Author Topic: Why did FH center core run out of TEA-TEB?  (Read 20116 times)

Offline hkultala

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Re: Why did FH center core run out of TEA-TEB?
« Reply #20 on: 02/19/2018 09:22 pm »

I am suspicious of the idea that backpressure in the engines causes relighting difficulty.  The stagnation pressure during the landing relight is 17 kPa (boosters) and 13 kPa (core), according to Zach's simulation here.  That's less than 0.2 bar, which is going to be less than the pressure drop across whatever nozzle injects the TEA-TEB for relight.  My guess is the TEA-TEB tank is probably at least ten times that pressure, and if the nozzle is choked flow, no minor pressure fluctuation downstream like that is going to affect the mass flow.

The entry burn relight is at even lower stagnation pressure: under 300 Pa, so that's not a problem either.

I think it's interesting that the peak stagnation pressure during the entry burn is so low, about 10 kPa. 
These values seemed way too low to me, based strictly on experience of sticking my hand out the window of a car and feeling the forces.

But rockets are light for their size.   If a landing rocket masses 30 tonnes, and is falling at terminal velocity, then the force must be 300 kN.

Terminal velocity changes with altitude. As the rocket starts with high velocity, and atmosphere is getting thicker and terminal velocity is getting lower, it's all the time coming at considerably higher velocity than the "theoretical" terminal velocity for that altitude, and is constantly decelerating.

This means that the aerodynamical forces affecting it are higher than 9.81 N/kg.

Offline IainMcClatchie

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Re: Why did FH center core run out of TEA-TEB?
« Reply #21 on: 02/19/2018 10:07 pm »
The aerodynamic forces on the rocket when it lights up for reentry are far less than 9.81 N/kg.  It's basically in freefall at that point, and dynamic pressure is miniscule.

For the booster landing, Zach has the booster relighting at 174 m/s at an altitude of 900 m.  Standard air density at that altitude is 1.13 kg/m^3.  Dynamic pressure is 17106 Pa.

Now what is interesting is that Zach has the deceleration, just before booster landing relight, at 1.194 G, and the mass at 24605 kg.  So that says there is 288 kN aerodynamic force on the booster.  If Cd = 1, area is 16.8 m^2.  The vehicle base is 10.7 m^2, and I think the rest is the body (which is still at a bit of an angle), the grid fins, and perhaps a Cd a little higher than 1.

I don't know that the booster's velocity profile has much to do with terminal velocity at any given point.  At the start of reentry, it is well below its terminal velocity and accelerating toward the ground.  After the reentry burn, it's still in freefall (well below terminal velocity). As it drops into the thicker air, the terminal velocity comes way down and the rocket experiences up to 7 G deceleration (so its way above terminal velocity).  During the landing burn, it briefly matches terminal velocity and then drops below, all the way to zero velocity at the ground obviously.

Offline Roy_H

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Re: Why did FH center core run out of TEA-TEB?
« Reply #22 on: 02/22/2018 03:13 am »
I wonder if, as part of the throttle-down of the center core immediately after launch, they actually did a shutdown of some of the engines,
no shutdowns
I know, Jim, that you are one of the best authorities on this web site, and if I have my info correct, you are a NASA employee, not a SpaceX employee. I certainly agree that the likelihood of FH center core engines being shut down during ascent is slim, but the argument makes sense. So how do you know for sure it didn't happen? If SpaceX did want to try this, would they be required to inform NASA even though it was not a NASA payload?
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Offline docmordrid

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Re: Why did FH center core run out of TEA-TEB?
« Reply #23 on: 02/22/2018 03:39 am »
I wonder if, as part of the throttle-down of the center core immediately after launch, they actually did a shutdown of some of the engines,
no shutdowns
>
So how do you know for sure it didn't happen?

Only 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.
« Last Edit: 02/22/2018 03:41 am by docmordrid »
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Offline speedevil

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Re: Why did FH center core run out of TEA-TEB?
« Reply #24 on: 02/22/2018 03:46 am »
Only 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.

