SpaceX Falcon Heavy Discussion (Thread 6)

[LouScheffer]

LouScheffer: can you explain something above? (I won't quote in interests of brevity)

How do you come up with 185 km = 100 miles? Is there something I am missing in the unit conversion? I get 115 miles. Is using nautical miles standard for orbit calculations?

Yes, the Falcon 9 user's guide states:

A perigee altitude of 185 km (100 nmi) is baselined for GTO

Likewise, the Atlas 5 manual says:

A park orbit perigee altitude of 167 km (90 nmi) is assumed for the reference cases.

I'd guess this is because aircraft use nautical miles, presumably because ships use nautical miles.  It would be helpful if everyone would switch to metric....

[docmordrid]

Last night Musk tweeted Falcon Heavy  Block 5 thrust numbers; about 2550t,  25MN, or 5,621,788 lbf.

Elon Musk ✔ @elonmusk
Max thrust of 2550 tons will be almost 10% higher than Falcon Heavy demo mission last year
1:16 PM - Apr 5, 2019

https://twitter.com/elonmusk/status/1114215249517981702

[Slarty1080]

Last night Musk tweeted Falcon Heavy  Block 5 thrust numbers; about 2550t,  25MN, or 5,621,788 lbf.

Elon Musk ✔ @elonmusk
Max thrust of 2550 tons will be almost 10% higher than Falcon Heavy demo mission last year
1:16 PM - Apr 5, 2019

https://twitter.com/elonmusk/status/1114215249517981702

Key question then what does this translate to in terms of payload? Is there a linear relationship between payload and thrust or more complex than that (suspect the later)?

[speedevil]

Last night Musk tweeted Falcon Heavy  Block 5 thrust numbers; about 2550t,  25MN, or 5,621,788 lbf.

Elon Musk ✔ @elonmusk
Max thrust of 2550 tons will be almost 10% higher than Falcon Heavy demo mission last year
1:16 PM - Apr 5, 2019

https://twitter.com/elonmusk/status/1114215249517981702

Key question then what does this translate to in terms of payload? Is there a linear relationship between payload and thrust or more complex than that (suspect the later)?

As you suspect, the latter.
Naively, the FH stack will weigh 1450 tons or so.
If it got off the pad at 2550 tons, it would be doing 17.5m/s upwards of which 10 is gravity losses, leaving 7.5m/s of real upwards acceleration.
At 2300 tons, this reduces to only 16m/s, or 6ms upwards.

So, 10% less thrust means 6, not 7.5m/s, or 20% less upwards acceleration.

The next gross approximation you could make is that a F9 first stage burns for 180 seconds or so, and during that time it's fighting gravity.

If you burn the fuel in 160, not 180 seconds, the stage is exposed to gravity for 20 seconds less, or 200m/s extra delta-v.

In reality, computing the exact gain is nasty.
It will probably not come off the pad at maximum throttle, and will only throttle up once it's cleared the tower, and then will throttle back down at some point near mach 1 (30 seconds into flight) to keep aerodynamic loads down.
Then throttle back up.
The exact profile of the throttle settings as well as trajectory matters. In reality, if you're thrusting partially sideways, as the first stage is most of its flight, not all of the 10m/s gravitational acceleration is a loss.

And, of course, it's a FH, not a F9, which adds additional complexities.

But, 'considerably less than 200m/s' is a safe bet to what it adds..

[Slarty1080]

Last night Musk tweeted Falcon Heavy  Block 5 thrust numbers; about 2550t,  25MN, or 5,621,788 lbf.

Elon Musk ✔ @elonmusk
Max thrust of 2550 tons will be almost 10% higher than Falcon Heavy demo mission last year
1:16 PM - Apr 5, 2019

https://twitter.com/elonmusk/status/1114215249517981702

Key question then what does this translate to in terms of payload? Is there a linear relationship between payload and thrust or more complex than that (suspect the later)?

As you suspect, the latter.
Naively, the FH stack will weigh 1450 tons or so.
If it got off the pad at 2550 tons, it would be doing 17.5m/s upwards of which 10 is gravity losses, leaving 7.5m/s of real upwards acceleration.
At 2300 tons, this reduces to only 16m/s, or 6ms upwards.

