SpaceX FH : Falcon Heavy Demo : Feb 6, 2018 : Discussion Thread 2

[yokem55]

We can finally get a good idea of the timings for each event now that the press kit's here. Some changes that I can see are:

1- Filling RP-1 and LOX begins 15 and 10 minutes earlier, respectively.

2- The engines on the side boosters ignite two seconds before the ones on the core do.

3- MaxQ occurs 10 seconds or so earlier than usual.

4- MECO for the core occurs about 30 seconds after it occurs for the side boosters.

5- The press kit seems to suggest that the side boaster landings may occur simultaneously, with the core stage landing some 15 seconds later!

This will be intense for sure!

The timing of the second stage burns (in particular burn 2 at T+28:XX for a dozen seconds) seems to indicate that the first two burns will insert the stage into a standard GTO, cruise to 36000 km high and then do the Earth escape burn there.

Or, roughly half a GTO burn to a ~6 hour orbit, but then do the Earth escape burn once it's swung back around to perigee. That would get the Tesla the biggest kick out of town.

Edit: The GovSat GTO burn lasted a little over a minute. A dozen seconds won't get the second stage to a very high altitude. It may even only be a  3 hour orbit and the Earth escape happens after the second go around.

[LouScheffer]

Edit: I also noticed that the burn to GTO here seems to be only 30 seconds long compared to the usual 55-60 seconds. Does this suggest a fairly elliptical parking orbit?

It's even less delta-V, since the second stage is more loaded with fuel than usual, and the first half of the burn produces less delta-V.

From an orbital mechanics point of view this makes perfect sense.   You want the Mars insertion burn to happen as deep in the gravity well as possible, for maximum delta-V at infinity.   So the first GTO-is burn should target a 6 hour orbit, then the Mars insertion when it comes back Earth perigee 6 hours later.

Russian Proton missions do exactly this to maximize their delta-V, although for a different reason.   Their engine is low thrust, so it can't complete the burn while staying low.   But if you can spare the time, it's just as efficient to do it a little at each perigee.   The FH does not need to do this, but is doing so to demonstrate the 6 hour coast.

[scdavis]

From an orbital mechanics point of view this makes perfect sense.   You want the Mars insertion burn to happen as deep in the gravity well as possible, for maximum delta-V at infinity.

That's counter-intuitive... which I've heard here several times is a good description of orbital mechanics! It would seem that the burn would have the most effect when the rocket is furthest away from earth, so it experiences less force of gravity, thus higher final velocity.

Could someone explain further or point to an article that explains this?

[tvg98]

Edit: I also noticed that the burn to GTO here seems to be only 30 seconds long compared to the usual 55-60 seconds. Does this suggest a fairly elliptical parking orbit?

It's even less delta-V, since the second stage is more loaded with fuel than usual, and the first half of the burn produces less delta-V.

From an orbital mechanics point of view this makes perfect sense.   You want the Mars insertion burn to happen as deep in the gravity well as possible, for maximum delta-V at infinity.   So the first GTO-is burn should target a 6 hour orbit, then the Mars insertion when it comes back Earth perigee 6 hours later.

Russian Proton missions do exactly this to maximize their delta-V, although for a different reason.   Their engine is low thrust, so it can't complete the burn while staying low.   But if you can spare the time, it's just as efficient to do it a little at each perigee.   The FH does not need to do this, but is doing so to demonstrate the 6 hour coast.

So what you're saying is that instead of going to GTO, they are targeting an orbit which has a period of 6 hours. That way, when the US returns to perigee 6 hours later it can make the most of out the fuel it has by being deep within Earth's gravity well when it's time for TMI. Did I get all that right? I'm a bit of a noob when it comes to orbital mechanics but I've heard it's related to something called the Oberth Effect?

[Lee Jay]

From an orbital mechanics point of view this makes perfect sense.   You want the Mars insertion burn to happen as deep in the gravity well as possible, for maximum delta-V at infinity.

That's counter-intuitive... which I've heard here several times is a good description of orbital mechanics! It would seem that the burn would have the most effect when the rocket is furthest away from earth, so it experiences less force of gravity, thus higher final velocity.

Could someone explain further or point to an article that explains this?

At the bottom of the well, it's at maximum speed.  Since the thrust of a rocket is independent of the speed at which it's traveling and the power imparted to the system by the rocket is proportional to that speed, the same thrust at a higher speed will add kinetic energy faster, thus using the energy of the fuel more efficiently.