Merlin 1D can probably throttle to 40% or so of maximum.
Shutting down 3/9 engines gets you to 26%.

So, for it to be worth it, it would have to be valulable to operate in that 14% range. This seems not hugely likely on the face of it.

Online smndk

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Re: Why did FH center core run out of TEA-TEB?
« Reply #25 on: 02/22/2018 08:32 am »
I wonder if, as part of the throttle-down of the center core immediately after launch, they actually did a shutdown of some of the engines,
no shutdowns
>
So how do you know for sure it didn't happen?

Only 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.

How are the engines that are not tied to the rockets internal TEA-TEB system initially startet?

/Svend

Offline docmordrid

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Re: Why did FH center core run out of TEA-TEB?
« Reply #26 on: 02/22/2018 08:42 am »
I wonder if, as part of the throttle-down of the center core immediately after launch, they actually did a shutdown of some of the engines,
no shutdowns
>
So how do you know for sure it didn't happen?

Only 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.

How are the engines that are not tied to the rockets internal TEA-TEB system initially startet?

/Svend

On the ground TEA-TEB is supplied externally.

https://forum.nasaspaceflight.com/index.php?topic=42705.msg1745087#msg1745087
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Offline Roy_H

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Re: Why did FH center core run out of TEA-TEB?
« Reply #27 on: 02/22/2018 06:44 pm »
Only 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.

Merlin 1D can probably throttle to 40% or so of maximum.
Shutting down 3/9 engines gets you to 26%.

So, for it to be worth it, it would have to be valulable to operate in that 14% range. This seems not hugely likely on the face of it.
You guys are missing the point here. Rocket engines are less efficient when throttled. Now I don't know how much but at 40% throttle maybe it uses 60% fuel rate. Anybody know actual figures?
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Offline hkultala

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Re: Why did FH center core run out of TEA-TEB?
« Reply #28 on: 02/23/2018 03:53 am »
Only 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.

Merlin 1D can probably throttle to 40% or so of maximum.
Shutting down 3/9 engines gets you to 26%.

So, for it to be worth it, it would have to be valulable to operate in that 14% range. This seems not hugely likely on the face of it.
You guys are missing the point here. Rocket engines are less efficient when throttled.

yes, but...

Quote
Now I don't know how much but at 40% throttle maybe it uses 60% fuel rate. Anybody know actual figures?

No, isp does not drop from 300 seconds to 200 seconds because of throttling to 40%.

it drops maybe something like from 300 seconds to 290-294 seconds, like 2-3% instead of 33%.
« Last Edit: 02/23/2018 04:01 am by hkultala »

Offline envy887

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Re: Why did FH center core run out of TEA-TEB?
« Reply #29 on: 02/23/2018 04:31 am »
Only 3 engines can restart because they're the only ones tied to the rockets internal TEA-TEB system. As the FH center core proved you don't want to exhaust the TEA-TEB. Better to throttle.

Merlin 1D can probably throttle to 40% or so of maximum.
Shutting down 3/9 engines gets you to 26%.

So, for it to be worth it, it would have to be valulable to operate in that 14% range. This seems not hugely likely on the face of it.
You guys are missing the point here. Rocket engines are less efficient when throttled.

yes, but...

Quote
Now I don't know how much but at 40% throttle maybe it uses 60% fuel rate. Anybody know actual figures?

No, isp does not drop from 300 seconds to 200 seconds because of throttling to 40%.

it drops maybe something like from 300 seconds to 290-294 seconds, like 2-3% instead of 33%.

Depends on altitude. I'll plot isp curves for Merlin as a function of throttle and altitude in RPA tomorrow. If I remember :D

OK, here's the plot. Some assumptions here but should be pretty close. At SL throttling to 40% costs about 15% in isp. But this very rapidly drops with altitude and at optimal expansion (only 2.4 km up!) it only costs about 1%.
« Last Edit: 02/23/2018 02:27 pm by envy887 »

Offline IainMcClatchie

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Re: Why did FH center core run out of TEA-TEB?
« Reply #30 on: 02/23/2018 05:02 am »
How are the engines that are not tied to the rockets internal TEA-TEB system initially startet?
On the ground TEA-TEB is supplied externally.

