Author Topic: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion  (Read 414089 times)

Offline sevenperforce

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #760 on: 01/02/2018 02:06 PM »
First time poster.

Regarding the 92% thrust.

Is this not simply because it's an LV capable of lifting 16000kg to MTO that is in fact only lifting off with a 1500kg payload. An LV that can hoist 60000kg off the pad only lifting 1500kg (or whatever the weight of the tesla + mounting is) would go into a massive over g situation within seconds lifting that light a load. As it is I would imagine the 92% will only be until it clears the pad, then a massive throttle down until well past max q and throttling down all the way up. A lot of rockets auto throttle to constant 5g acceleration for structural reasons IIRC. This baby in its maiden config will have plenty of horses to spare.

Or am I wrong on that?

Edit: Does anyone know if there is extra ballast in the payload to counteract this? Or what the actual final payload weight actually will be?

You need to consider payload as a portion of gross lift of mass. Expecting the early stages of flight to be noticeably different due to the small payload is akin to being able to spot a HGV accelerating more quickly just because the driver lost some weight.
Putting this to rest...

A Falcon 9 first stage masses roughly 22.2 tonnes dry plus 411 tonnes of propellant. A Falcon 9 upper stage masses roughly 4 tonnes with 107.5 tonnes of propellant.

FH's theoretical maximum payload to LEO is 63.8 tonnes.

Thus, a Falcon Heavy with the maximum payload would come in at 1,475 tonnes. A Falcon Heavy with no payload (just a bare upper stage) would come in at 1411 tonnes. Thus, the TWR difference between launching with the maximum expendable LEO payload and no payload at all is just 4.3%. Nothing you'd ever notice.

Offline rsdavis9

First time poster.

Regarding the 92% thrust.

Is this not simply because it's an LV capable of lifting 16000kg to MTO that is in fact only lifting off with a 1500kg payload. An LV that can hoist 60000kg off the pad only lifting 1500kg (or whatever the weight of the tesla + mounting is) would go into a massive over g situation within seconds lifting that light a load. As it is I would imagine the 92% will only be until it clears the pad, then a massive throttle down until well past max q and throttling down all the way up. A lot of rockets auto throttle to constant 5g acceleration for structural reasons IIRC. This baby in its maiden config will have plenty of horses to spare.

Or am I wrong on that?

Edit: Does anyone know if there is extra ballast in the payload to counteract this? Or what the actual final payload weight actually will be?

You need to consider payload as a portion of gross lift of mass. Expecting the early stages of flight to be noticeably different due to the small payload is akin to being able to spot a HGV accelerating more quickly just because the driver lost some weight.
Putting this to rest...

A Falcon 9 first stage masses roughly 22.2 tonnes dry plus 411 tonnes of propellant. A Falcon 9 upper stage masses roughly 4 tonnes with 107.5 tonnes of propellant.

FH's theoretical maximum payload to LEO is 63.8 tonnes.

Thus, a Falcon Heavy with the maximum payload would come in at 1,475 tonnes. A Falcon Heavy with no payload (just a bare upper stage) would come in at 1411 tonnes. Thus, the TWR difference between launching with the maximum expendable LEO payload and no payload at all is just 4.3%. Nothing you'd ever notice.

and for the second stage the payload fraction at stage 2 ignition is 63.8/(107.4+4)=.57 so much more significant here.
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Offline CyndyC

cyndy- That tweet references how the rocket thrust profile will work in general, not how the individual mission thrust profile will be deployed.

The thrust profile tweet was part of the thread announcing the maiden launch when it was planned for November, easily seen in its entirety by clicking on the link I included.

That profile obviously adds up to less than 100%, just as the maiden launch has been confirmed will. Although there is one customer payload currently on the manifest and I don't know what the thrust requirements will be for that one, I doubt they can plan on less than 100% for all time without knowing what their other future payloads will be.

There are at least 2 of us here who think the most logical reason for the lower center thrust is to match or coordinate with a lower maximum thrust on the much older side cores, and it appears the idea of a need to accommodate an underweight payload has been put to rest.

