Advanced Question Thrust Fluctuation

[fred18]

Hi all,

I was thinking of adding (as an option) a bit of bigger realism to ms2015 and I think (but I am not sure) that it would be realistic to have a bit of fluctuation in main thruster power.

Would it be realistic? If so what could be a good model for it and how large should be the fluctuation?

Thanks in advance to all the rockets experts that will help me

Fred

 

[Urwumpe]

Depends on the kind of rocket motor. solid rocket motors fluctuate for example much more and stronger than bipropellant engines.

Generally speaking, everything from tank to nozzle has effects on the frequency and amplitude of the fluctuations, more fluctuations than usual are a sign of a developing engine failure. There are various behaviours called buzzing or screaching for example, that describe abnormal engine thrust.

If I remember correctly, the thrust fluctuations of the SRBs of the Space Shuttle had been as high as 2% at times.

 

[fred18]

Thank you, I was counting on a reply from you

I can make the thrust varying on a sinusoid with maximum 2% amplitude, i can even make the fluctuations frequency much more intense getting closer to the failure instant, but I was thinking that maybe it's not worth it to pay computational capacity for this small feature (that would be optional anyway of course).

What do you think?

 

[Urwumpe]

Thank you, I was counting on a reply from you

I can make the thrust varying on a sinusoid with maximum 2% amplitude, i can even make the fluctuations frequency much more intense getting closer to the failure instant, but I was thinking that maybe it's not worth it to pay computational capacity for this small feature (that would be optional anyway of course).

What do you think?

Well, it can disturb your PEG autopilot as well (acceleration estimate), so it is really just a "gimmick" feature. For the Space Shuttle SSMEs, the oscillations by the ATVC steering loop is causing much stronger acceleration changes than the SSME oscillations.

You don't need much computational power there, two sine functions for representing the two dominating oscillation modes, more for more modes. But I hardly know any spectrum of a real engine that is significant more than two major modes and some noise. The standard FPU sine could actually be faster than a look-up table.

 

[fred18]

Well Peg gets acceleration in realtime and on staging the estimation will be of a very low order, less than plus or minus 1%. There is more approximation not accounting burn delay of next stage actually.

You're right that it's not complicated at all. Maybe I'll give it a try, if results are not satisfactory I'll leave it for future release!

Apart from general failures and thrust fluctuation I don't see any other factor to be simulated that could randomize the flight.

 

[Urwumpe]

Apart from general failures and thrust fluctuation I don't see any other factor to be simulated that could randomize the flight.

You could simulate hot and cold SRMs maybe, their performance is strongly influenced by propellant temperature.

Another option would be including an optional battery life for upper stages. Many here would coast with their upper stage to Mars, while in reality, the batteries would likely be dead after 12 hours of flight. Not random, but has a good impact on flight planning.

Or you allow flawed navigation sensors, though this could become terrible.

 

[Notebook]

GO for it, the more engine paramaters the better...

Mind you never know who's listening:
https://www.cia.gov/library/center-for-the-study-of-intelligence/kent-csi/vol8no4/pdf/v08i4a03p.pdf

N.

 

[White Owl]

Another option would be including an optional battery life for upper stages. Many here would coast with their upper stage to Mars, while in reality, the batteries would likely be dead after 12 hours of flight. Not random, but has a good impact on flight planning.

I really like this idea. Random failures in simulations tend not to be too much fun. Realistic limitations prompting careful planning, however... that's good stuff.

 

[fred18]

this is very easy to implement, I will simply put a new item in the ini file called Battery_Life=XXX, that will be expressed in hours. when the battery will be finished all the thrusters will be erased, so the vehicle will be dead. In case the users will not define this param the battery life will be infinite.

 

[Donamy]

Will solar panels recharge them ?

 

[Urwumpe]

Will solar panels recharge them ?

Well, Solar arrays are a bit rare on upper stages. Such a configuration would rather be called space tug and could better be done as genericvessel vehicle...