Monday, July 20, 2015

GPS on several fronts...

This week end I flew an open source Piksi GPS on an HPR to see if it could be made to keep
lock at high acceleration without any imu aiding. I modified the tracking constants of the piksi software .(actually used # defined for wide bandwidth tracking already in the latest software)

It lost lock at boost, got lock again at apogee (strong winds at FAR apogee horizontal V was 204knots!)  This pulled the electronics bay out of the vehicle when the drogue deployed at that speed.  Fortunately its a sturdy rocket and it survived a no main chute deploy with minor damage.

There were some issues I think I set the lock detection too too optimistic and it tracked some noise...
I will fly a Piksi again recording the Raw RF some time in early Aug.
(I'm also going to change out the TCXO for one with lower G sensitivity see discussion below)

On a 2nd GPS front PSAS flew a software GPS receiver on their rocket on Sunday.
They gathered raw data, but they don't have the full GPS solution working yet.
They posted the following graph

It shows doppler shift in the tracked sats.
I actually question that a little bit...

Doppler is what makes a trains whistle change tone as if goes past.
If you change the velocity with respect to a radio transmitter the frequency will shift.

There is another effect that can also effect this data... the RF system on a GPS uses a very precise clock (usually a TXCO)  in the receiver.  That clock may have acceleration sensitivity.
IE the frequency of this oscillator may vary with applied acceleration...

I'm somewhat suspicious that the graph above shows this more than Doppler shift.
I have not looked up the orbits of the gps sats at the time of flight or the position of PSAS flight, but I'll make some assumptions...

The GPS sats are scattered randomly about, ie some are almost straight up and some are at the horizon.  If the rocket flies straight up one would expect a bigger doppler shift for the sat straight over head and much less shift for the sats at the horizon as their relative velocity changes less.
Also if the rocket turns into the wind and has some horizontal velocity one might even expect a doppler shift in the other direction for the sat you are flying toward/away from.

However if the frequency shift is due to g effects on the reference clock one would expect all the doppler shifts to move the same direction....

So it looks like they got a shift of about 1400 hz  at 1.5Ghz that would be a velocity of  140 m/sec or so... at 1.5 Ghz. So the magnitude is withing the realm of possibility...

(The graph also shows an overshoot at the end of the rocket burn this is the acceleration changing direction from the perspective of the GPS receiver.)

There are vendors that specifically sell clock oscillators with low G sensitivity...

Given their data the PSAS should be able to determine this.
With a full GPS solution derived from the data (they have not done this yet)
They will get position velocity and time....
They also have sample of data precisely clocked out via their reference oscillator...
The difference between the time in the solution and the time via their local clock can
be measured and will give an idea of clock drift.

They can also calculate the expected doppler between them and the individual sats and see how that compares with the   measured doppler any difference is clock error...

All of these data extractions require they get a full solution running....

There is a quick and dirty test they can do with just what they have...

Run their gps sampler system on the ground with a fixed  stationary antenna...

Now take the GPS board and orient it it differently in the 1g gravity field.
6 combinations IE board component side up, component side down, tipped on its end, pointing up, down right left....

 and generate the doppler graph above...  if the doppler shifts with orientation of the sampler board then they probably need a more stable clock.

  At  bare minimum this test will show them what axis has the lowest clock sensitivity....

In any case I really applaud what the PSAS guys are doing!

Monday, July 06, 2015


Over the last two weeks John and I have built and failed 10 or so short tank samples.
We have a tank solution... fully allocated with fittings and end caps we have a MR of about
15 at 795 PSI burst. (16 at 500 psi)

Now we have to put together the tooling to make flight length tanks , ie longer than the samples.

I did a 3d printed regen motor design and sent off to have a SLS nylon version printed to evaluate.
That should be here today.

Not sure if I want to put gimbal at the nozzle (as test print) or up at the top of the motor...
Been busy....

Tuesday, June 16, 2015

Tanks again

I finished fabricating all the parts for the tank test V2.
I should be able to assemble and test this evening.
This is a short piece of test carbon tube with reinforced ends for additional load bearing,
The end caps are 0.075" domes with AN-8 o-ring ports on them.  The outside of the dome had to be cut with two different cutting tools, so you can see a line/step of about 0.010" where I changed tools.

