Author Topic: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion Thread 1  (Read 593445 times)

Offline CyndyC

There are certainly some untested aspects of this vehicle, especially..... pogo.....  History is a guide.

I don't think "torque", or force of rotation around an axis, is the right word or focus. Where is everyone seeing so many spinning parts that can go wrong?

From everything I've Googled today, pogo oscillations appear to be the real risk, up and down variations in thrust & LOX & fuel flow. They brought down the 4th & final 30-engine N-1 launch, and have a long history of plaguing US launches as well. They also have a long history of mitigation by various kinds of engineering, the latest written up by our own Philip Sloss just this month, a 3D-printed POGO Suppression Accumulator built by Aerojet Rocketdyne for NASA's Space Launch System R-25 engines. In the past NASA instituted a safety level of G forces to be tolerated by such oscillations, +/-0.25G.

Here's one description related to the 4th N-1 launch:

Quote
Pogo oscillation is exactly what it sounds like – a violent up-and-down motion that could make anybody sick, but makes rockets really really sick. It results from a variation in thrust from different engines, and with thirty unreliable ones, you can get a lot of variation. Once it starts, it's very hard to correct, as the variable acceleration leads to variable fuel pump pressures, which leads to more variables acceleration, and so on and so on until eventually you match the vehicle's resonance frequency and vibrate the whole thing to death.
https://jalopnik.com/this-insane-rocket-is-why-the-soviet-union-never-made-i-1448356326
« Last Edit: 12/30/2017 02:58 AM by CyndyC »
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Offline Grandpa to Two

There are certainly some untested aspects of this vehicle, especially..... pogo.....  History is a guide.

I don't think "torque", or force of rotation around an axis, is the right word or focus. Where is everyone seeing so many spinning parts that can go wrong?

From everything I've Googled today, pogo oscillations appear to be the real risk, up and down variations in thrust & LOX & fuel flow. They brought down the 4th & final 30-engine N-1 launch, and have a long history of plaguing US launches as well. They also have a long history of mitigation by various kinds of engineering, the latest written up by our own Philip Sloss just this month, a 3D-printed POGO Suppression Accumulator built by Aerojet Rocketdyne for NASA's Space Launch System R-25 engines. In the past NASA instituted a safety level of G forces to be tolerated by such oscillations, +/-0.25G.

Here's one description related to the 4th N-1 launch:

Quote
Pogo oscillation is exactly what it sounds like – a violent up-and-down motion that could make anybody sick, but makes rockets really really sick. It results from a variation in thrust from different engines, and with thirty unreliable ones, you can get a lot of variation. Once it starts, it's very hard to correct, as the variable acceleration leads to variable fuel pump pressures, which leads to more variables acceleration, and so on and so on until eventually you match the vehicle's resonance frequency and vibrate the whole thing to death.
https://jalopnik.com/this-insane-rocket-is-why-the-soviet-union-never-made-i-1448356326

I also was researching pogo and believe it could be the greatest risk to this first flight. It’s impossible to check until the Heavy is running. Maybe we’ll see a much longer static fire so they can get an idea of how the vibrations affect the rocket. Once airbourne will come the real test for pogo. I’m glad Elon is going to keep thrust down to 92% for this flight.
"All truths are easy to understand once they are discovered; the point is to discover them" Galileo Galilei

Offline georgegassaway

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #642 on: 12/30/2017 04:16 AM »
Hmm, the historic examples of Pogo were with non-throttled engines (AFAIK). Makes me wonder if the rate of throttle response might be fast enough to reduce some Pogo effect., if programmed to do so.  But I do not know anything about the throttle response rate. 

Ideally of course they want a design/physical fix for Pogo as the primary solution, regardless.
« Last Edit: 12/30/2017 04:16 AM by georgegassaway »

Offline Lars-J

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #643 on: 12/30/2017 04:16 AM »
I see very little (if any) risk for POGO for FH, since they are flying the same stages as F9. Same engines, same tank lengths. POGO - if present - will show up in early launches.

All this talk of possible failure points - one more dramatic than the next - starts to border on concern trolling, IMO. “Surely SpaceX has not thought of *this*?”
« Last Edit: 12/30/2017 04:20 AM by Lars-J »

Offline Surfdaddy

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #644 on: 12/30/2017 04:23 AM »
I see very little (if any) risk for POGO for FH, since they are flying the same stages as F9. Same engines, same tank lengths. POGO - if present - will show up in early launches.

All this talk of possible failure points - one more dramatic than the next - starts to border on concern trolling, IMO. “Surely SpaceX has not thought of *this*?”

I'm no expert, but wouldn't the pogo risk be increased because of potential flexibilities in the triple stage cluster that could impart motions/resonances and vibrations and flexing that would NOT be the same as the relative simplicity of single boosters?

Put simply. I disagree with the assumption that "since we know the F9 works fine, clustering 3 together will work fine without those problems as well".