However, an interesting side note is that the Shuttle boosters were routinely responsible for 83% of the full assembly's thrust, if WikiP is to be believed. Whether or not the FH attachment points could or will withstand more discrepancy after the maiden launch has yet to be known.

https://twitter.com/elonmusk/status/890810308326940672
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Online gongora

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #763 on: 01/02/2018 04:19 PM »
That profile obviously adds up to less than 100%, just as the maiden launch has been confirmed will.

Throttling down the center core early in flight will happen on every FH launch, that is what the vehicle is designed to do.  That part has nothing to do with the max thrust of the vehicle on any particular flight.

Offline CyndyC

That profile obviously adds up to less than 100%, just as the maiden launch has been confirmed will.

Throttling down the center core early in flight will happen on every FH launch, that is what the vehicle is designed to do.  That part has nothing to do with the max thrust of the vehicle on any particular flight.

ok
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Offline UKobserver

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #765 on: 01/02/2018 04:47 PM »
The 92% thrust level might only be for this mission.

I've wondered if the thrust "reduction" on this mission is just because he's giving thrust numbers for FH Block 5 and this vehicle is running at Block 3 levels.

There was some discussion about this in the Merlin 1D update thread a few days ago;

https://forum.nasaspaceflight.com/index.php?topic=41014.200

It was prompted by the following Instagram post from a SpaceX employee, showing off a new Block 5 booster engine, which he claimed was rated at 205,000lbf.

https://www.instagram.com/p/Bc8pqAng9rH/

While there have been suggestions that these new engines may have actually been tested on the stand at thrust levels as high as 245,000lbf, presumably to establish safety margins, it looks like they will be introducing a new flight rating of 205,000lbf when these engines debut on the new block 5 booster.

This first FH is a composite of B3 side boosters and a B4 core, whose Merlin 1Ds I believe run at 190,000lbf thrust. AncientU pointed out that 27 x 190,000lbf is 5.1Mlbf, which matches the thrust that Elon said FH would have at liftfoff.

190,000 is 92% of 205,000, so those facts combined would suggest that this first test will run at what are normal thrust levels for each of the respective cores (albeit that the centre core will throttle down after lift-off), but that the total thrust available on this flight is only 92% of what a full-up Block 5 Falcon Heavy will be capable of.
« Last Edit: 01/02/2018 04:49 PM by UKobserver »

Online nacnud

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #766 on: 01/02/2018 04:58 PM »
Good sleuthing! 

Online gongora

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #767 on: 01/02/2018 05:05 PM »
There was some discussion about this in the Merlin 1D update thread a few days ago;

https://forum.nasaspaceflight.com/index.php?topic=41014.200

It was prompted by the following Instagram post from a SpaceX employee, showing off a new Block 5 booster engine, which he claimed was rated at 205,000lbf.

https://www.instagram.com/p/Bc8pqAng9rH/

While there have been suggestions that these new engines may have actually been tested on the stand at thrust levels as high as 245,000lbf, presumably to establish safety margins, it looks like they will be introducing a new flight rating of 205,000lbf when these engines debut on the new block 5 booster.

On the SpaceX web site it shows thrust at sea level of 190k lbf and thrust in vacuum of 205k lbf.
« Last Edit: 01/02/2018 05:06 PM by gongora »

Offline CyndyC

Good sleuthing!

No kidding! That certainly explains everything! Good to know & understand!
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Online Formica

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #769 on: 01/02/2018 05:17 PM »

This first FH is a composite of B3 side boosters and a B4 core, whose Merlin 1Ds I believe run at 190,000lbf thrust. AncientU pointed out that 27 x 190,000lbf is 5.1Mlbf, which matches the thrust that Elon said FH would have at liftfoff.


1033.1, the center core, is definitely based on Block 3 tech. 1023.2 and 1025.2 started life as Block 2, and may have been upgraded to Block 3 tech in the same way that 1021.2 was for its reflight. There was an intern from KSC who posted on Reddit about upgrading 1021.2 during its refurbishment, describing the process of replacing Block 1 parts with Block 3 parts. I'm still looking for a link to that post.

