## Thursday, 18 April 2013

### GPOPS - Optimisation Software

One of the major things missing from the CADAC4 software is the ability to calculate the optimum trajectory. This comes down to finding the optimal path given the known initial conditions (position and velocity) and the desired final conditions (orbital velocity and height). I have been doing a bit of reading of research papers dealing with this problem, however making the leap from the theory to the actual coding has been difficult. Mainly because I am not good enough at the maths to understand it properly.

Since it is unlikely that I will ever be able to code this myself, I did some more searching for an existing solution. I found the GPOPS software, which is a Matlab program for solving these sort of optimal control problems. Although there is still a lot to learn, one of the examples is a rocket launch with multiple stages. My aim is to learn this example and then create a clone of it with the DH-1 details. The input details will be determined by running CADAC4 and getting the velocity and position at stage separation. These will then be the inputs for the GPOPS run to determine the optimal path. It will be interesting to see how close the figures are to the book numbers! I will post some diagrams when I have gotten this done.

### Running GPOPS

1. GPOPS can be downloaded from Sourceforge. I am running it under Matlab R2009b under Ubuntu.
2. Ensure that you have a large amount of swap space. I have had to increase mine to 16Gb.
3. Run the 'gpopsSetup' command from the unzipped directory.
4. Change to the 'examples/launch' directory and run 'launchMain' to start the run.

## Wednesday, 10 April 2013

### Developement Pathway to the DH-1

The two biggest barriers to getting the funding for building the DH-1 are:

1. The acceptance of simple engineering in the rocket area
2. Getting over he believability hump

### Simple Engineering

The simple engineering approach is to cut back all the unnecessarily complex approaches to aerospace engineering and focus on building things that will work reliably. Accept the compromises where they need to be made instead getting bogged down trying to develope new technology to solve all the aspects of a problem. Focusing on the achievability of the whole mission, not any single piece of technology. An example of this is the design of the U2 spyplane with its unstable flight characteristics and unusual landing gear, but over the last 50  years it has proven to be one of the most effective ways of doing that high altitude spy missions. Being able to accept the compromises in design to achieve the mission is a big mental hurdle.

Instead of having the design drivers being low weight and maximum efficiency, have the design drivers as being the simplest approach which will enable to job to get done. This means that instead of trying to build the space shuttle, with its incredible complexity, we instead try to build a DC-3. Overengineering the first vehicle is not something to be avoided but encouraged! We simply dont know what the working environment will be for a reusable launch vehicle which operates on a daily or weekly basis. We therefore need to focus on getting a vehicle which will survive for a sufficiently long number of launches to prove the concept. The very first DH-1 will likely have a very small or zero payload. But it will truly be reusable, and the experience gained will be invaluable.

### The Believability Hump

The second barrier is the believabily hump. Most people will not be able to understand how simple and robust the DH-1 concept is until they see it in operation. Therefore we need to have a way of starting small while still keeping all the important aspects of the design. One idea which I have recently found is to first create a smaller reusable sounding rocket. This would be a single stage vehicle that combines both the first and second stage ideas. There is a very good article by Yoshifumi
Inatani over at www.spacefuture.com detailing the design of a reusable VTOVL design and the emphasis on reliable operability. Although it doesnt have the capability to reach orbital velocity it can still reach an altitude of over 300 km.

This rocket also has a payload of 100kg, which is a lot more than most sounding rockets, with a much larger payload bay volume as well! Because it is reusable with quick turnaround times it would greatly reduce the whole cycle of developing an experiment through to launching. I feel that if this cycle can be reduced to months instead of years then it will allow a lot more interesting work to be done. Instead of having a sounding rocket launch be a once in an education experience, it could be a once a semester type of experience. Most importantly it would show that the model of having much simpler yet more robust launch vehicles being turned around rapidly is a significant advantage.

I would envisage a small single stage reusable sounding rocket such as this being built for a couple of million dollars. The aim would be to assemble a small team of engineers to design and build this, operating on the X plane development model. This would enable the software and analysis to be done in smaller easier steps and verify the concept before too much money needs to be spent. This experience would then enable better engineering and design choices to be made once the full DH-1 developement occurs.

### The Side Note

There is some great video footage from onboard the Armadillo Aerospace Stiga sounding rocket. An interesting look at what the world is like from 40km up!

Another talk which is interesting is by Kim Ennico (SETI), talking about Commercial Suborbital Spaceflight.