Offline vaporcobra

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #645 on: 12/30/2017 04:52 AM »
If the question starts with "I'm no expert...," the answer is almost invariably, "Yes, dozens of highly skilled and experienced engineers considered it and implemented the best possible solution over the course of many iterations of analysis, modeling, and testing."
« Last Edit: 12/30/2017 04:53 AM by vaporcobra »

Offline cscott

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #646 on: 12/30/2017 05:20 AM »
Emily Lakdawalla points out that the FH demo launch is too early for an optimal earth-mara trajectory:
https://twitter.com/elakdawalla/status/946941586373451776

Can someone with the technical chops compute if FH has to capability to put a roadster-sized payload arbitrarily close to Mars (ie, for purposes of computation, on a path impacting Mars) even if launched mid-January?

Offline Lars-J

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #647 on: 12/30/2017 05:47 AM »
Emily Lakdawalla points out that the FH demo launch is too early for an optimal earth-mara trajectory:
https://twitter.com/elakdawalla/status/946941586373451776

Can someone with the technical chops compute if FH has to capability to put a roadster-sized payload arbitrarily close to Mars (ie, for purposes of computation, on a path impacting Mars) even if launched mid-January?

It’s not going to Mars. It’s going out to Mars orbit. It will get close to Mars *eventually*, but the launch window(s) are not as rare as she thinks.
« Last Edit: 12/30/2017 05:50 AM by Lars-J »

Offline pargoo

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #648 on: 12/30/2017 06:05 AM »
So...let me get this strait.  We have the most eagerly-awaited rocket roll-out in decades, and all we get are long-distance shots and no up-close hi-res's from SpaceX?

Offline vaporcobra

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #649 on: 12/30/2017 06:05 AM »
Yeah, judging from some coincidentally accurate porkchop plots and a few articles, Roadster can certainly make it to a rather close encounter with Mars, it just might take anywhere from a 12-24 month coast period to get there. Rather improbable that S2 can remain functional over that period without some intense modifications (and solar panels).

Edit: Found a very convenient table of trajectory calculations over the 2017-2018 period and they look quite promising. Jan 2018 is certainly less efficient than the May that Emily mentions, but it's a minor improvement compared to 2017 (500-900 days, 5000-8000 m/s dV). The random forum user's calculations show that a launch to TMI from LEO in Jan 2018 would have a coast period of ~260 days and a dV cost of ~4500 m/s.

InSight's May 2018 launch will have a coast of ~250 days with a dV cost of ~3550 m/s.

https://www.orbiter-forum.com/showthread.php?t=37063
« Last Edit: 12/30/2017 06:30 AM by vaporcobra »

Offline vaporcobra

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #650 on: 12/30/2017 06:06 AM »
So...let me get this strait.  We have the most eagerly-awaited rocket roll-out in decades, and all we get are long-distance shots and no up-close hi-res's from SpaceX?

Patience, young Padawan.

Offline deruch

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #651 on: 12/30/2017 06:21 AM »
Emily Lakdawalla points out that the FH demo launch is too early for an optimal earth-mara trajectory:
https://twitter.com/elakdawalla/status/946941586373451776

Can someone with the technical chops compute if FH has to capability to put a roadster-sized payload arbitrarily close to Mars (ie, for purposes of computation, on a path impacting Mars) even if launched mid-January?

My search results: It could do a flyby of Mars (view the 1st one with a Jan departure date [C3=20.1]) with a launch opportunity on January 2nd.  So, I guess, given the actual, likely date being mid-month, it probably still could if the payload had the ability to do some course correction (which it doesn't).  LSP's elvperf calculator says that a Falcon Heavy (with recovery) can launch 2960kg to the C3 that trajectory would require.  The roadster had a curb weight of 1305kg (though this one may have been modified, either for Elon's driving pleasure or in the effort to turn it into a spacecraft).  So, that is well within the capability of FH even if you added some weight to enable trajectory maneuvers, etc.
« Last Edit: 12/30/2017 06:50 AM by deruch »
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Offline Ben the Space Brit

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #652 on: 12/30/2017 06:53 AM »
I don't think that there is any realistic prospect of SpaceX confirming actively that the roadster gets to its destination orbit. All they will be able to do is monitor the second stage for as long as possible to confirm its trajectory. All they'll ever be able to say is: "With a high degree of confidence..."
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Offline pargoo

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #653 on: 12/30/2017 10:32 AM »
OK, I guess it's coming up to the New Year, but still..

Offline Oersted

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #654 on: 12/30/2017 11:14 AM »
So...let me get this strait.  We have the most eagerly-awaited rocket roll-out in decades, and all we get are long-distance shots and no up-close hi-res's from SpaceX?

To the general public it would look weird with a headline saying "the rocket went to the launch pad and was then carted back into its hangar again". They'll roll out the PR train when it comes out for launch.