Update I put it all together and hydro tested it this evening.
It failed at about 850PSI.  The bag and end caps leaked profusely until about 300psi when the o-rings seated. At 500 PSI it held pressure with zero leakage.  The primary failure seemed to be a radial  split down the center.


Wednesday, June 10, 2015

Slightly OT How to get from old world to new world (parts and vendors)

Twenty years ago everything slightly technical came through a distributor that provided technical support.  Today the internet is a source of that support, so what function does the distributor provide?

I was looking to buy some tooling for my lathe. (Names not included to protect the innocent)
I found the exact thing I wanted where a vendor had a nice description of how to use the tool and had captured the exact thing I was looking for...

I emailed the vendor, they entered my information in their system and one of their technical people provided me with a quote and some nice technical information on the proper usage of the part....

So far we are happy everything is golden....

The problem is there was no way to order the part.  On their website their was a list of 29 distributors for their stuff in CA.  I called one at 2:15 pm they said the factory is closed I can't get you delivery info.... I'll call you back on Wedensday.... After some more searching I found no on-line vendor for this "Exactly what I wanted part"  So I went to MSC direct and figured out what I needed to order to do the job, probably not as well as the part I wanted, but I placed the order  at 4:59 pm.

Today I received a call back from the distributor at 1:15 pm saying the part was not in stock.
I got the parts from MSC delivered to my house (UPS ground from NV) at 1:57pm.

So here is a vendor with good marketing an a quality product that failed to make the sale.
Why?  They aren't stupid they know this process is killing them why no online ordering or stock check?

They are a successful company that was actually profitable and doing well before the internet.  100% of their revenue comes via distribution and if they open an online store they kill all their distributors and loose most of their existing business flow.   If they offer their product line through some existing on-line distributor they upset all of their existing customers. Its sad, but they are trapped.

In my day to day life I see a lot of these....

Its one of the things that's changing in the world,  it takes a lot less people to run a few centralized where houses set up to ship quickly than it does to have a real human at a distributor in every city.

Just this past weekend our dryer died, in years past this would have involved a drive down to the appliance parts store in Chula Vista on Monday morning..... This time I ordered a new dryer belt from amazon Saturday at 9PM and had it at 9:30am Monday morning.

The internet and automation is going to eat a lot more jobs before its done. Its not just automation replacing specific workers, its a new structure of doing business that is decimating a whole layer in the supply chain that used to employ millions of people.  May you live in interesting times...

Tuesday, June 09, 2015


One of the failings of silver ball was that I never got an accurate hardware in the loop simulator running.
On this project I'm not going to make the same error. 

After evaluating a number of possible solutions I've decided to use JSBSim  and I've secured the assistance of the 
Jon Berndt,  Development coordinator and chief architect for the JSBSIM, in getting that done. (I've made an arrangement where he will help me get started for a consulting fee, his time permitting)

One of my questions to him was:

>A details question.... I realize that JSBSim was built to be a flight simulator.
>I'm trying to repurpose it as a rocket simulator....

>Internally you seem to use ECEF coordinates.... I have not dug deep enough to figure out 
>if you model the inertial effects of a rotating earth...
>will one see a performance difference between an equatorial launch and aa polar launch?

>If I have a vehicle attached to the surface of the earth will it show the rotation rates of 0, or will it show the 
>the rotations rates one would see in an Earth centered inertial frame?

His answer greatly increased my confidence that I made the correct selection...
And, yes, JSBSim had a full overhaul of its EOM many years ago for full and complete EOM. It was the only non-NASA sim used in a large check case development effort that is published here:

The EOM integration takes place in ECI, full rotation effects are factored in and – yes – you would see the benefit from an equatorial launch compared to a higher-latitude launch. It is a full, vehicle simulator that can model anything from a rocket to a ball to a blimp to a sub/supersonic aircraft.

The vehicle sitting on the launch pad should show no Earth relative motion, but should show a translation and rotation relative to inertial. This was one aspect that was compared across NASA sims and JSBSim in the check case effort, and the comparison effort investigated tiny differences to make sure we were all on the same page and using the same Earth rotation rate. At this stage, I am very confident in JSBSim. That’s one nice thing about open source and broad use: the more people use it in different ways, the better the development effort goes, and the more robust the code becomes.

