Author Topic: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion Thread 1  (Read 586065 times)

Offline the_other_Doug

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #720 on: 01/01/2018 02:33 PM »
From memory, it has been said that the first launch will be at 82% thrust.

And from actuality (not your memory), it will fly at 92% thrust.  Not 82%.  And not just "has been said," but tweeted by Elon Musk.

It's refreshing to see other people have even worse memories than I do... :)
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Online speedevil

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #721 on: 01/01/2018 03:12 PM »
From memory, it has been said that the first launch will be at 82% thrust.

And from actuality (not your memory), it will fly at 92% thrust.  Not 82%.  And not just "has been said," but tweeted by Elon Musk.

It's refreshing to see other people have even worse memories than I do... :)

Thanks. This would put required thrust for other engines at ~103% on a core, and remove capability for playing with the thrust levels to bring the centre of thrust to the same place.
One engine out on any or even all cores is probably quite managable.
More than one a core especially early in flight might lead to problems.
 

Offline Semmel

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #722 on: 01/01/2018 04:32 PM »
If the engine failure at the side booster happens before the throttle back of the center core, the situation may become hairy as the thrust difference between center and side boosters might not work with the structure of the heavy assembly. Maybe the center would have to throttle back prematurely.
The thrust to weight at launch should be around 1.5. With 11% thrust missing it should still be launching without much problems.

Online AncientU

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #723 on: 01/01/2018 05:08 PM »
If the engine failure at the side booster happens before the throttle back of the center core, the situation may become hairy as the thrust difference between center and side boosters might not work with the structure of the heavy assembly. Maybe the center would have to throttle back prematurely.
The thrust to weight at launch should be around 1.5. With 11% thrust missing it should still be launching without much problems.

One engine is 3.7%
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Offline Gotorah

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #724 on: 01/01/2018 05:30 PM »
This is not comparing Falcon Heavy 1 vs Falcon Heavy 2. It is comparing right side vs left side for balanced thrust. If one is lost on one side, those remaining on that side can be throttled up a few percent and if needed the ones on the opposite booster can be throttled down a few.  I bet a dollar to a doughnut that the SpaceX Flight Software engineers have it all programmed. All we have to do is watch in awe !

Offline Semmel

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #725 on: 01/01/2018 05:50 PM »
If the engine failure at the side booster happens before the throttle back of the center core, the situation may become hairy as the thrust difference between center and side boosters might not work with the structure of the heavy assembly. Maybe the center would have to throttle back prematurely.
The thrust to weight at launch should be around 1.5. With 11% thrust missing it should still be launching without much problems.

One engine is 3.7%

If center and opposite core need to match the loss of thrust, it's 11.

Online Lars-J

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #726 on: 01/01/2018 07:22 PM »
If the engine failure at the side booster happens before the throttle back of the center core, the situation may become hairy as the thrust difference between center and side boosters might not work with the structure of the heavy assembly. Maybe the center would have to throttle back prematurely.
The thrust to weight at launch should be around 1.5. With 11% thrust missing it should still be launching without much problems.

One engine is 3.7%

If center and opposite core need to match the loss of thrust, it's 11.

Since running at 92%, the remaining engines in a core can throttle up to compensate - at any point in the flight.

Offline Johnnyhinbos

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #727 on: 01/01/2018 08:34 PM »
I think this is an interesting conversation. However I believe it would be better suited to be on the general Falcon Heavy discussion thread rather than on this mission specific thread?
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Offline Nomadd

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #728 on: 01/01/2018 09:25 PM »
I think this is an interesting conversation. However I believe it would be better suited to be on the general Falcon Heavy discussion thread rather than on this mission specific thread?
The 92% thrust level might only be for this mission.

Online gongora

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #729 on: 01/01/2018 09:33 PM »
The 92% thrust level might only be for this mission.

I've wondered if the thrust "reduction" on this mission is just because he's giving thrust numbers for FH Block 5 and this vehicle is running at Block 3 levels.
« Last Edit: 01/01/2018 09:33 PM by gongora »

Online ZachS09

The 92% thrust level might only be for this mission.

