SpaceX Falcon 9 v1.1 - SES-8 - DISCUSSION THREAD

[Norm38]

Why do people suddenly imagine there is a soot problem with these engines? C'mon...

Because Elon tweeted:

Rocket engines are healthy, but cleaning turbopump gas generators will take another day. Aiming for Mon eve launch.

If they're not cleaning soot, what are they cleaning out?

[Lar]

If they're not cleaning soot, what are they cleaning out?

Unicorns. The pesky things climbed up from the flame trench and won't go home. Also Gremlins.

And incompletely combusted fuel, maybe. An engine can "not have a soot problem" when operating normally but still maybe have some things gunking it up if the start was cancelled early??

[John-H]

If the engine is shut down during the startup sequence, would that leave some extra contamination? If it wasn't a 'clean ' start... sorry

[Jason1701]

Fuel-rich RP engines collect soot, so if SpaceX had on-pad static fire and a post-ignition launch abort it might be time to take a brush to the turbine exhaust.

The engines also had a test fire at McGreggor. So the abort was the third ignition, correct?  How often do they have to be cleaned?  Two firings is okay but three is too much?  Were they cleaned after McGreggor testing?
And is startup inherently more sooty?  If the engines have to be cleaned after 2-3 three second firings, how dirty are they after a full three minute burn?  And does that have implications for restarting first stage engines for landing?  Seems the cleaning has to be more precautionary than strictly necessary.

The engines undergo an acceptance test and a multiple-firing stage acceptance test, so they have likely been fired 5-10 times each so far.

[king1999]

http://www.wacotrib.com/blogs/joe_science/the-tea-teb-glitch-can-t-light-a-falcon-without/article_1b7c4ae6-5a16-11e3-afbb-0019bb2963f4.html?mode=jqm

Good explanation about the ignition issue.

[hrissan]

If some expert stumbles upon this post...

1. Why mix of TEA-TEB in specific ratio? Lowest freezing point (like eutetic Na-K)?
2. Does ignition fluid goes into chamber through separate "hole", or is supplied into RP-1 line and then through main injector?
3. Is there a single TEA-TEB tank per first stage or per engine?
4. Is it a membrane tank with He pressurization?

[guckyfan]

Just got back in and catching up... “If” the TEA-TEB for the first and second stages is from the same source, how can they be sure that the second stage is not contaminated as well at this point?

Just an observation, if it was contamination was caused by air leaking in, the second stage is in vacuum when the TEA-TEB is introduced.

Do they do a nitrogen purge before filling tanks?

I just had another thought. Possibly it was not contaminated at the pad but a bad batch was delivered. In that case the TEA-TEB in the second stage would also be affected. But I am sure SpaceX has not missed that possibility and checked on it.

[mlindner]

Can we keep speculation to within information that was actually stated? This is getting a bit much.

If it was not in the second stage because its on a "separate circuit" then the TEA-TEB on the upper stage is filled through a normal tanking process, presumably using different lines than the lower stage and the contamination occurred either later, or in these separate lines.

No freezing of TEA-TEB, no contamination of second stage (already explicitly stated).

[AJA]

http://www.wacotrib.com/blogs/joe_science/the-tea-teb-glitch-can-t-light-a-falcon-without/article_1b7c4ae6-5a16-11e3-afbb-0019bb2963f4.html?mode=jqm

Good explanation about the ignition issue.

Thanks for that. Here's the key bit (as to why it aborts).

On a vehicle with multiple engines, if 8 out of 9 lit but one was just dumping un-lit propellant out the end, the fire from the others would ignite that propellant. The fire would then travel up into the engine where it would create a massive pressure spike, definitely destroying the engine and possibly destroying engines nearby. In propulsion testing the euphemism for this is a "hard start" leading to "rapid unplanned disassembly".

Hypothesis: If the flow to each engine was controlled separately, and coupled with sensor measurements for thrust ramp up in each engine, then the Fuel/Oxidiser flow rate would remain unchanged, unless ignition and intitial thrust build up was detected on said engine. In this case, it's immaterial if the ignition happens because of TEA-TEB, or due to the flame travelling upstream as Ben Brockert mentions.