Here's a community based spreadsheet, with references, that keeps track of the blocks. It is as accurate as it can be without input from SpaceX, as far as I can tell.

I'm just a space fan, please correct me if I'm wrong!

Online gongora

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #770 on: 01/02/2018 05:31 PM »
Good sleuthing!

It's really not.  The 190k lbf is the sea level thrust of Block 5, not Block 3.

Online nacnud

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #771 on: 01/02/2018 05:51 PM »
Well the current booster engine has 190k lbf, the instagrammed booster engine has 205k lbf. One is 92% of the other.

I don't think we ever got to the bottom of the instagrammed engine, but Elons 92% tweet makes me think that it really had been uprated to 205k. Which I was doubting as there could have been some confusion between booster and vacuum engines.

The only info we have is from keeping a close watch on the public internet and waiting for more into from Spacex/Elon. But in reference to FH it's reasonable to think that this launch will have 190k lbf Merlin D engines while then next block 5 FH will have 205k lbf Merlin D engines.

We'll have to keep trawling social media for clues! :)
« Last Edit: 01/02/2018 05:52 PM by nacnud »

Online gongora

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #772 on: 01/02/2018 05:54 PM »
Well the current booster engine has 190k lbf, the instagrammed booster engine has 205k lbf. One is 92% of the other.

I don't think we ever got to the bottom of the instagrammed engine, but Elons 92% tweet makes me think that it really had been uprated to 205k. Which I was doubting as there could have been some confusion between booster and vacuum engines.

The only info we have is from keeping a close watch on the public internet and waiting for more into from Spacex/Elon. But in reference to FH it's reasonable to think that this launch will have 190k lbf Merlin D engines while then next block 5 FH will have 205k lbf Merlin D engines.

We'll have to keep trawling social media for clues! :)

The Block 5 values as shown on the SpaceX web site are 190k/sea level, 205k/vacuum.  For the same first stage engines.
« Last Edit: 01/02/2018 05:54 PM by gongora »

Online nacnud

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #773 on: 01/02/2018 06:12 PM »
I took a look at the falcon 9 page but couldn't determine the model of F9 it was referring to, so I assumed it was the current one, not B5.

I could be wrong though.

I also noted that Vac M1D has a thrust of 210k lbs, when I thought it had 205k, so yeah I could be getting rather confused.
« Last Edit: 01/02/2018 06:25 PM by nacnud »

Offline RocketLover0119

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #774 on: 01/02/2018 06:20 PM »
Chris B on the ZUMA update thread says there is a potential 2nd SF coming up, if so, that would definitely push back the FH SF this week.....
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Offline UKobserver

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #775 on: 01/02/2018 07:04 PM »
Well the current booster engine has 190k lbf, the instagrammed booster engine has 205k lbf. One is 92% of the other.

I don't think we ever got to the bottom of the instagrammed engine, but Elons 92% tweet makes me think that it really had been uprated to 205k. Which I was doubting as there could have been some confusion between booster and vacuum engines.

The only info we have is from keeping a close watch on the public internet and waiting for more into from Spacex/Elon. But in reference to FH it's reasonable to think that this launch will have 190k lbf Merlin D engines while then next block 5 FH will have 205k lbf Merlin D engines.

We'll have to keep trawling social media for clues! :)

The Block 5 values as shown on the SpaceX web site are 190k/sea level, 205k/vacuum.  For the same first stage engines.

I'm not disagreeing, as it's completely plausible that the SpaceX website is showing B5 stats and that the employee was referring to thrust level in a vacuum, but are you suggesting that it's just a massive co-incidence that they are running at 92% this time? Why 92%? And 92% of what?

