Launch News SpaceX Falcon 9 v1.2 with JCSAT-14 NET 5 May 2016

[RisingFury]

I give them a ten on that landing. Dead center !!!

X marks the spot! And a really nasty difficult descent.

I still don't know what went wrong on the previous GTO landing that they said will probably fail. Now I wonder if they tested the three engine approach...

 

[Andy44]

Unlike the last landing, this one doesn't seem to have bounced or skated sideways after touchdown. It was right in the X ring.

 

[Donamy]

The last landing seemed to be at an angled approach, and straightened out in the last seconds. Though it didn't show it, maybe this one was a less angled approach.

 

[fraxudemspas]

The last landing seemed to be at an angled approach, and straightened out in the last seconds. Though it didn't show it, maybe this one was a less angled approach.

If anything, wasn't it the other way around? The first stage followed a longer trajectory this time, without a boostback burn (only reentry and landing burns), so it came in at an even more of a sideways angle as opposed to mostly vertical.

 

[Col_Klonk]

Besides the obvious problem if the engine's thrusts are not exactly tuned,
I take it they're firing the engines at a lower altitude.. this makes me think:-

- Higher G-forces to slow down.
- 3x engines running at lower thrust = lower efficiency = more fuel.

Compared to a single engine where...

- Higher thrust = more efficiency = less fuel.
- You can start higher with a more gradual G-load - preserving the stage more.
- Redundancy by way of only one engine failure is necessary in either case.
??

Maybe they're just testing different configurations.. but then again it's not rocket science to work this out..

 

[RisingFury]

Besides the obvious problem if the engine's thrusts are not exactly tuned,
I take it they're firing the engines at a lower altitude.. this makes me think:-

- Higher G-forces to slow down.
- 3x engines running at lower thrust = lower efficiency = more fuel.

Compared to a single engine where...

- Higher thrust = more efficiency = less fuel.
- You can start higher with a more gradual G-load - preserving the stage more.
- Redundancy by way of only one engine failure is necessary in either case.
??

Maybe they're just testing different configurations.. but then again it's not rocket science to work this out..

Dead wrong.

1.) The stage can easily take the G-forces. Keep in mind that during liftoff, it has to support the entire weight of the second stage and its propellant, even through Max-Q - highest load conditions.

2.) Running on three engines actually saves fuel because you spend less time almost hovering, so you end up with lower gravity losses. Keep in mind that if a rocket wants to hover, it has to produce 1 g worth of thrust, but goes nowhere, so the efficiency is 0/1. If you double the thrust, it'll produce 2 g of thrust, but will only accelerate upwards at 1 g, with efficiency of 1/2. If you triple the thrust, the efficiency rises to 2/3.

The reason this wasn't done straight off the bat is because they weren't confident they could do it. Now they've got better data, more knowledge of how the vehicle behaves and better understanding of how the control system behaves during landing, so they one-upped it.

 

[Thunder Chicken]

Dead wrong.

1.) The stage can easily take the G-forces. Keep in mind that during liftoff, it has to support the entire weight of the second stage and its propellant, even through Max-Q - highest load conditions.

2.) Running on three engines actually saves fuel because you spend less time almost hovering, so you end up with lower gravity losses. Keep in mind that if a rocket wants to hover, it has to produce 1 g worth of thrust, but goes nowhere, so the efficiency is 0/1. If you double the thrust, it'll produce 2 g of thrust, but will only accelerate upwards at 1 g, with efficiency of 1/2. If you triple the thrust, the efficiency rises to 2/3.

The reason this wasn't done straight off the bat is because they weren't confident they could do it. Now they've got better data, more knowledge of how the vehicle behaves and better understanding of how the control system behaves during landing, so they one-upped it.

On top of this, the engines are much more efficient at their design thrust. Removing the need to deep throttle the engines by burning at higher thrust for shorter periods is an additional win over and above beating the gravity losses.

---------- Post added at 07:02 PM ---------- Previous post was at 05:48 PM ----------

I so want to see a picture of a hangar full of used Falcon 9 stages in all their burned-in glory.

 

[Col_Klonk]

Dead wrong.

Not at all...
You have two scenarios with the returning stage.