[WindyCity]

In short:
 * All liked it up to the point that the Roadster appeared.
 * Those who were long term space advocates, didn't like the gimmick
 * Those who were professional advertising, gave ton's of detailed criticism as an ad, being conflicted in what it was selling
 * Those who were in the music industry didn't think the Bowie number worked
 * Those in the financial services industry thought it trivialized the advantages of the booster reuse
 * Those in planetary science got irked when Mars came into view ("it's going to hit MAVEN?", "take out MRO", "planetary protection")
 * Those in aerospace wanted to see a real payload that made that mission worthwhile. (Missed the FSS too.)
 * Those with Tesla's really loved (and only focussed on) the car driving to Mars.
 * Those uncolleged youth also loved it for the thought of driving to Mars.
 * Those younger Gen X going in to venture to pitch this morning, thought it was fantastic, wanted to work it into their pitch.
 * Those in the robotics field ignored the car, focused on the four robotic vehicles as a transport system, loved it as well.
 * Those mathematicians/data scientists ... liked it but were irked by it conveying the math of such a mission all wrong.
 * Those into astronomy noted that the moon is last quarter now, not first quarter, wrong portion of sky.
 * Those who knew about orbital dynamics complained about the impossibility of the trajectory and lack of coast/second burn.

Everyone had an opinion. Most summed it up as an appeal to use commercial space to reach to the moon and Mars to those who voted for the current majority in office.

One possible benefit of the whimsical, juvenile stunt is that it might attract the attention of young people, interesting them in space, which could draw some of them into STEM fields. Putting aside all thoughts about its utilitarian value (aside from serving as off-world ballast), the spectacle it presents is fun! It brought a smile to my face when I saw "The Stig" strapped in the driver's seat. Think of all the children's books that will come, featuring the adventures of that masked explorer in his interplanetary sports car. Eat your heart out, Little Prince!

[Herb Schaltegger]

From an orbital mechanics point of view this makes perfect sense.   You want the Mars insertion burn to happen as deep in the gravity well as possible, for maximum delta-V at infinity.

That's counter-intuitive... which I've heard here several times is a good description of orbital mechanics! It would seem that the burn would have the most effect when the rocket is furthest away from earth, so it experiences less force of gravity, thus higher final velocity.

Could someone explain further or point to an article that explains this?

The best explanations will all involve math. If you have the mathematical background for it, there are a number of really good sites that can walk you through the equations.

If you don’t, well ... it’s kind of too late for this Falcon Heavy flight, but Kerbal Space Program is the best, most-accessible-to-laypeople introduction to orbital mechanics I’ve ever run across. But even so, it requires a number of hours to get the hang of how trajectories work, let alone how to actually do anything in the game.

[DigitalMan]

The recording of the press conference had some interesting notes:

1. If BFS is not progressing fast enough could do crew on FH around the moon and some other things (what do they have in mind?)
2. BFS tests expected to start next year

[LouScheffer]

From an orbital mechanics point of view this makes perfect sense.   You want the Mars insertion burn to happen as deep in the gravity well as possible, for maximum delta-V at infinity.

That's counter-intuitive... which I've heard here several times is a good description of orbital mechanics! It would seem that the burn would have the most effect when the rocket is furthest away from earth, so it experiences less force of gravity, thus higher final velocity.

Could someone explain further or point to an article that explains this?

There are lots of ways to think about this, this is one that works for me:

An orbit is defined by constant energy - a spacecraft in an elliptical orbit is always trading potential energy from gravity vs kinetic energy from speed.  The sum is always constant, with the spacecraft going fastest at closest approach.

Also, getting out of the gravity well takes a certain amount of energy.  If there is energy left over, then that determines your speed at infinity.

Since both orbits and gravity wells are defined in terms of energy, you can add energy anywhere along the orbit and get exactly the same final result.

But rocket burns do not add a constant amount of energy, they add a constant amount of speed (this is called delta-V, for change in velocity).  So the energy added from a burn depends on the speed you are already going, and it's highest when the speed is fastest.  So you add the most energy by burning at perigee, where the speed is the highest.

Here's an extreme example.  Suppose you have a really, really elliptical orbit around the Earth.  Then at the top you are hardly moving (perhaps 1 m/s or so) and near Earth you are going about 11,000 m/s, just under escape speed.    Now you can fire your rocket to add 1000 m/s to your speed.

If you fire it at the top, when you are almost out of the gravity well, your speed at that time is basically 0.  You've added 1000*1000/2 = 500,000 joules/kg,  and you'll now be moving away from the Earth at just under 1000 m/s.

But if you fire at the bottom, you change the speed from 11,000 to 12,000 m/s.   You've added (144,000,000 - 121,000,000)/2 = 11,500,000 joules/kg.   Gravity still extracts the same amount of energy on the way out, so now, when you get to the top, you've got 11,500,000 joules left over - 23 times the excess energy as when you fired at the top.   So now you speed away from Earth at just under 4,800 m/s.  You gained 4.8 times as much speed at infinity by firing at the bottom as opposed to the top.