There would be no pressure spike beyond what you'd normally expect at startup, because the mixture/amounts of fuel/oxidiser in the engine is the same as what you're injecting at ignition (minus the TEA-TEB).

Isn't this the case with the Falcon 9's first stage?

[MP99]

http://www.wacotrib.com/blogs/joe_science/the-tea-teb-glitch-can-t-light-a-falcon-without/article_1b7c4ae6-5a16-11e3-afbb-0019bb2963f4.html?mode=jqm

Good explanation about the ignition issue.

Great - many thanks.

cheers, Martin

[MP99]

4. Launch was aborted due to lower than expected thrust ramp. Is this the thrust ramp of the entire stage, or of each individual Merlin? If the former, it makes sense how one engine not having started at all would give a low ramp. If it's for each individual engine - then surely the thrust in the culprit wouldn't ramp up at all, since it wouldn't even ignite?! Why does Merlin need TEA-TEB once combustion has been initiated? How long is the transient, before this combustion is self-sustaining? Way shorter than 3 seconds?

Jim has said they monitor pressure in the individual combustion chambers, and derive thrust from that.

cheers, Martin

[sugmullun]

I found this to help me understand the possible issues, and hope it might help some others:
Modern Engineering for Design of Liquid-propellant Rocket Engines, Volume 147
 By Harry Arbit, page 121,122
in Google Books
http://goo.gl/dpVq7G
Based on the description of how the TEB-TEA work, I'd guess that the oxygen degraded the aluminum based part of the mix,since it's the more aggressive and actually used to decrease delay, greatly slowing the reaction time of the hypergolic to the lox.

[Nilof]

On a side note, does anyone here know why this mission has such a short launch window? Since it's a GEO launch, I don't see how the time of the launch would matter.

Does it have anything to do with keeping the satellite in sunlight at apropriate times?

[Jim]

On a side note, does anyone here know why this mission has such a short launch window? Since it's a GEO launch, I don't see how the time of the launch would matter.

Does it have anything to do with keeping the satellite in sunlight at apropriate times?

it is explained earlier in the thread

[Avron]

Jim..

How are turbo-pumps ignited? Trying to understand the rational for cleaning of the pumps.

[deltaV]

Jim..

How are turbo-pumps ignited? Trying to understand the rational for cleaning of the pumps.

Turbo-pumps are not ignited. Do you mean to ask how the gas generators that produces the hot gas that drives the turbo-pumps are ignited?

[Avron]

Jim..

How are turbo-pumps ignited? Trying to understand the rational for cleaning of the pumps.

Turbo-pumps are not ignited. Do you mean to ask how the gas generators that produces the hot gas that drives the turbo-pumps are ignited?

Yes, sir I do. Spark ignition ?

[sublimemarsupial]

Jim..

How are turbo-pumps ignited? Trying to understand the rational for cleaning of the pumps.

Turbo-pumps are not ignited. Do you mean to ask how the gas generators that produces the hot gas that drives the turbo-pumps are ignited?

Yes, sir I do. Spark ignition ?

Nope, they use TEA-TEB as well.

[smoliarm]

If some expert stumbles upon this post...

1. Why mix of TEA-TEB in specific ratio? Lowest freezing point (like eutetic Na-K)?
2. Does ignition fluid goes into chamber through separate "hole", or is supplied into RP-1 line and then through main injector?
3. Is there a single TEA-TEB tank per first stage or per engine?
4. Is it a membrane tank with He pressurization?

I can make a chemical guess on #1:
TEA - it reacts with O2 much faster (more hyperholic so to speak), but it has lower enthalpy of reaction.
TEB - it reacts slower, but it has higher enthalpy of reaction - it is the component which produces most of the heat necessary for ignition.
So, the ratio in TEA-TEB mix - I guess it's a mass-wise optimum (or volume-wise)

[Jim]

The TEA-TEB is squirted into it