1) It's on target (x) so at the right moment..
a) The 3 engines fire at max thrust increasing the g-load 3x that of a single engine.
b) Deep throttling the engines to reduce g-load, accumulates 3x less efficiency than a single engine (more fuel needed)
c) A single engine fires higher up at max thrust (1x G-load), but only produces effiency losses at 1x

All aircraft frame lifetimes are determined by active lifetime use and g-loading => G-loading being the prominent factor.
(Maybe SpaceX are not interested in this at the moment)

2) The stage is slightly off target...
a) All three engines will have to start deep throttling to give it time to adjust it's flight path (efficiency losses as point b) above)
b) A single engine as in c) above, will give time for flight adjustments already, with the same resulting efficiency losses as c) above.

This is over and above the launch phase.. so the end result with 3 engine landing is basically either 1 or 2 below.
1) Higher ('impulse') g-load decreasing the rocket frame's lifetime even faster.
2) Up to 3x the fuel wastage.

SpaceX must be toying with ideas at the moment.

 

[ADSWNJ]

2.) Running on three engines actually saves fuel because you spend less time almost hovering, so you end up with lower gravity losses. Keep in mind that if a rocket wants to hover, it has to produce 1 g worth of thrust, but goes nowhere, so the efficiency is 0/1. If you double the thrust, it'll produce 2 g of thrust, but will only accelerate upwards at 1 g, with efficiency of 1/2. If you triple the thrust, the efficiency rises to 2/3.

The reason this wasn't done straight off the bat is because they weren't confident they could do it. Now they've got better data, more knowledge of how the vehicle behaves and better understanding of how the control system behaves during landing, so they one-upped it.

Could we see a 9 engine suicide burn, using this same argument? Can you imagine the 360 deg camera for that landing?!

 

[DaveS]

Could we see a 9 engine suicide burn, using this same argument? Can you imagine the 360 deg camera for that landing?!

Only three engines have the TEA-TEB starter kit. So only those three can be reignited. Pre-launch, the TEA-TEB comes from a ground source.

 

[RisingFury]

Not at all...
You have two scenarios with the returning stage.

1) It's on target (x) so at the right moment..
a) The 3 engines fire at max thrust increasing the g-load 3x that of a single engine.
b) Deep throttling the engines to reduce g-load, accumulates 3x less efficiency than a single engine (more fuel needed)
c) A single engine fires higher up at max thrust (1x G-load), but only produces effiency losses at 1x

You don't deep throttle the engines, you keep high thrust. That's the point.

All aircraft frame lifetimes are determined by active lifetime use and g-loading => G-loading being the prominent factor.
(Maybe SpaceX are not interested in this at the moment)

Aircraft lifetimes are measured in decades and 100 000's of flight hours. If a reusable rocket gets a total of 10 hours of flight time with engines on, it'll be lucky. Structural concerns aren't as much of an issue as payload.

On top of that, the first stage can take the loads - it carries the second stage up.

2) The stage is slightly off target...
a) All three engines will have to start deep throttling to give it time to adjust it's flight path (efficiency losses as point b) above)
b) A single engine as in c) above, will give time for flight adjustments already, with the same resulting efficiency losses as c) above.

Or you could just make sure that your guidance is better.

SpaceX must be toying with ideas at the moment.

Either that, or they know better than the armchair rocket scientists. Something makes me think the latter is more likely, given that they employ thousands of people that actually know what they're doing.

 

[Andy44]

Either that, or they know better than the armchair rocket scientists. Something makes me think the latter is more likely, given that they employ thousands of people that actually know what they're doing.

Bingo.

 

[Col_Klonk]

You don't deep throttle the engines, you keep high thrust. That's the point.

I know that, and threw that in to give various options.
It will give less time for corrections.. and when guidance is inaccurate you'll see a rocket turn into a 'submarine'. :tiphat:

Aircraft lifetimes are measured in decades and 100 000's of flight hours. If a reusable rocket gets a total of 10 hours of flight time with engines on, it'll be lucky. Structural concerns aren't as much of an issue as payload.

Bend an aircraft/rocket frame and that's the end of it's life... bingo :lol:

Or you could just make sure that your guidance is better.

Statistically it won't be accurate all the time... thus the comparisons.