Is S/L thrust also 92% of vacuum thrust for previous versions of the M1D, such as on the B2 and B3 boosters? Is that due to the expansion ratio of the nozzle? If so, does Elon mean that they are going to gradually throttle down each engine as this FH climbs so as to keep the maximum thrust from each engine at S/L thrust levels all the way through the flight, perhaps for structural reasons?
« Last Edit: 01/02/2018 07:07 PM by UKobserver »

Online gongora

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #776 on: 01/02/2018 07:11 PM »
I'm not disagreeing, as it's completely plausible that the SpaceX website is showing B5 stats and that the employee was referring to thrust level in a vacuum, but are you suggesting that it's just a massive co-incidence that they are running at 92% this time?
Yes

Quote
Why 92%?
I don't know

Quote
And 92% of what?
5.1M lbf

Quote
Is S/L thrust also 92% of vacuum thrust for previous versions of the M1D, such as on the B2 and B3 boosters?
The previous value I see from their site is 170k lbf @ SL/185k lbf @ vacuum, so yes.

Quote
Is that due to the expansion ratio of the nozzle? If so, does Elon mean that they are going to gradually throttle down each engine as this FH climbs so as to keep the maximum thrust from each engine at S/L thrust levels all the way through the flight, perhaps for structural reasons?
I'm assuming Elon was talking about thrust at lift-off, period.  Engines throttle to various settings during flight as needed (around Max-Q, limiting acceleration as the vehicle gets lighter, etc.)

Offline edkyle99

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #777 on: 01/02/2018 07:21 PM »
Elon's tweet specified sea-level thrust, as in "max thrust at liftoff is 5.1 million pounds", which is 188,889 lbs force per engine.  He then said that this launch will run at 92%, which would be 173,778 lbs (4,692,000 lbs total).  That's how I read things any way.

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« Last Edit: 01/02/2018 07:21 PM by edkyle99 »

Offline envy887

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #778 on: 01/02/2018 07:24 PM »
Quote
Why 92%?
I don't know

92% of what the website lists is about what all the 2017 flights have been running at - see the sims thread. I'm guessing it's what Block 3 and 4 cores are rated for, since SpaceX used to list Merlin at 170 klbf at liftoff (89.5% of 190 klbf). This would seem to apply to all early blocks of FT/v1.2, with only Block 5 getting a thrust upgrade.

https://web.archive.org/web/20160101081059/http://www.spacex.com/falcon9
« Last Edit: 01/02/2018 07:25 PM by envy887 »

Offline UKobserver

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #779 on: 01/02/2018 07:29 PM »
I'm not disagreeing, as it's completely plausible that the SpaceX website is showing B5 stats and that the employee was referring to thrust level in a vacuum, but are you suggesting that it's just a massive co-incidence that they are running at 92% this time?
Yes

Quote
Why 92%?
I don't know

Quote
And 92% of what?
5.1M lbf

Quote
Is S/L thrust also 92% of vacuum thrust for previous versions of the M1D, such as on the B2 and B3 boosters?
The previous value I see from their site is 170k lbf @ SL/185k lbf @ vacuum, so yes.

Quote
Is that due to the expansion ratio of the nozzle? If so, does Elon mean that they are going to gradually throttle down each engine as this FH climbs so as to keep the maximum thrust from each engine at S/L thrust levels all the way through the flight, perhaps for structural reasons?
I'm assuming Elon was talking about thrust at lift-off, period.  Engines throttle to various settings during flight as needed (around Max-Q, limiting acceleration as the vehicle gets lighter, etc.)

Thanks, that's useful to know re earlier models/expansion ratio. I really doubt that 92% is a co-incidence though; I can't imagine anyone picking that exact number unless it matched up with something, like the difference between maximum thrust levels of the side and core boosters, or structural limits, or something else. I also think Elon would have explicitly stated that S/L thrust was 92% of that produced in a vacuum if he meant it that way. I could be completely wrong but I interpreted his statement as meaning that at some point in flight the thrust levels will be only 92% of either, a) what these engines are actually capable of producing, or b) what the B5 engines will be capable of producing, at that same point in flight. But who knows.
« Last Edit: 01/02/2018 07:47 PM by UKobserver »

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