I've wondered if the thrust "reduction" on this mission is just because he's giving thrust numbers for FH Block 5 and this vehicle is running at Block 3 levels.

Maybe this Falcon Heavy is running at 92% of the Block 3 thrust level, which is about 4.14 million pounds as opposed to 4.5 million pounds.
Because the Falcon Heavy Test Flight was successful, it has inspired thousands of people to consider changing the future of space travel.

Offline CyndyC

The 92% thrust level might only be for this mission.

I've wondered if the thrust "reduction" on this mission is just because he's giving thrust numbers for FH Block 5 and this vehicle is running at Block 3 levels.

Here's the tweet that was threaded with the photos. Agree the older side cores wouldn't be capable of taking this flight to 100% of 5.1

https://twitter.com/elonmusk/status/943590152181448704
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Online butters

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #732 on: 01/02/2018 12:22 AM »
It's safe to say that this will be only Falcon Heavy ever to use pre-Block 5 cores. After re-scoping Falcon Heavy to chase F9 iterations to v1.1 and then to FT specifications, now there's Block 5 coming. SpaceX needed to make the decision whether to bring the FH demo vehicle up to Block 5 spec with bolted octowebs etc. and accept further delays to a program already repeatedly delayed by F9 iteration churn -- or to simply launch the damn thing already even though it will end up being a one-off. This will forever be the unique "albino" FH demo vehicle.

Offline jpo234

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #733 on: 01/02/2018 12:55 AM »
It's safe to say that this will be only Falcon Heavy ever to use pre-Block 5 cores. After re-scoping Falcon Heavy to chase F9 iterations to v1.1 and then to FT specifications, now there's Block 5 coming. SpaceX needed to make the decision whether to bring the FH demo vehicle up to Block 5 spec with bolted octowebs etc. and accept further delays to a program already repeatedly delayed by F9 iteration churn -- or to simply launch the damn thing already even though it will end up being a one-off. This will forever be the unique "albino" FH demo vehicle.
As mentioned in the STP - 2 thread, there might only be one other FH. Reason: B5 cores should have more flights in them than there are FH missions on the manifest.
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Offline allio

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #734 on: 01/02/2018 01:24 AM »
First time poster.

Regarding the 92% thrust.

Is this not simply because it's an LV capable of lifting 16000kg to MTO that is in fact only lifting off with a 1500kg payload. An LV that can hoist 60000kg off the pad only lifting 1500kg (or whatever the weight of the tesla + mounting is) would go into a massive over g situation within seconds lifting that light a load. As it is I would imagine the 92% will only be until it clears the pad, then a massive throttle down until well past max q and throttling down all the way up. A lot of rockets auto throttle to constant 5g acceleration for structural reasons IIRC. This baby in its maiden config will have plenty of horses to spare.

Or am I wrong on that?

Edit: Does anyone know if there is extra ballast in the payload to counteract this? Or what the actual final payload weight actually will be?
« Last Edit: 01/02/2018 01:29 AM by allio »

Online Comga

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #735 on: 01/02/2018 02:42 AM »
First time poster.

Regarding the 92% thrust.

Is this not simply because it's an LV capable of lifting 16000kg to MTO that is in fact only lifting off with a 1500kg payload. An LV that can hoist 60000kg off the pad only lifting 1500kg (or whatever the weight of the tesla + mounting is) would go into a massive over g situation within seconds lifting that light a load. As it is I would imagine the 92% will only be until it clears the pad, then a massive throttle down until well past max q and throttling down all the way up. A lot of rockets auto throttle to constant 5g acceleration for structural reasons IIRC. This baby in its maiden config will have plenty of horses to spare.

Or am I wrong on that?

Edit: Does anyone know if there is extra ballast in the payload to counteract this? Or what the actual final payload weight actually will be?