Offline Herb Schaltegger

I don't think that there is any realistic prospect of SpaceX confirming actively that the roadster gets to its destination orbit. All they will be able to do is monitor the second stage for as long as possible to confirm its trajectory. All they'll ever be able to say is: "With a high degree of confidence..."

Doppler tracking and telemetry from the stage itself within milliseconds of the S2 injection burn cutoff will tell them EXACTLY what the final trajectory will be, absent any unplanned venting events (debris impact or valve failure leading to overpressure). They don't need an active second stage, broadcasting for days, weeks or months to "follow" the stage and watch.
« Last Edit: 12/30/2017 12:45 PM by Herb Schaltegger »
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Online clongton

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #656 on: 12/30/2017 01:13 PM »
I know I am not going to make any friends by saying this but it needs to be said.

I would majorly hate to be a member of the generation that just can't stand to wait until it [whatever "it" may be] actually happens. I get the impression that they would never be able to stand the wait time of several YEARS while a probe makes its way to Jupiter, Saturn, Uranus, Neptune, Pluto or the Ort cloud. Their heads would explode. Try launching a probe like that knowing that you yourself may very well not live long enough to see it arrive and that children who are in 4th grade at launch date would likely be the scientists that would monitor and record the arrival. Do you have the patience for that?

Jeepers people. The solar system isn't your back yard that can be crossed in a leap and a bound.
SpaceX will provide photographs when it wants to and not one second before - they don't owe any of us a thing.
Falcon Heavy will launch when it is ready. - Give it a rest and have a cup of tea or coffee or latte-mocha-chi-whatever.
Chill. It will happen when it happens.
THIS site will be the first to let you know it's happening so just stay tuned and quit complaining.
« Last Edit: 12/30/2017 01:14 PM by clongton »
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Offline SimonFD

I know I am not going to make any friends by saying this but it needs to be said.

I would majorly hate to be a member of the generation that just can't stand to wait until it [whatever "it" may be] actually happens. I get the impression that they would never be able to stand the wait time of several YEARS while a probe makes its way to Jupiter, Saturn, Uranus, Neptune, Pluto or the Ort cloud. Their heads would explode. Try launching a probe like that knowing that you yourself may very well not live long enough to see it arrive and that children who are in 4th grade at launch date would likely be the scientists that would monitor and record the arrival. Do you have the patience for that?

Jeepers people. The solar system isn't your back yard that can be crossed in a leap and a bound.
SpaceX will provide photographs when it wants to and not one second before - they don't owe any of us a thing.
Falcon Heavy will launch when it is ready. - Give it a rest and have a cup of tea or coffee or latte-mocha-chi-whatever.
Chill. It will happen when it happens.
THIS site will be the first to let you know it's happening so just stay tuned and quit complaining.

Ah, the impatience of youth, assisted by a culture hell bent on instant gratification, topped by the photos-or-it-didn't-happen dogma.

 ;) 8)

Just in case that wasn't clear - I agree with Chuck
« Last Edit: 12/30/2017 01:26 PM by SimonFD »
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Offline AncientU

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #658 on: 12/30/2017 01:24 PM »
I know I am not going to make any friends by saying this but it needs to be said.

I would majorly hate to be a member of the generation that just can't stand to wait until it [whatever "it" may be] actually happens. I get the impression that they would never be able to stand the wait time of several YEARS while a probe makes its way to Jupiter, Saturn, Uranus, Neptune, Pluto or the Ort cloud. Their heads would explode. Try launching a probe like that knowing that you yourself may very well not live long enough to see it arrive and that children who are in 4th grade at launch date would likely be the scientists that would monitor and record the arrival. Do you have the patience for that?

Jeepers people. The solar system isn't your back yard that can be crossed in a leap and a bound.
SpaceX will provide photographs when it wants to and not one second before - they don't owe any of us a thing.
Falcon Heavy will launch when it is ready. - Give it a rest and have a cup of tea or coffee or latte-mocha-chi-whatever.
Chill. It will happen when it happens.
THIS site will be the first to let you know it's happening so just stay tuned and quit complaining.

Ah, the impatience of youth, assisted by a culture hell bent on instant gratification, topped by the photos-or-it-didn't-happen dogma.

 ;) 8)

The flip side is waiting patiently, like an 'adult,' for decades -- all the while knowing deep down that it will never happen. 

Seems to me, it takes a healthy portion of each to get something useful done.
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Offline cscott

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #659 on: 12/30/2017 01:30 PM »


Yeah, judging from some coincidentally accurate porkchop plots and a few articles, Roadster can certainly make it to a rather close encounter with Mars, it just might take anywhere from a 12-24 month coast period to get there.

What's with the asymptote in that porkchop plot?  It seems like the required dV around Jan 15 is actually extremely sensitive to the exact date selected.  Is that just an artifact of some other choice made in the plot, or is there an actual orbital mechanics reason why Jan 15 would be so much worse than Jan 1?

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