So, 10% less thrust means 6, not 7.5m/s, or 20% less upwards acceleration.

The next gross approximation you could make is that a F9 first stage burns for 180 seconds or so, and during that time it's fighting gravity.

If you burn the fuel in 160, not 180 seconds, the stage is exposed to gravity for 20 seconds less, or 200m/s extra delta-v.

In reality, computing the exact gain is nasty.
It will probably not come off the pad at maximum throttle, and will only throttle up once it's cleared the tower, and then will throttle back down at some point near mach 1 (30 seconds into flight) to keep aerodynamic loads down.
Then throttle back up.
The exact profile of the throttle settings as well as trajectory matters. In reality, if you're thrusting partially sideways, as the first stage is most of its flight, not all of the 10m/s gravitational acceleration is a loss.

And, of course, it's a FH, not a F9, which adds additional complexities.

But, 'considerably less than 200m/s' is a safe bet to what it adds..

Slightly confused that you said 10% less thrust in your workings?

[speedevil]

But, 'considerably less than 200m/s' is a safe bet to what it adds..

Slightly confused that you said 10% less thrust in your workings?

The 2550 ton figure is '10% more'. Current is '10% less'.

[Alexphysics]

Those are US tons, not metric tons. Converted to MN it gives a total of 22.686MN (~5.1m lbf) which is similar to the numbers on the website. If you go by it as being metric tons, then it would have a total thrust of 25.056MN (~5.633m lbf) and that would mean each M1D+ engine produces about 928kN (~208.6k lbf) of thrust each one of them. If you compare numbers it is clear that he was refering to US tons and not metric tons.

[envy887]

Those are US tons, not metric tons. Converted to MN it gives a total of 22.686MN (~5.1m lbf) which is similar to the numbers on the website. If you go by it as being metric tons, then it would have a total thrust of 25.056MN (~5.633m lbf) and that would mean each M1D+ engine produces about 928kN (~208.6k lbf) of thrust each one of them. If you compare numbers it is clear that he was refering to US tons and not metric tons.

Each Merlin does produce 914 kN in vacuum, according to the SpaceX website. So it's not at all clear that he was using short tons. It makes just as much sense to be vacuum metric tons.

[Alexphysics]

Those are US tons, not metric tons. Converted to MN it gives a total of 22.686MN (~5.1m lbf) which is similar to the numbers on the website. If you go by it as being metric tons, then it would have a total thrust of 25.056MN (~5.633m lbf) and that would mean each M1D+ engine produces about 928kN (~208.6k lbf) of thrust each one of them. If you compare numbers it is clear that he was refering to US tons and not metric tons.

Each Merlin does produce 914 kN in vacuum, according to the SpaceX website. So it's not at all clear that he was using short tons. It makes just as much sense to be vacuum metric tons.

So how did the Merlin get the extra 14kN that there are until 928kN which would be what Elon said?

[Slarty1080]

Last night Musk tweeted Falcon Heavy  Block 5 thrust numbers; about 2550t,  25MN, or 5,621,788 lbf.

Elon Musk ✔ @elonmusk
Max thrust of 2550 tons will be almost 10% higher than Falcon Heavy demo mission last year
1:16 PM - Apr 5, 2019

https://twitter.com/elonmusk/status/1114215249517981702

Key question then what does this translate to in terms of payload? Is there a linear relationship between payload and thrust or more complex than that (suspect the later)?

As you suspect, the latter.
Naively, the FH stack will weigh 1450 tons or so.
If it got off the pad at 2550 tons, it would be doing 17.5m/s upwards of which 10 is gravity losses, leaving 7.5m/s of real upwards acceleration.
At 2300 tons, this reduces to only 16m/s, or 6ms upwards.

So, 10% less thrust means 6, not 7.5m/s, or 20% less upwards acceleration.

The next gross approximation you could make is that a F9 first stage burns for 180 seconds or so, and during that time it's fighting gravity.

If you burn the fuel in 160, not 180 seconds, the stage is exposed to gravity for 20 seconds less, or 200m/s extra delta-v.

