...Continue Reading

3 months after landing it.

Landed Falcon 9 First Stage Test Firing (2 min 43 sec)

Now that's something which has never been done before. They didn't say how many engines were the ones which flew on the last mission, but there's always hope it is legit. Also intriguing is how they strap it down from the top, instead of holding it down from the bottom like liftoff.


We all know about the work hardening of metal as it's repeatedly contracted & expanded. It remanes to be seen if the 1st stage can survive all the stresses of a 2nd launch, the aerodynamic stresses of max Q, the vibration, the heating of reentry. Metal contracts & expands quite a bit as it goes from -340F to thousands of degrees.


The shuttle components were reused, but it took years. The mane engines had to be completely disassembled, boroscoped, checked for cracks, tested again. The boosters needed to be completely disassembled, packed with solid propellant & parachutes, transported across the country twice & stacked again. Auxillary power units, landing gear, & wiring usually was only good for 1 mission.


The only way they could launch every 3 months was by processing many components from past launches in parallel. The orbiter had to be rebuilt from scratch using components from many launches.

So the front differential was never used since it was converted to 2WD mode. It came out in a single piece suitable for directly inserting in the rear. The mane spur gear was on the right. This got it driving in the right direction again, with no grinding.

The wheel base of the Ruckus is about 1/2 of the Lunchbox, so the Lunchbox wouldn't be a good replacement. The Ruckus is just narrow enough to fit on curbs in the city. The day job probably won't last another 400 miles, so the Lunchbox may still be a suitable replacement wherever the next day job is.

For better stability, a stiffer suspension would be the next idea. Metal gears would solve the differential issues. It's going to need new tires by 800 miles. Helas, the plastic wheel bearings are completely worn out. It can't steer with the current wheel bearings, so like any servicing of an old car, what started as a grinding sound turned into a stream of endless repairs.

3 days of dissection between commutes revealed the differential to be stripped. 400 miles in 2WD mode was all it lasted. In 4WD mode, it might have lasted longer. After all the effort to extend the range by converting it to 2WD mode, it never went over 1/2 its range. At least this left a spare differential full of parts. The decision was made to get another 400 miles out of the spare differential parts. With the tires going bald, it'll then be time for another vehicle.

The thought had occurred of using a hoverboard or a boosted board as the next vehicle, but before hoverboards went out of style, they couldn't balance themselves. Boosted boards can't steer themselves. Any other vehicle would require a new motor. The answer most likely is another ECX Ruckus left in 4WD mode to protect the differentials.

So SpaceX has been varying the MECO for all of its LEO missions, based on payload. Each mission is customized to get the most reserve for landing, but they might leave margin for engine failures. CRS-9 had the lightest payload, which made a big difference in the burn marks. They could have burned the 2nd stage longer & MECOed sooner, but didn't.

Mane engine cutoff times for the LEO missions:

Orbcommmm-2:
2m25s 6012km/h 74.3km return to land
6m50s stage 2

CRS-8:
2m34s 6658km/h 67.5km return to ocean
7m7s stage 2

CRS-9:
2m22s 5688km/h 59.6km return to land
6m29s stage 2

The only trend is they got lower. Speed wasn't on a downward trend. Perhaps they experimentally found lower & lower altitudes where drag could be managed by the 2nd stage, so they could save fuel by accumulating velocity lower in the atmosphere.

Revisiting the anticogging table generated a few months ago revealed a bug in its calculation. Measured again the hall sensor readout for a wide range of rotation & found the cogging was fairly consistent around the motor's entire range & power levels. A little manual tweeking just might make a table which defeats the cogging.

Calculated a new table of phase offsets to correct the cogging. This looked a lot more ordered than the previous table. With the new table applied, the rotation was a lot smoother. It still wasn't perfect, but the months had proven any other method would be inferior.

Recursively creating a new anticogging table by testing itself didn't improve the results. A plot of a complete rotation using the anticogging table didn't show any areas where an equal offset could be applied to all parts of the rotation.

Finally, making an anticogging table for the motor's entire rotation rather than a single sine wave period showed some improvement but took too much memory. It means there's some variability in the reluctance or the stiction for different parts of the rotation.

Anticogging a brushless gimbal (0 min 28 sec)

It wasn't surprising that a piecewise linear scaling of the PWM could fix the current for all but its least negative point. That was only for a single winding with no rotor. Adding a rotor or changing the angle of the rotor changed the scale factors.

To simultaneously smooth out the 3 sine waves of 3 windings would be a bit harder. The parameters of 1 PWM's linear scaling depend on the linear scaling of the other 2 PWM's. The easiest way was to have the computer shift all 3 sine waves in time so they would play back the desired waveforms. The shift amounts could hopefully be translated into a piecewise linear scaling. With only 2 current sense resistors, there was no way to know the 3rd sine wave or what the positive half looked like. There was only the hope a sensible linear scaling would emerge that could be applied to all the sine waves.

That got 1 sine wave to play back as a sine wave, but not the other 2. The time shifting ended up a messy curve instead of a simple linear scaling.