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

[Jeff Lerner]

Could it be that the vehicle flew "hotter" than expected (simulations only tell you so much, the wind was blowing at altitude, etc.) during the booster/core phase, requiring the core to use up more propellant than expected while trying to aim for OCISLY?  Just a thought.

 - Ed Kyle

Quite possibly. Musk said they were 0.3 sigma off on propellant usage, which might have been enough to run out of props on the landing burn.

In this scenario, does the stage recognize it can't make it to the ship and bail on the landing, plopping into the ocean ?...I know this hasn't happened in the past...

[AnnK]

I wonder if the USAF paid part of the bill to test the fairings? I am wondering why the heavy is being used to launch Arab Sat A? At 6000 kg of mass it is well overkill.

[nicp]

I'm wondering if Elon forgot and left his wallet in the glove compartment.

[abaddon]

I wish someone had asked Elon if the live view would remain public up to and through the TMI burn.  Guess we will find out in a couple of hours.

[Mark Lattimer]

I think I read earlier that the final burn is scheduled for 9:45pm, is that correct? And, does anyone know the scheduled duration of the burn?

[Space Ghost 1962]

Could it be that the vehicle flew "hotter" than expected (simulations only tell you so much, the wind was blowing at altitude, etc.) during the booster/core phase, requiring the core to use up more propellant than expected while trying to aim for OCISLY?  Just a thought.

 - Ed Kyle

Quite possibly. Musk said they were 0.3 sigma off on propellant usage, which might have been enough to run out of props on the landing burn.

In this scenario, does the stage recognize it can't make it to the ship and bail on the landing, plopping into the ocean ?...I know this hasn't happened in the past...

Like with land, it targets off target, to the side disposal areas (with land offshore), so if it doesn't make landing burn (which adjusts to target), it disposes without destroying landing area/collateral damage.

[LouScheffer]

How does the core run out of igniter ??...there are only so many engines to be restarted and they only restart so many times...Spacex has done this many time before and they hadn't run out of igniter...

Does this imply they had a problem restarting a couple of the engnes a number of times on this flight and ran out of,igniter,??

I'm just speculating on how this could happen ??

It's unlikely, for the reasons you stated. Each restart uses a known quantity of TEA/TEB.

We have, however, seen stages run out of propellant during the landing burn.

My guess - the rocket does not take the same amount of TEA/TEB, it goes until it ignites.  Normally that's the same amount.  However in this case the rocket is going faster, tail first, when the re-entry burn happens.  That causes more back pressure -> harder to start -> takes more TEA/TEB.    So it runs out, the stage can't ignite for the 3 engine burn, and kablooey.

This theory is based entirely on the hypothesis that SpaceX are not idiots, so there must be some reason it used more TEA/TEB.

[Kabloona]

Could it be that the vehicle flew "hotter" than expected (simulations only tell you so much, the wind was blowing at altitude, etc.) during the booster/core phase, requiring the core to use up more propellant than expected while trying to aim for OCISLY?  Just a thought.

 - Ed Kyle

Quite possibly. Musk said they were 0.3 sigma off on propellant usage, which might have been enough to run out of props on the landing burn.

In this scenario, does the stage recognize it can't make it to the ship and bail on the landing, plopping into the ocean ?...I know this hasn't happened in the past...

It's possible in theory, if it runs out of propellant early enough in the landing burn. But in practice, if it runs out at too low an altitude and/or too high speed, there may not enough time/control authority with the grid fins to take evasive action.

More likely they just got lucky and ran out of propellant early enough that the stage missed the ASDS. Remember, it's approaching from an angle, not from straight above.

[tvg98]

Could it be that the vehicle flew "hotter" than expected (simulations only tell you so much, the wind was blowing at altitude, etc.) during the booster/core phase, requiring the core to use up more propellant than expected while trying to aim for OCISLY?  Just a thought.

 - Ed Kyle

Quite possibly. Musk said they were 0.3 sigma off on propellant usage, which might have been enough to run out of props on the landing burn.

In this scenario, does the stage recognize it can't make it to the ship and bail on the landing, plopping into the ocean ?...I know this hasn't happened in the past...

Like with land, it targets off target, to the side disposal areas (with land offshore), so if it doesn't make landing burn (which adjusts to target), it disposes without destroying landing area/collateral damage.

Sounds like these zones need to be adjusted then, since it took out two engines on OCISLY. Hopefully this won't an impact on the next mission.