In reality, computing the exact gain is nasty.
It will probably not come off the pad at maximum throttle, and will only throttle up once it's cleared the tower, and then will throttle back down at some point near mach 1 (30 seconds into flight) to keep aerodynamic loads down.
Then throttle back up.
The exact profile of the throttle settings as well as trajectory matters. In reality, if you're thrusting partially sideways, as the first stage is most of its flight, not all of the 10m/s gravitational acceleration is a loss.

And, of course, it's a FH, not a F9, which adds additional complexities.

But, 'considerably less than 200m/s' is a safe bet to what it adds..

OK so the 10% thrust gain does not provide much more orbital capability due to the limitations of the engine configuration and thrust profile. So what good is it other than bragging rights for Musk? Entering amateur Kerbal calculation mode, (interested in feedback on my musings as a learning opportunity not as a practical proposition!):

10% more thrust could lift 10% more mass (all other things being equal, which they are not but bear with me) but it would not because:

1) It would burn through the fuel quicker and run out earlier and there is no room for extra fuel
2) Stretching the upper stage (to take the full mass difference) would not work because
a.   The core stage would not be able to take the extra mass of the upper stage
b.   The rocket would become too long and would be unstable to lateral forces etc
c.   It would make the second stage too heavy for the single merlin vac engine and there is only room for one merlin vac engine.

Assuming all that could be sorted out and multiple merlin sl engines could be used in the second stage, the extra thrust might be useful, but at that point I have gone too far into Kerbal/lego territory and it’s a new rocket.

[TrueBlueWitt]

Last night Musk tweeted Falcon Heavy  Block 5 thrust numbers; about 2550t,  25MN, or 5,621,788 lbf.

Elon Musk ✔ @elonmusk
Max thrust of 2550 tons will be almost 10% higher than Falcon Heavy demo mission last year
1:16 PM - Apr 5, 2019

https://twitter.com/elonmusk/status/1114215249517981702

Key question then what does this translate to in terms of payload? Is there a linear relationship between payload and thrust or more complex than that (suspect the later)?

As you suspect, the latter.
Naively, the FH stack will weigh 1450 tons or so.
If it got off the pad at 2550 tons, it would be doing 17.5m/s upwards of which 10 is gravity losses, leaving 7.5m/s of real upwards acceleration.
At 2300 tons, this reduces to only 16m/s, or 6ms upwards.

So, 10% less thrust means 6, not 7.5m/s, or 20% less upwards acceleration.

The next gross approximation you could make is that a F9 first stage burns for 180 seconds or so, and during that time it's fighting gravity.

If you burn the fuel in 160, not 180 seconds, the stage is exposed to gravity for 20 seconds less, or 200m/s extra delta-v.

In reality, computing the exact gain is nasty.
It will probably not come off the pad at maximum throttle, and will only throttle up once it's cleared the tower, and then will throttle back down at some point near mach 1 (30 seconds into flight) to keep aerodynamic loads down.
Then throttle back up.
The exact profile of the throttle settings as well as trajectory matters. In reality, if you're thrusting partially sideways, as the first stage is most of its flight, not all of the 10m/s gravitational acceleration is a loss.

And, of course, it's a FH, not a F9, which adds additional complexities.

But, 'considerably less than 200m/s' is a safe bet to what it adds..

OK so the 10% thrust gain does not provide much more orbital capability due to the limitations of the engine configuration and thrust profile. So what good is it other than bragging rights for Musk? Entering amateur Kerbal calculation mode, (interested in feedback on my musings as a learning opportunity not as a practical proposition!):

10% more thrust could lift 10% more mass (all other things being equal, which they are not but bear with me) but it would not because:

1) It would burn through the fuel quicker and run out earlier and there is no room for extra fuel
2) Stretching the upper stage (to take the full mass difference) would not work because
a.   The core stage would not be able to take the extra mass of the upper stage
b.   The rocket would become too long and would be unstable to lateral forces etc
c.   It would make the second stage too heavy for the single merlin vac engine and there is only room for one merlin vac engine.

Assuming all that could be sorted out and multiple merlin sl engines could be used in the second stage, the extra thrust might be useful, but at that point I have gone too far into Kerbal/lego territory and it’s a new rocket.

Elon also said that the newer bolted Octaweb allowed them to transfer a lot more load from side boosters to the core. That is probably a much bigger deal to overall performance. Can fully take advantage of that higher thrust!