Either that, or they know better than the armchair rocket scientists. Something makes me think the latter is more likely, given that they employ thousands of people that actually know what they're doing.

A bit touchy when somebody questions the 'experts'
:thumbup:

 

[Andy44]

A bit touchy when somebody questions the 'experts'
:thumbup:

There's a reason why they're the "experts" and you're not. The "experts" have landed 3 rocket stages, including 2 on a pitching deck at sea. Even when they fail to make landings, they still manage to deliver the payloads to orbit. The only major failure had nothing to do with the recovery scheme.

The "experts" put a lot of work into the engineering behind all this and actually built working machines which function better on each iteration.

It's very clear that no matter what they accomplish, it will never be enough to satisfy you. You just continue to double down on your criticism and offer nothing concrete in turn.

Here's a smiley. :hello:

 

[RGClark]

Surprising fact about the Merlin 1D in an Elon tweet:

In reply to Luke
Elon Musk ‏@elonmusk May 6
@lukealization Max is just 3X Merlin thrust and min is ~40% of 1 Merlin. Two outer engines shut off before the center does.
https://twitter.com/elonmusk/status/728753234811060224

Never before heard of a Merlin being able to throttle down to 40%. I thought the lowest was 60%. Is this a new capability of the Merlins?

Bob Clark

 

[Urwumpe]

Never before heard of a Merlin being able to throttle down to 40%. I thought the lowest was 60%. Is this a new capability of the Merlins?

I thought those pintle injector engines can throttle down to 1/12.5th of the maximum thrust. or was that another engine family of SpaceX? :blink:

SpaceX said minimal 70% for the initial batch of Merlin-1D engines and 39% for later with the introduction of the vacuum version.

 

[Col_Klonk]

There's a reason why they're the "experts" and you're not. The "experts" have landed 3 rocket stages, including 2 on a pitching deck at sea. Even when they fail to make landings, they still manage to deliver the payloads to orbit. The only major failure had nothing to do with the recovery scheme.

The "experts" put a lot of work into the engineering behind all this and actually built working machines which function better on each iteration.

It's very clear that no matter what they accomplish, it will never be enough to satisfy you. You just continue to double down on your criticism and offer nothing concrete in turn.

Here's a smiley. :hello:

What are you working yourself up for..
All I asked was whether is would be more sensible to continue using a single engine descent than a 3 engine one.. and you couldn't even work that out.. and all you could do was 'play the man'. (tut tut)

Sorry!! that I just don't accept everything I see, or that's fed over the media.
:facepalm:

---------- Post added at 12:43 PM ---------- Previous post was at 12:31 PM ----------

Never before heard of a Merlin being able to throttle down to 40%. I thought the lowest was 60%. Is this a new capability of the Merlins?

It's not clear whether he means the total thrust of all three engines, at a minimum is equal to 40% of 1x Merlin (which would take the throttle down to approx 13% for each Merlin) ....Or the Merlin's minimum is 40% ??

 

[Unstung]

 

[BrianJ]

I'm scratching my head about this "three-engine landing burn". Are we sure Elon doesn't actually mean "three-engine reentry burn" ? (which would be the same as the last GTO mission). I've been staring at the videos of the landing but I can't really make out whether there is more than one engine operating. However, I can see that the landing burn takes about 13 seconds (from the light visible on the barge at 0:02 to landing at 0:15). If you assume the stage is about 25000kg, then the deceleration from 3 engines at full throttle would be about 80m/s^2 and increasing. If you burn for 13s, that implies a dV of >1000m/s, which implies that the stage is supersonic when it starts the burn. In my Orbiter world the 1st stage has a terminal velocity < 200m/s at 1km alt. If you throttle down to 40%, you get deceleration of about 25m/s^2, which is similar to a single engine at 100% throttle giving about 20m/s^2, and fits better with a 13s burn but doesn't seem to confer much advantage over a single-engine strategy.
I do wonder if Elon didn't mis-speak, or rather mis-tweet! Then again, he is the man on the spot so he should know.

 

[Kyle]

It's actually both! The landing burn starts off with three engines burning, but the two outer engines shut down just before landing, leaving the center engine burning until touchdown.