[Kabloona]

How does the core run out of igniter ??...there are only so many engines to be restarted and they only restart so many times...Spacex has done this many time before and they hadn't run out of igniter...

Does this imply they had a problem restarting a couple of the engnes a number of times on this flight and ran out of,igniter,??

I'm just speculating on how this could happen ??

It's unlikely, for the reasons you stated. Each restart uses a known quantity of TEA/TEB.

We have, however, seen stages run out of propellant during the landing burn.

My guess - the rocket does not take the same amount of TEA/TEB, it goes until it ignites.  Normally that's the same amount.  However in this case the rocket is going faster, tail first, when the re-entry burn happens.  That causes more back pressure -> harder to start -> takes more TEA/TEB.    So it runs out, the stage can't ignite for the 3 engine burn, and kablooey.

This theory is based entirely on the hypothesis that SpaceX are not idiots, so there must be some reason it used more TEA/TEB.

Do we know for a fact that it was the TEA/TEB that ran out? I missed that somewhere.

I would have guessed the TEA/TEB injection runs on a preset time of "valves open" that injects a known quantity of TEA/TEB, with the timing preset so that the TEA/TEB burn is long enough to cover, say, 3-sigma dispersions in engine ignition timing, with tanks sized accordingly. That way you design the system to pretty much guarantee you can't run out of TEA/TEB.

[sewebster]

Do we know for a fact that it was the TEA/TEB that ran out? I missed that somewhere.

That was certainly the impression I got from the press conference...

[AlanF]

I keep looking, but can’t find it. So we know when the TMI burn will occur?  Sorry for being dense.

[PeterAlt]

So, how long will it stay attached to the second stage? If it’s not attached when it makes the first Mars flyby, we won’t have any of the cool pics of the Roadster against the backdrop of the Martian atmosphere, or will we?

It's not flying by Mars and the stage will be long dead by then.

So, the backdrops of Earth and the sun are the best (and only) celestial backdrops we’ll get?

I'm pretty sure that in the news conference, Musk said that the batteries are only good for six hours (maybe it was eight). After that, it will be inert. So no ability to take photos with other backdrops, regardless of it;s location.

This is really disappointing. How much extra would it have been to attach a communication and power package to the Roadster?

[Gliderflyer]

Do we know for a fact that it was the TEA/TEB that ran out? I missed that somewhere.

That was certainly the impression I got from the press conference...

The audio is hard to hear, but you can hear him say triethylborane.

[Kabloona]

Do we know for a fact that it was the TEA/TEB that ran out? I missed that somewhere.

That was certainly the impression I got from the press conference...

The audio is hard to hear, but you can hear him say triethylborane.

Thanks, I missed that part.

[envy887]

I keep looking, but can’t find it. So we know when the TMI burn will occur?  Sorry for being dense.

Presumably at the next perigee, which will be at 9:45 pm EST.

[Herb Schaltegger]

T+5 hours. I wonder how SpaceX is getting continuous coverage? TDRS?

My supposition is that since this extended-coast part of the mission is to demonstrate the ability to satisfy DOD requirements, SpaceX has arranged to use TDRS for continuous telemetry and video coverage, since presumably a priority national security payload going straight to GEO would have access to or might itself require such coverage.

[envy887]

This is really disappointing. How much extra would it have been to attach a communication and power package to the Roadster?

Communicating at Mars' distances is really hard, so a lot extra. This is a mass simulator, remember - not a science payload.

Plus it wouldn't get anywhere near Mars for years. There's not much else to see in interplanetary space.

[envy887]

T+5 hours. I wonder how SpaceX is getting continuous coverage? TDRS?

My supposition is that since this extended-coast part of the mission is to demonstrate the ability to satisfy DOD requirements, SpaceX has arranged to use TDRS for continuous telemetry and video coverage, since presumable a priority national security payload going straight to GEO would have access to or might itself require such coverage.

They lost coverage for about 10 minutes before and after the last perigee. When it's not close to perigee, it's above the horizon for much of the world (see how much of the surface is visible in the Tesla cams), so they might be using ground tracking and comms.

So we might not get to see the TMI burn, if it does happen at the next perigee.

[PeterAlt]

This is really disappointing. How much extra would it have been to attach a communication and power package to the Roadster?

Communicating at Mars' distances is really hard, so a lot extra. This is a mass simulator, remember - not a science payload.

Plus it wouldn't get anywhere near Mars for years. There's not much else to see in interplanetary space.

Could they have used one of the existing satellites orbiting Mars for a relay?