[envy887]

Those are US tons, not metric tons. Converted to MN it gives a total of 22.686MN (~5.1m lbf) which is similar to the numbers on the website. If you go by it as being metric tons, then it would have a total thrust of 25.056MN (~5.633m lbf) and that would mean each M1D+ engine produces about 928kN (~208.6k lbf) of thrust each one of them. If you compare numbers it is clear that he was refering to US tons and not metric tons.

Each Merlin does produce 914 kN in vacuum, according to the SpaceX website. So it's not at all clear that he was using short tons. It makes just as much sense to be vacuum metric tons.

So how did the Merlin get the extra 14kN that there are until 928kN which would be what Elon said?

The numbers on the website have been there since early 2016. Maybe they uprated it slightly since then.

[punder]

I'd guess this is because aircraft use nautical miles, presumably because ships use nautical miles.  It would be helpful if everyone would switch to metric....

Can't remember offhand where statute miles come from, but clue in the name, probably a tradition codified into law at some point, most likely for surveying (i.e. where does Lord X's property end and Baron Y's property begin).

But nautical miles are a particular subdivision of the earth's circumference, and map square latitudes are based on them, which simplifies navigation for ships and planes--and missile trajectories. (The squares shrink in longitude as you approach the poles though; hope I'm not getting mixed up here, but I probably am...)

Of course now it's "follow the little pink line, follow the little pink line, follow follow follow follow, follow the little pink line"!   

[launchwatcher]

Time-lapse of Falcon Heavy final assembly prior to static fire:

https://twitter.com/SpaceX/status/1114611309180411905

Fair bit of elbow room even with four cores in the hangar.

[Lar]

Not to nitpick, but 10% more is 9% less (to one decimal point)

[litton4]

Not to nitpick, but 10% more is 9% less (to one decimal point)

Beat me to it.

Technically 9.0909 recurring %

[hkultala]

c.   It would make the second stage too heavy for the single merlin vac engine and there is only room for one merlin vac engine.

Merlin Vacuum has lots of thrust, the upper stage could be stretched considerably without making it too heavy for the engine.

Especially when used with BLEO lauches on Falcon Heavy which stages much higher than F9, the stage has much less gravity losses on FH than on F9, and the stage is optimized for F9 (much earlier staging)

With 8.5-tonne payload (typical heavy comm satellite launch to GTO) Falcon second stage has T/W ratio of about 0.75.

With similar payload, Atlas V-551 Centaur has T/W ratio of about 0.35, though it stages later.

[su27k]

https://www.nbcnews.com/mach/science/nasa-s-17-billion-moon-rocket-may-be-doomed-it-ncna991061

"I'll never forget being at Marshall with the leadership team the day that SpaceX announced the Heavy," said Lori Garver, NASA's deputy administrator from 2009 to 2013. She recalls NASA officials telling her: "Lori, you have got to tell your friend Elon he can't do that. He's in our lane. You made us get out of low-Earth orbit, so we've given him that lane, but this is our lane. We build the big rockets."

[punder]

https://www.nbcnews.com/mach/science/nasa-s-17-billion-moon-rocket-may-be-doomed-it-ncna991061

"I'll never forget being at Marshall with the leadership team the day that SpaceX announced the Heavy," said Lori Garver, NASA's deputy administrator from 2009 to 2013. She recalls NASA officials telling her: "Lori, you have got to tell your friend Elon he can't do that. He's in our lane. You made us get out of low-Earth orbit, so we've given him that lane, but this is our lane. We build the big rockets."

I can just imagine Elon's reaction, but since this is a family-friendly website I can't speculate on the wording.

[Draggendrop]

https://www.nbcnews.com/mach/science/nasa-s-17-billion-moon-rocket-may-be-doomed-it-ncna991061

"I'll never forget being at Marshall with the leadership team the day that SpaceX announced the Heavy," said Lori Garver, NASA's deputy administrator from 2009 to 2013. She recalls NASA officials telling her: "Lori, you have got to tell your friend Elon he can't do that. He's in our lane. You made us get out of low-Earth orbit, so we've given him that lane, but this is our lane. We build the big rockets."

I can just imagine Elon's reaction, but since this is a family-friendly website I can't speculate on the wording.

He may have just said....."hold my beer!"....and Jeff may have said it as well....