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I think it's TEA-TEB. Spark igniters are external
I think it's TEA-TEB. Spark igniters are external (?right?) and there's no evidence of one present. There's also no illumination that would suggest sparks prior to ignition. SpaceX is very comfortable with and experienced at using TEA-TEB. The idea they would use it for the prototype engine seems very unsurprising to me as a result.As far as the green flash in the middle of the previous fire, maybe some residue somehow? That seems unlikely. Or a leak. That would be troubling, not really for the engine itself, but more for the test program. So that's definitely a data point going the other direction.
Could the green be a very slight engine rich combustion?
Quote from: notsorandom on 11/15/2017 05:10 amCould the green be a very slight engine rich combustion?Most likely it's just a camera artifact from the sudden increase in brightness.
Quote from: jpo234 on 11/15/2017 08:45 amQuote from: notsorandom on 11/15/2017 05:10 amCould the green be a very slight engine rich combustion?Most likely it's just a camera artifact from the sudden increase in brightness.I think so too. I remember distinctly that Elon said in the 2016 presentation that Raptor has a little torch inside that is spark ignited which in turn ignites the main combustion cycle. Creating a prototype with hypergolics makes no sense since this is one of the core problems in engine development.
[...]The LOX and RP-1 tanks are pre pressurized with helium. High pressure helium spins up the turbo pump. LOX and RP-1 are ignited by TEA-TEB in the gas generator and takes over from the helium. The propellants meet in the combustion chamber and are also ignited by TEA-TEB.[...]
They said it uses autogenous pressurization, so use some of those gases.
Spinning the wheel a bit further (pun intended), how does the Raptor actually start? I mean, spark ignition or not, it needs to spin up its turbines. Following the ongoing discussion on the Merlin:Quote from: Jim on 11/23/2017 03:08 am[...]The LOX and RP-1 tanks are pre pressurized with helium. High pressure helium spins up the turbo pump. LOX and RP-1 are ignited by TEA-TEB in the gas generator and takes over from the helium. The propellants meet in the combustion chamber and are also ignited by TEA-TEB.[...]But the Raptor doesnt have high pressure helium available. Its tanks are autogenous pressurization. So how do the turbine wheels of Raptor start? I do have ideas how it could be done but I dont want to wildly speculate. Does anyone has info on that?
This ignition approach would make all Raptors restartable assuming their propellants had enough head pressure.Head pressure and an electrical power source is all that is required to start.The start sequence is something like the following:- crack valves and dribble in propellants to pre-chill the engine.- open valves and propellants flow into their respective pre-burners.- spark ignites stoichiometric mixture in torches.- torches ignite pre-burners- pre-burner exhaust spins turbines attached to propellant pumps. (one for methane, one for LOX)- main chamber torch ignites gaseous propellants entering chamber.- pumps start increasing pressure above head pressure and quickly climb to design pressure.This requires detailed understanding of the combustion processes and the dynamics of the pumps, turbines and valves. It is a tightly choreographed dance.John
Quote from: livingjw on 11/23/2017 01:06 pmThis ignition approach would make all Raptors restartable assuming their propellants had enough head pressure.Head pressure and an electrical power source is all that is required to start.The start sequence is something like the following:- crack valves and dribble in propellants to pre-chill the engine.- open valves and propellants flow into their respective pre-burners.- spark ignites stoichiometric mixture in torches.- torches ignite pre-burners- pre-burner exhaust spins turbines attached to propellant pumps. (one for methane, one for LOX)- main chamber torch ignites gaseous propellants entering chamber.- pumps start increasing pressure above head pressure and quickly climb to design pressure.This requires detailed understanding of the combustion processes and the dynamics of the pumps, turbines and valves. It is a tightly choreographed dance.JohnThats exactly what I am interested in. So initially, the propellant flows through the not-jet-rotating pumps until it reaches the preburner, is than ignited. It therefore puts pressure onto the turbine which starts to turn. But at the same time, the preburner also puts pressure back up the pumps and into the tanks. Because the pumps are not yet rotating. They are about to start rotating but they dont do it yet. It looks to me like a hen and a egg problem. How can you start the turbines/pumps under these conditions? Are there valves in front of the preburner that quickly close once some propellant is in the preburners and push it out the turbine only to open a fraction of a second later to allow new fuel to reach the preburner and further turn the turbine? And now my thought process looks like a moebius strip...
Everything downstream of the pumps has a lower total pressure. That's why the propellants flow. This is true as long as there is pressure in the tanks even if the pumps are not rotating. Preburner combustion greatly increased the volume of the propellants and hence their velocity (dynamic pressure) not their static pressure. Dynamic pressure spins the turbine.