We have not yet had opportunity to examine data, parts or footage.

A tentative speculation on root cause would be simple LOX overload. In other words a problem with measuring the correct amount of LOX in the tank, which we have encountered previously, and worked on solving via several methods. We will look into this.

Too much LOX will result in a too small gas pocket in the tank – which equals too little gas propellant energy and a premature loss of tank pressure in the LOX tank, again resulting in an increasing O/F-ratio mismatch. The engine subsequently extinguished.
The GNC didn’t detect acceleration below minimum, as this detection was removed from the algorithm prior to the mission, based on the fact that the acceleration, through the use of pressure blow-down at relatively low altitude, would be very low before the occurrence of MECO.
The GNC instead used the chamber pressure. This was low at an unusually early time. The GNC treated this as a sensor failure, and instead an estimated pressure, based on a table generated from earlier tests, was used.
Thus the GNC didn’t signal the deployment of the parachute, as it was under the (false) impression that the engine was still operating as expected.

Categories: Blog

Published by Rasmus Agdestein on

13 Comments

Bryan Kilgallin · 24th July 2016 at 4:34 pm

Thanks for the analysis. There seem to have been incorrect assumptions made about failure modes. What changes might be made?

Eric · 24th July 2016 at 5:47 pm

If you use a fill tube inside of the top of the LOX tank this will guarantee that you will always have at least a minimum amount of ullage space in the tank. Close the bottom of the fill tube and drill radial holes in the tube at the bottom so the pressurant gas doesn’t directly impinge on the liquid oxygen.

With the tube in place you fill your LOX tank until you have a steady stream of liquid oxygen blowing out of your fill vent.

Two helium regulators might be in order also to keep your tank pressures more constant as long as possible. If you’re using helium then you can probably can use an off the shelf regulator since your flow rates with helium are several times higher then with other gases.

Here is a photo of the helium regulation section on the rocket I flew back in 2006. The valve on the right is the LOX vent. The circle seal helium regulator on the left. A secondary utility pressure regulator is attached to run the pressure operated vent valves, etc. A 4500psi helium tank was used to provide the source pressure for the rocket. This was filled remotely using a 6000psi helium bottle.

https://s32.postimg.org/7co9lorid/He_Reg.jpg

https://s31.postimg.org/z48jqmniz/DSC_0051.jpg

Good luck with your future flights!

    Jesper · 24th July 2016 at 6:51 pm

    Sounds like DPR. that’s already planned for nexo II 🙂

Flo · 24th July 2016 at 8:02 pm

Regarding tank filling and determining the correct amount of liquid, have you ever thought of an optical system based on refraction index? Place an led and a photo sensitive element on top of the tank, attached to those two optical “conductors” e.g. pieces of an optical audio cable. The open ends of those cables should be suspended at the expected height of the liquid surface with a little air gap In between. You should then measure a specific brightness one the receiver element which will change when liquid fills the air gap between the open ends. Led and receiver at the top of the tank with the optical fiber due to the cold temperature of the Lox

All the best for the next trial, and don’t forget, it’s awesome times 1000 what you have achieved so far!

Andrew · 24th July 2016 at 8:25 pm

Also I would recommend a radio-actuated parachute release system as a fail-safe for events like these.

Christian · 24th July 2016 at 10:50 pm

I would like to congratulate the team to the successful launch. It appears you have created a very capable team/project setup.

In my impression all activities taking place on your launch vessel are occurring in a very difficult environment. It is not trivial to check things out there since the clock is ticking and it is an hazardous area because of the rocket and the seas.

In my opinion all processes affecting the rocket occurring in this difficult environment should be verified ideally by two additional independent measurements. 3 Measurements are ideal to make decisions. For example you could measure the mass flow (and thereby mass) of gas used to pressurize the Lox tank. Such things can be achieved by adding direct measurements by sensors (expensive) or performing indirect measurements (pressure change in inert gas tank). A third independent reference could be a time domain simulation of the process.

regards,
Christian

Sebastian · 25th July 2016 at 2:07 am

Congratulations on the launch! It is spectacular to see all these systems functioning together.

Some thoughts on sensing the LOX level: for laboratory cryostats with LHe, superconducting wires are commonly used and make for excellent continuous level sensors.

Obviously none of the metallic superconductors will work for LOX, but have you considered using a short segment of a ceramic high Tc superconductor as a discrete level indicator? For example, some Bi-2223 segments are available as demo kits at a modest price:
http://shop.can-superconductors.com/index.php?id_product=11&controller=product

Although, I have to admit I know nothing about the LOX compatibility of the various Tc > 90 K superconductors.

I have also seen LN2 level sensors consisting of a parallel plate capacitor. The relative permittivity of LN2 and LOX is roughly 1.5 compared to 1 for air/GOX/GN2, so the capacitance can be used to continuously infer the liquid level along the length of the plates.

Perhaps these options have been considered and disregarded, but I thought I’d mention them just in case. 🙂

All the best for the next flight!

Paw Vastrup-Suddergaard · 25th July 2016 at 7:42 am

Congratulations on a successful launch! This will be a great learning experience for the Nexø II. Already looking forward for the next launch!

Respect to the whole copsub crew!

Steve Ghioto · 25th July 2016 at 1:47 pm

Congrats on the mission, second Andrew’s comment on the radio actuated parachute.

Niels Foldager · 25th July 2016 at 8:43 pm

Thank you for your kind words and support.

We will consider all suggestions mentioned above.

@ Andrew:
Actually we had radio activated, manually commanded parachute release, and this worked for the parachute itself. However, the signal to nose cone separation did not lead to separation for a reason not yet understood.

Dan · 26th July 2016 at 2:01 am

Use two pressure sensors for the chamber. If they both have near the same reading, believe them. If one is close to what you expected and the other is not, believe the first one.

Recommend you reconsider the distance between the rocket and the ships and plane. When the GN&C is working well and a failure occurs, as we just saw, it comes almost straight down. Drones would be a safer option.

I’d also hang the rocket from the top connected to a load cell to get mass readings. Remove before flight…

Great flight!

Anima Mundi · 26th July 2016 at 1:33 pm

With turbopumps this could not had happend. Will Nexo II have one and can you please make a report on how the development is going and how it works.

Parachute should be able to deploy remotely.
Actually all the systems that can be controlled on the Rocket.
Use and implement good crypto.

Why do you not trust the sensor? Would it make sense to make it parallel redundant 3 times and take a 2/3 positive?
I think its a standard in aerospace?

Congratulation for this achievement! You really inspired a lot of people by making them believe in that something like open source human spaceflight is possible!

Niels Foldager · 26th July 2016 at 3:42 pm

@Anima Mundi:

Thank you.

> Parachute should be able to deploy remotely.

As I wrote above, we had radio activated, manually commanded parachute release, and this worked for the parachute itself. However, the signal to nose cone separation did not lead to separation for a reason not yet understood.

> Why do you not trust the sensor?

Whenever you have a discrepancy between reported and expected measurement, you have to decide which to trust. In this case the a priori probability for such a low chamber pressure that early in the flight was low.

> Would it make sense to make it parallel redundant 3 times and
> take a 2/3 positive?

Yes. If we had a lot of money.

As mentioned in the blog it was special for this pressure blow-down driven, low-altitude mission that we removed acceleration from the algorithm for parachute deployment. Later missions will not have that problem.

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