocket module:

- Around 40cm in diameter
- ~2.5m long
- GLOW = 305.9Kg
- Empty mass + 20Kg payload of 48.8Kg
- 80% H2O2 and Kerosene propellants

- The rocket must have at least more thrust than weight at liftoff, so therefore more than 3kN thrust.
- Most liquid fueled rocket motors can't be throttled by a large amount before combustion instability sets in and deep throttle capability is complexity we can do without. However the maximum period of acceleration of the rocket is just before burnout when weight is at its lowest point (due to fuel consumption), assuming constant thrust from liftoff to space the higher the initial thrust level the higher the final acceleration and the heavier the structure and payload will have to be to deal with it.
- So we need at least 3kN but with just 3kN the initial acceleration will technically be 0m/s^2 which is equally useless to us, so I set the initial acceleration requirement at 1.5g's and rounded up to the nearest whole number. That's how I get 5kN initial thrust resulting in 1.7g's off the pad.

As can be seen the 3 module system should be ok for making orbit, at least with this simple model. A much more accurate flight simulation system which will be able to handle control system interaction and other nice things is in the works but a ways off yet. Interesting to note the final burnout acceleration spikes up to around 10 g's which is quite violent, too much for a person but ok for pretty much all electronics. Remember how we chose the initial thrust level? If we'd chosen a value set to give higher initial acceleration this value would probably be significantly higher and perhaps too high to work. In reality the rocket motor will probably be slightly throttlable to help reduce this peak acceleration but we now don't absolutely have to which is a good thing.

From this simulation we can start to outline the structural design of the rocket, which will be influenced by the load cases we choose to test the design against. I've broken the various load cases into a few which I think will represent the primary and most demanding situations that will be encountered:

- Based on the simulation of the 3-1 orbital stack with a peak acceleration of 10g's, a compression limit load of 10g's for a full module and an additional load on the interstage for a possible 3rd stage. Using a margin of safety of 1.5 this gives an ultimate load case of 15gs + 1016.7Kg for a total compression load of 55002N (about 5.6 tons, quite high).
- The second load case is for a torque load, traditionally longitudinal loads are primarily taken by stringers/spars running the length of the rocket, however these large structures effectively take no torques. The torque load case was abritrarily set as the force required to accelerate/decellerate a fully loaded spinning rocket from/to 300 rpm in 1 second (for example the roll control system eliminating a high roll rate).
- Case 3 are for arbitraty ground-handling loads. Case 3a) is where the fully loaded module is simply supported at both ends and subjected to 2 lateral g's. Case 3b) is where the rocket is fixed at one end and subjected to 2 lateral g's.

Sorry the posts have been lacking in images to this point, unfortunately all this initial design and envelope-building is kinda boring but it's all part of the design process and will bear fruit in the long run. Next update will be on sizing the structure to deal with these load cases.

What is your "engine on" time assumption? I'm guessing 350 seconds from the graphs, but wanted to check. BTW, you might want to think about using 85% H202 in your calculations. If you can get 80% in bulk for your rocket, you can get 85% without too much trouble and the safety factors don't change that much. Being Australian and into space you probably know more about the British launches from down there that I do, but I do know they used 85% H202 and kerosene with solid cat packs to reach orbit.

ReplyDeleteHey Ray, there is no engine time on assumption, the motor run time is dictated by the engine propellant consumption and the propellant mass of each module. I haven't talked about the motor design yet but you can approximate total propellant mass flow rates using the ISP I stated below:

ReplyDeletePropellant mass flow = Motor thrust / ISP

That is for both propellants of course.

Yes you're right if you can get 80% you can almost certainly get 85%, I used 80% as a conservative number to build-in some tolerance on the peroxide concentration, if for example I had to concentrate it myself and I was unsure of the true purity.

The British rockets did indeed use peroxide and cat-packs however they did also inject kerosene to increase their ISP (peroxide is first decomposed in the catalyst packs then kerosene is injected into the super-heated steam). For this design I'd like to steer clear of silver catalysts, they are easily poisoned by impurities in the peroxide and I'm assuming fairly dirty peroxide.

Is your Matlabs code available for this project? I'm attempting to learn Matlabs for the purpose of rocket modeling, such code would be very valuable as a learning tool.

ReplyDelete