The simulator in general is designed to be data driven via external XML files. So one can model things like autopilot loops etc...I really want this to be a hardware in loop simulator so one of the modifications I'm asking Jon to help me put my c/c++ code into  the control loop. Eventually this will entail me taking the current state, simulating the IMU and GPS outputs feeding it into flight avionics and then taking the response from the flight avionics and giving it back to the sim to have it operate on the commanded changes.

Monday, June 08, 2015

Update on the week...

During the week I got my "paper" 4" otrag rocket to orbit.
This was a 2D rotating earth model including blow down pressure effects and atmospheric drag.
I assumed drag was the same as a sphere with the same frontal area as my otrag stack.
This was from 30deg latitude.

The classic otrag square 4:12:20, triangle otrag 3:12:20  and single tube last stage 1:6:12 all got to orbit. The single tube unit had the most margin.

I broke the trajectory into 2 segments for first and 2nd stage and 3 for third then did a brute force search of the space for optimum trajectory.... where my optimization fitness was horizontal velocity at 250Km with vertical velocity >=0.

I have a couple of other people doing modeling to "check my work" Ed Lebouthillier is one of the people helping me check my work with performance sims.  He put together the following picture...

I also reopened a discussion with the FAA on what it takes to do part 101 launches offshore...
and got a really favorable answer. On their request I'll publish more when the FAA resolves one remaining question. (Its really good news in any case)

Friday I got and assembled the pieces for a sunlight readable laptop... (pixel QI 10.1" screen and Samsung netbook) and swapped screens. Tested this in the Mojave on Saturday and it worked well.

Saturday I went out to FAR and almost finished cleaning up the old propellant.
This involved using a large "can opener" to open the tops of drums, removing the poly liner and getting the drums recycled.  The down side is that the can opener only works on about 50% of the drums. For the rest of the drums removal involved a  45 minutes, a cold chisel and a small sledge hammer... Did 9, have 5 left.

While I was at FAR I picked up a new carbon tube from John, with end reinforcement for better end cap retention at the desired 4" size. Will test this tank some time this week when I finish machining the end caps.

How to attach your plumbing to fabricated parts is always a challenge. With welded tanks we just welded on AN bungs.  But if your going for minimum weight welding really destroys the aluminum strength.  So the two remaining choices are machine AN fittings directly into the aluminum or put in an AN o-ring sealed port. Bought a port reaming tool and tried that Sunday night...
The tailstock on my lathe is a replacement and about 0.010" too high, with no adjustment.  So this caused the super rigid port reaming tool to chatter  screwed up the bore leading to bad threads... so I'll make another end cap tonight and probably do the port machining on the mill where I can indicate in center....

I may take the tailstock apart and try to machine off 0.010 between the base and the tail stock...
Need to set up to fully indicate the tailstock quill to see if its just too high or has parallel issues as well. (Quick run out test says parallel is ok and its just too high.)

Next up is to farther flesh out the detailed design of the paper rocket with respect to fill and drain, valves and gimbal actuators...

I'll try to post something at least once a week....

FAR has another event in two weeks, I may try to fly a PIKSI on my HPR to see how well it holds lock with with acceleration.

That's all for this week in unreasonable rocket land.

Sunday, May 31, 2015

Quick tank update...

The goal for the composite tanks are 250PSI operating 500 psi design burst and 375psi qualification.
Today the tank I tested failed at 375psi.  When I blew one end out, I did not re-bond the opposite end, the opposie end then failed. (Sort of expected)  When it failed it also split lengthwise.

John is gong to make me some new tubes to test.. with a peel ply layer at the bonding ends and an additional layer on the ends for additional strength on the end caps.
We may add enough material to have bolted ends as that allows you to disassemble and inspect etc...

I also need to slightly rework my seals as the tank seeped until the pressure came up and the seals seated hard. (IE it seeped at 50psi, but stopped at 125 or so)

The current conceptual launcher uses bundles of 4" tanks so the next tube samples will be 4" The current sample is 3"