I am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.

Offline Lar

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Re: Why did FH center core run out of TEA-TEB?
« Reply #31 on: 02/23/2018 05:11 am »
How are the engines that are not tied to the rockets internal TEA-TEB system initially startet?
On the ground TEA-TEB is supplied externally.

I am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.
Why? A larger tank (or tanks) would have to hold 9 more starts than the tank (or tanks, because we don't know if there is a central tank pair or one pair per engine) that only  has to support restarts.
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Offline Nomadd

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Re: Why did FH center core run out of TEA-TEB?
« Reply #32 on: 02/23/2018 03:48 pm »
How are the engines that are not tied to the rockets internal TEA-TEB system initially startet?
On the ground TEA-TEB is supplied externally.

I am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.
Why? A larger tank (or tanks) would have to hold 9 more starts than the tank (or tanks, because we don't know if there is a central tank pair or one pair per engine) that only  has to support restarts.
Somebody must have mentioned it somewhere, but I can't find it.
 How much fluid are we talking about to start an engine? And, is the fluid pressure fed or use pumps?
« Last Edit: 02/23/2018 03:50 pm by Nomadd »
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Offline IainMcClatchie

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Re: Why did FH center core run out of TEA-TEB?
« Reply #33 on: 02/23/2018 10:09 pm »
I am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.
Why? A larger tank (or tanks) would have to hold 9 more starts than the tank (or tanks, because we don't know if there is a central tank pair or one pair per engine) that only  has to support restarts.

Because scaling up a tank is very lightweight.  For small quantity things like this, the fittings and plumbing can chew up most of the mass.  I wouldn't be surprised to hear that there is enough TEA-TEB in the plumbing for a full restart.

Also note that the ground support equipment, and the rocket plumbing must have sufficient pipe diameter to deliver the shot of TEA-TEB in much less than 1 second.  That plumbing goes from two sources (ground and flight tanks) to 9 destinations.

Offline Herb Schaltegger

Re: Why did FH center core run out of TEA-TEB?
« Reply #34 on: 02/24/2018 12:29 am »
I am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.
Why? A larger tank (or tanks) would have to hold 9 more starts than the tank (or tanks, because we don't know if there is a central tank pair or one pair per engine) that only  has to support restarts.

Because scaling up a tank is very lightweight.  For small quantity things like this, the fittings and plumbing can chew up most of the mass.  I wouldn't be surprised to hear that there is enough TEA-TEB in the plumbing for a full restart.

Also note that the ground support equipment, and the rocket plumbing must have sufficient pipe diameter to deliver the shot of TEA-TEB in much less than 1 second.  That plumbing goes from two sources (ground and flight tanks) to 9 destinations.


I really love it when people are smarter than professionals like Tom Mueller.
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Offline Roy_H

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Re: Why did FH center core run out of TEA-TEB?
« Reply #35 on: 02/24/2018 03:27 am »
Depends on altitude. I'll plot isp curves for Merlin as a function of throttle and altitude in RPA tomorrow. If I remember :D

OK, here's the plot. Some assumptions here but should be pretty close. At SL throttling to 40% costs about 15% in isp. But this very rapidly drops with altitude and at optimal expansion (only 2.4 km up!) it only costs about 1%.
Thank you for clearing this up. Obviously I was wrong as full thrust from all 3 cores is required at lift-off and throttling only occurs when g-forces exceed desired limit. I don't know where that is, but I would expect it to be about the 2.4km altitude so at only 1% loss it becomes a non-issue. I think there would still be some gain by shutting down center core engines to save fuel until after side boosters have done their job, but now I think all 9 engines would have to be shut down and re-started to make any significant difference.
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Offline groundbound