There's also a wikipedia article on the Oberth effect, which is what this is called.  Also there's lots of textbooks.   If you read enough of these you may find an explanation that seems clearer to you, or at least makes it seem a little less counter-intuitive.

[DrRobin]

From an orbital mechanics point of view this makes perfect sense.   You want the Mars insertion burn to happen as deep in the gravity well as possible, for maximum delta-V at infinity.

That's counter-intuitive... which I've heard here several times is a good description of orbital mechanics! It would seem that the burn would have the most effect when the rocket is furthest away from earth, so it experiences less force of gravity, thus higher final velocity.

Could someone explain further or point to an article that explains this?

The best explanations will all involve math. If you have the mathematical background for it, there are a number of really good sites that can walk you through the equations.

If you don’t, well ... it’s kind of too late for this Falcon Heavy flight, but Kerbal Space Program is the best, most-accessible-to-laypeople introduction to orbital mechanics I’ve ever run across. But even so, it requires a number of hours to get the hang of how trajectories work, let alone how to actually do anything in the game.

HaHa!
I think LouScheffer can handle this -pretty rudimentary- math. (I recently realized we are fellow Caltech alums connected indirectly through some pretty interesting paths.) He's right that it's counter-intuitive, though. My favorite explainer for these sorts of things is Hopp's Blog (also an NSF regular and orbital mechanics auto-didact). Here is his take on the Oberth Effect: http://hopsblog-hop.blogspot.com/2013/10/what-about-mr-oberth.html

[edit: Oops! I misread the quote level. Not LouScheffer asking the question. Sorry! Anyway, I still like Hopp's visual explanation.]

[MATTBLAK]

https://www.geekwire.com/2018/elon-musk-explains-spacexs-falcon-heavy-rocket-risky-revolutionary/

“If we wanted to, we could actually add more two more side boosters and make it ‘Falcon Super-Heavy,’ and probably get thrust upwards of maybe 9 million pounds of thrust … but we think that the new BFR architecture is a better way to go,” Musk said.

...That's what some of us have been saying for awhile! 45x engine beast? (Aussie accent) "Nah, mate - tell him he's dreamin'..."

[dodo]

For any layman like me wondering what shockwave impingement is, here is an article by a retired engineer. See the diagram mid-article (the whole article is interesting):

http://exrocketman.blogspot.co.uk/2017/06/shock-impingement-heating-is-very.html

[Space Ghost 1962]

https://twitter.com/jeff_foust/status/960628075171106816

Musk: looks like development of BFR is moving quickly, and won’t be necessary to qualify Falcon Heavy for crewed spaceflight.

Why develop a CC vehicle further if you wish to advance its successor faster?

https://twitter.com/jeff_foust/status/960628677817044992

Musk: if we’re successful, offer near super-heavy-lift for little more than Falcon 9. “Game over” for all other heavy-lift rockets.

If BFR/BFS arrives ahead of (or coincident with) New Glenn, a fully reusable SHLV would render a partially reusable HLV obsolete.

https://twitter.com/jeff_foust/status/960629706826698752

Musk: if successful should be able to d another FH in three to six months. Can produce them at a pretty rapid rate.

They certainly integrated the first FH faster than DIVH, and the production rate of F9 far exceeds Atlas/Delta rates.

https://twitter.com/jeff_foust/status/960629934388588544

Musk: if we wanted to, we could add to more side boosters, make it Falcon Super Heavy.

Yup. 5x Falcon 9 boosters. Well into SLS territory. Like Angara, just bigger and cheaper.

https://twitter.com/jeff_foust/status/960630695788990465

Musk: we kind of tabled Crew Dragon on Falcon Heavy (including the cislunar mission announced last Feb.) and focus our energies on BFR.

Not long after giving up on Red Dragon and propulsive landing crew/cargo Dragon.

Clearly they don't want NASA/govt as a impediment to a large scale HSF vehicle, given the torturous CC program that deviated from the more successful CRS program. Too much uncontrolled politics in the HSF room with NASA, better to accept offers from NASA post-development on contracted services to be performed rather than providing a vehicle used for a NASA mission. Interesting.

https://twitter.com/jeff_foust/status/960631417792319488

Musk: it would be a “real huge downer” if Falcon Heavy blows up, but hope to learn a lot. It’s a win if it just clears the pad.

And, by declaring it separate from any HSF "need" or "use",  they don't have to deal with Zuma-like schadenfreude on the political side over CC, and being raked over the coals over not good enough for NASA astros to risk life on. Yup. Smart.

https://twitter.com/jeff_foust/status/960631901865283584

Musk: main reason for the six-hour coast is to demonstrate direct GEO insertion (for national security customers).