Not exactly
Most of the mass being carried by by the first stage, or the core in the case of Heavy, is the fueled second stage. Others can give you the numbers, but in general having a very light payload  doesnít change things dramatically until well into the second stage burn.
Looked at it another way, running at 92% thrust cuts it by about a half million pounds. The payload is not 100,000 pounds less than the advertised maximum. Even at the max acceleration before staging itís a smaller effect than the throttling. 
(Sorry for the archaic units. Trying to use one for all and I know the thrust in pounds.)
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline CyndyC

Here's a reminder of the thrust profile, tweeted way back in July. Took a minute to find again but I knew I'd seen it somewhere

https://twitter.com/elonmusk/status/890810308326940672
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Offline allio

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #737 on: 01/02/2018 03:31 AM »
there is abou 135000 lbs difference between max payload and that tesla :)

I may have overstated the throttling earlier :) but maybe not, as a quick read indicates that the fully loaded shuttles would be running at 65% thrust by max q, with the SRB's deigned to derate their thrust to 66% through propellant design after 50 seconds as well. And that's for a full load.

I believe the shuttle and falcon max q points are similar(13-14km).

at max q i'd expect the boosters in the FH configuration to have burned over half their fuel, with the core down by a third of it's fuel, at which point that 135k lbs in the nose becomes a much greater ratio of the remaining weight of the craft, which in turn adversely affects TWR forcing more throttling etc.

I thought throttling was used to maintain a specific TWR so as not to accelerate too early in the flight where the atmosphere is thicker where overspeed can lead to instability, extra structural loading and heating from excess aerodynamic resistance etc.

I would imagine that the core booster will throttle down earlier to offset the light payload.

anyway. just my basic understanding of rocketry

cyndy- That tweet references how the rocket thrust profile will work in general, not how the individual mission thrust profile will be deployed.

Offline Stan-1967

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #738 on: 01/02/2018 05:11 AM »
there is abou 135000 lbs difference between max payload and that tesla :)

I believe the shuttle and falcon max q points are similar(13-14km).

at max q i'd expect the boosters in the FH configuration to have burned over half their fuel, with the core down by a third of it's fuel, at which point that 135k lbs in the nose becomes a much greater ratio of the remaining weight of the craft, which in turn adversely affects TWR forcing more throttling etc.

I thought throttling was used to maintain a specific TWR so as not to accelerate too early in the flight where the atmosphere is thicker where overspeed can lead to instability, extra structural loading and heating from excess aerodynamic resistance etc.

I would imagine that the core booster will throttle down earlier to offset the light payload.

anyway. just my basic understanding of rocketry


I think you have the right concept about how throttling is used to manage Max-Q, heating, & loading, however you are in error regarding the importance you assign to the weight of the Tesla in comparison to the largest theoretical payload FH can carry when you focus on the 135k lbs difference. 

Keep in mind there is no payload on the FH manifest that exceeds 8t ( 17.6k lbs ).  The largest payload that Delta V Heavy can lift to GTO is around 14.2t ( 31.4k lbs), & FH hopes to enter the market for this payload class.  That is the ballpark maximum foreseeable payload for FH.    Also understand that the existing F9 or FH PAF can only accomodate up to 24k lbs.   The max payload of FH may never be used.

The thrust profile for the Tesla will not be radically different than what will be used for FH GTO launches.

Online hkultala

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Re: SpaceX FH : Falcon Heavy Demo : early 2018 : Discussion
« Reply #739 on: 01/02/2018 06:06 AM »
there is abou 135000 lbs difference between max payload and that tesla :)

No, there is not.

The 63 tonne number in spacex web site is for LEO (total delta-v about 9km/s), and even that number is notional as there actually currently is no a payload adapter capable of holding that mass.

The roadster will go to non-optimal trajectory that goes near Mars(over 13km/s needed).
On optimal trajectory, FH payload to Mars is about 17 tonnes.







I may have overstated the throttling earlier :) but maybe not, as a quick read indicates that the fully loaded shuttles would be running at 65% thrust by max q, with the SRB's deigned to derate their thrust to 66% through propellant design after 50 seconds as well. And that's for a full load.

I believe the shuttle and falcon max q points are similar(13-14km).
[/quote]

FH should initially accelerate much faster than STS, as it has considerable higher T/W ratio at liftoff.

On the other hand, to make booster RTLS use less delta-v, FH will have higher trajectory than STS, making it also rise from the dense air to the thinner air earlier.

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