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Re: Why did FH center core run out of TEA-TEB?
« Reply #36 on: 02/24/2018 05:03 am »
Depends on altitude. I'll plot isp curves for Merlin as a function of throttle and altitude in RPA tomorrow. If I remember :D

OK, here's the plot. Some assumptions here but should be pretty close. At SL throttling to 40% costs about 15% in isp. But this very rapidly drops with altitude and at optimal expansion (only 2.4 km up!) it only costs about 1%.
Thank you for clearing this up. Obviously I was wrong as full thrust from all 3 cores is required at lift-off and throttling only occurs when g-forces exceed desired limit. I don't know where that is, but I would expect it to be about the 2.4km altitude so at only 1% loss it becomes a non-issue. I think there would still be some gain by shutting down center core engines to save fuel until after side boosters have done their job, but now I think all 9 engines would have to be shut down and re-started to make any significant difference.

I'l chime in by repeating something I was schooled on in another NSF thread a couple of years ago that impinges on this as well. Merlin is a GG engine, not SC. The GG losses are not necessarily a constant percentage as you go to higher chamber pressures. So if GG losses go up at higher throttle, that comes out of the higher ISP you thought you would get from higher chamber pressure.

Offline IainMcClatchie

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Re: Why did FH center core run out of TEA-TEB?
« Reply #37 on: 02/24/2018 11:35 pm »
I am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.

I really love it when people are smarter than professionals like Tom Mueller.

Let me say it differently then.

Can someone point out the non obvious thing that makes having the post-ignition quick disconnect preferrable to a bigger tank?

For instance, deleting the quick disconnect means that, should they start the engines and then shut down on the pad, they probably can't try an engine restart without reconnecting, which requires draining the fuel tanks and so on.  Maybe that's the reason.

Your statement contained no information about rockets.

Offline Herb Schaltegger

Re: Why did FH center core run out of TEA-TEB?
« Reply #38 on: 02/25/2018 12:28 am »
I am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.

I really love it when people are smarter than professionals like Tom Mueller.

Let me say it differently then.

Can someone point out the non obvious thing that makes having the post-ignition quick disconnect preferrable to a bigger tank?

For instance, deleting the quick disconnect means that, should they start the engines and then shut down on the pad, they probably can't try an engine restart without reconnecting, which requires draining the fuel tanks and so on.  Maybe that's the reason.

Your statement contained no information about rockets.

The tail service masts don't release until the hold-downs do. And fluid QD's are very light, very easy to service/replace and when they leak, it's very obvious, neither of which are true for internal fluid tanks and lines.

(DISCLAIMER: I have spec'd/designed fluid QD for crewed spaceflight applications in life-critical operations; I'm not talking out of my ass).
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Offline Perchlorate

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Re: Why did FH center core run out of TEA-TEB?
« Reply #39 on: 02/25/2018 12:33 am »
I am amazed that the post-ignition quick disconnect for a hypergolic fluid is lighter weight than just having a larger tank on board.

I really love it when people are smarter than professionals like Tom Mueller.

Let me say it differently then.

Can someone point out the non obvious thing that makes having the post-ignition quick disconnect preferrable to a bigger tank?

For instance, deleting the quick disconnect means that, should they start the engines and then shut down on the pad, they probably can't try an engine restart without reconnecting, which requires draining the fuel tanks and so on.  Maybe that's the reason.

Your statement contained no information about rockets.

The tail service masts don't release until the hold-downs do. And fluid QD's are very light, very easy to service/replace and when they leak, it's very obvious, neither of which are true for internal fluid tanks and lines.

(DISCLAIMER: I have spec'd/designed fluid QD for crewed spaceflight applications in life-critical operations; I'm not talking out of my ass).

Actually, Herb, I believe that qualifies more as a CLAIMER than a DISCLAIMER.   :)
Pete B, a Civil Engineer, in an age of incivility.

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