Yes, direct insertion with long lived F9US. Hand in hand for NSS. Still need VI.

This will be the penultimate Falcon mission. Nothing more to be developed for this architecture. When Dragon 2 flies with crew, it becomes a sustainment program. No looking back.

[scdavis]

From an orbital mechanics point of view this makes perfect sense.   You want the Mars insertion burn to happen as deep in the gravity well as possible, for maximum delta-V at infinity.

That's counter-intuitive... which I've heard here several times is a good description of orbital mechanics! It would seem that the burn would have the most effect when the rocket is furthest away from earth, so it experiences less force of gravity, thus higher final velocity.

Could someone explain further or point to an article that explains this?

...
But rocket burns do not add a constant amount of energy, they add a constant amount of speed (this is called delta-V, for change in velocity).  So the energy added from a burn depends on the speed you are already going, and it's highest when the speed is fastest.  So you add the most energy by burning at perigee, where the speed is the highest.
...
There's also a wikipedia article on the Oberth effect, which is what this is called.  Also there's lots of textbooks.   If you read enough of these you may find an explanation that seems clearer to you, or at least makes it seem a little less counter-intuitive.

Thanks Lou, and everyone else who responded! The explanations made sense and the math isn't bad.

For anyone else still struggling, the wikipedia article is very helpful. It confirmed a hunch I got reading Lou's response... the counterintuitive bit is that it seems like the rocket gets free energy just for higher initial speed. That extra energy impacting the rocket is "taken" from the kinetic energy of the propellant which is moving way faster at perigee.

[Wolfram66]

From an orbital mechanics point of view this makes perfect sense.   You want the Mars insertion burn to happen as deep in the gravity well as possible, for maximum delta-V at infinity.

That's counter-intuitive... which I've heard here several times is a good description of orbital mechanics! It would seem that the burn would have the most effect when the rocket is furthest away from earth, so it experiences less force of gravity, thus higher final velocity.

Could someone explain further or point to an article that explains this?

...
But rocket burns do not add a constant amount of energy, they add a constant amount of speed (this is called delta-V, for change in velocity).  So the energy added from a burn depends on the speed you are already going, and it's highest when the speed is fastest.  So you add the most energy by burning at perigee, where the speed is the highest.
...
There's also a wikipedia article on the Oberth effect, which is what this is called.  Also there's lots of textbooks.   If you read enough of these you may find an explanation that seems clearer to you, or at least makes it seem a little less counter-intuitive.

Thanks Lou, and everyone else who responded! The explanations made sense and the math isn't bad.

For anyone else still struggling, the wikipedia article is very helpful. It confirmed a hunch I got reading Lou's response... the counterintuitive bit is that it seems like the rocket gets free energy just for higher initial speed. That extra energy impacting the rocket is "taken" from the kinetic energy of the propellant which is moving way faster at perigee.

http://jwilson.coe.uga.edu/EMAT6680Fa05/Bacon/hohmanntransfers.html Is more of what the burn at max distance from earth.

[sanman]

There’s a second Roadster and astronaut - on the dash. (I didn’t discover this).  And who knows, maybe... (here you go Lar)

[1] ... maybe there’s a tiny Roadster and astronaut on IT’S dash and... [1]

Very funny, young man - but it's Teslas all the way down!

[Johnnyhinbos]

Lol - and I love turtles (all the way down)!

[refsmmat]

Jonathan McDowell is reporting that the escape burn will be at apogee 33600km. A large burn of Merlin Vacuum at that altitude 6 hrs after launch should produce a cloud of kerosene soot and water vapor that will appear sunlit from the ground after local nightfall. A ground track may indicate the most likely observing sites.

[Galactic Penguin SST]

Jonathan McDowell is reporting that the escape burn will be at apogee 33600km. A large burn of Merlin Vacuum at that altitude 6 hrs after launch should produce a cloud of kerosene soot and water vapor that will appear sunlit from the ground after local nightfall. A ground track may indicate the most likely observing sites.

Do you have a source for that?

[Kabloona]

Source for Jonathan McDowell quote:

https://mobile.twitter.com/planet4589/status/960635478159560704

Correction: probably SECO-1 is 180 x 200 km or so, SECO-2 is 200 x 35800 km to demo GTO, SECO-3 is 35800 x Escape . Hopefully we'll know more after the event.

However, reading that Twitter thread, Jonathan seems not to be aware that the final burn would be much more efficiently done at perigee instead of apogee, as yokem55 and Lou Scheffer have described above, so I wouldn't take that as Jonathan's final guesstimate. I expect he'll follow up and come to the same conclusion as the aforementioned NSF'ers.