Author Topic: F9 - S2 reusable modification as evolution steps to BFS(ITS)  (Read 39089 times)

Offline Lars-J

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Of course you can use retro-propulsion to slow down, but why would you want to? A heat shield is much much lighter way to slow down in an atmosphere at high speed. S2 will be using aerodynamic drag to slow down until just before landing.

Two reasons:
1. One Mars you will impact the ground at supersonic speed using only your heat shield
2. On Earth, the atmosphere is so thick at surface level that if you come in from the wrong angle, it is like hitting a wall. Too much G forces. (Which is why the F9 first stage does a braking burn)

Offline livingjw

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Of course you can use retro-propulsion to slow down, but why would you want to? A heat shield is much much lighter way to slow down in an atmosphere at high speed. S2 will be using aerodynamic drag to slow down until just before landing.

Two reasons:
1. One Mars you will impact the ground at supersonic speed using only your heat shield
2. On Earth, the atmosphere is so thick at surface level that if you come in from the wrong angle, it is like hitting a wall. Too much G forces. (Which is why the F9 first stage does a braking burn)

1. Aero-braking with very modest L/D and low ballistic coefficient will allow you to successfully enter the Martian atmosphere with relatively low max g's and slow to  about 1500 m/s. The final landing burn reduces this to zero. 
2. The F9 does a retro braking burn to limit its maximum reentry Mach to limit heating.

Online wannamoonbase

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I'm really excited to hear EM confirm yesterday that they are going to be pursuing US reuse.

I'm looking forward to following the development and see what technology they come up with.
Excited to be finally into the first Falcon Heavy flow, we are getting so close!

Offline mikelepage

I'm really excited to hear EM confirm yesterday that they are going to be pursuing US reuse.

I'm looking forward to following the development and see what technology they come up with.

There was actually quite a lot to unpack in his statements yesterday.

He used to think that a "base heat shield plus landing legs" would be the best way to land on Mars, he no longer thinks this, so that's part of the reason why they nixed the landing legs/propulsive landing with Dragon 2.  Maybe that's referring specifically to landing legs that pop out of the heat shield and not ruling out F9 Booster style fold out landing legs, but still, it was interesting to hear him say it.

Very curious now to hear the ITS update talk.

Offline mikelepage



Btw.  I feel like the last dozen comments or so would be better informed if you watched from 15 minutes to 21 minutes of this video.


Offline livingjw

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Thank you. I watched it again. 5% reduction in landing fuel with more control over landing point, if done with peripheral rockets as apposed to center mounted engines. Is that your take on it as well?

John

Offline mikelepage

Thank you. I watched it again. 5% reduction in landing fuel with more control over landing point, if done with peripheral rockets as apposed to center mounted engines. Is that your take on it as well?

John

Yes. Except high level control over landing point is not just a "desirable", but absolutely critical for all human architectures, because of pre-deployed assets.
 
Studies have been done which show that a rocket firing from the center does not increase drag, but that drag can be increased if rockets are fired from around the perimeter. I cannot remember if heating was reduced. I will see if I can't dig up the study.

No, no, no. People keep misunderstanding this. This observed effect was only at low throttle levels. It does not mean that braking is impossible with a center mounted engine. Surely you have observed the F9 braking burns?

Those do not rely to any significant extent on drag. Drag is most important in the hypersonic and high supersonic phases of entry.

Right. But the problem is that some people interpreted the paper on drag reduction using a central thruster as proof that retro propulsion at supersonic speeds is impossible with a central thruster. ("you'll never get thrust, only reduce drag!")

Why wouldn't you get thrust? That defies conservation of momentum. I've never seen this claim before.

Hence my suggestion above Envy887, to watch the video (actually from 14 minutes - 21 minutes) - low level thrust displaces the bow shock from the vehicle, paradoxically reducing drag by more than the force of the thrust.

But most importantly, the video was showing that supersonic retropropulsion in specific cases can also increase the area of the bow shock front and double the effective thrust through drag when performed during this particular energy envelope.  This saves 5% landing fuel and increases cross range control to upwards of 100km - which will be virtually necessary if you want to land next to pre-deployed assets as will be the case in human landings.

Having said all that, this also answers my earlier question: The heat shield solves a completely different problem from the one supersonic retropropulsion solves, because peak heating occurs at much earlier in EDL, at mach 18 or so - when the vehicle is still moving at velocities far too fast to start retropropulsion (or deploy IADs for that matter).
 

Offline Robotbeat

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Of course you can use retro-propulsion to slow down, but why would you want to? A heat shield is much much lighter way to slow down in an atmosphere at high speed. S2 will be using aerodynamic drag to slow down until just before landing.

Two reasons:
1. One Mars you will impact the ground at supersonic speed using only your heat shield
2. On Earth, the atmosphere is so thick at surface level that if you come in from the wrong angle, it is like hitting a wall. Too much G forces. (Which is why the F9 first stage does a braking burn)

1. Aero-braking with very modest L/D and low ballistic coefficient will allow you to successfully enter the Martian atmosphere with relatively low max g's and slow to  about 1500 m/s. The final landing burn reduces this to zero. 
2. The F9 does a retro braking burn to limit its maximum reentry Mach to limit heating.
1) 1500m/s is definitely supersonic. So you need to still do Supersonic Retropropulsion.
2) Not just heating. Forces, too.
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Offline mikelepage

Getting back to the thread topic, I do wonder if, now that they've invested so much into improving the Falcon Heavy core stage, the way to test all the ITS technologies will take the form of a Falcon Heavy specific upper stage.

I'm picturing something spade shaped, which would sit across the top of all three FH boosters, since this would improve the aerodynamics/vibration environment during the boost phase.  If you put the heat shield on one face of the "spade" you get a much larger area across which you spread the energy of reentry, with recessed superdracos and legs to allow a belly landing. 

The idea would be to keep the Merlin Vac, RP1/Lox tankage exactly the same, but not simply have wings, but rather use the extra volume for superdraco tanks + landing apparatus, and potentially payload bays or RP1/Lox tanks to practice on-orbit refuelling.

PS: I made a crappy photoshop mockup to show it better.

Offline Req

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My impression, given the timing of Musk tweeting that they were again thinking about trying S2 recovery, was that it will just be TPS re-entry to a parafoil(drogues first?), then bouncy castle landing.

Beyond just the timing of the statement, this forum has been all up and down the topic of propulsive/winged/etc S2 recovery over the years, and it's hard to imagine any of those methods delivering enough payload to be used except for very infrequently, making it hard to justify development.  Superdracos in particular would be a real showstopper, they aren't going to add hypergolic handling to the flow for every rocket whether it has Dragon or not.  The problem has never been that it's not possible, just that Falcon 9 at it's size would have to eat up most of it's payload capability to do it.

Using the fairing recovery method, all you're taking away from the payload is the mass of the PICA-X and any flaps/aero surfaces that are needed, the weight of the propellant to do the de-orbit burn, the weight of systems to keep the stage alive(all of this so far is needed for ANY S2 recovery method), and the weight of the chutes.  It's a lot easier to imagine that closing both in payload penalty and development resources, especially given the already existing fairing recovery program.  It's also a method that could be kitted relatively easily, rather than having substantially different second stages for expendable and reusable.
« Last Edit: 07/29/2017 06:02 AM by Req »

Offline mikelepage

Getting back to the thread topic, I do wonder if, now that they've invested so much into improving the Falcon Heavy core stage, the way to test all the ITS technologies will take the form of a Falcon Heavy specific upper stage.

I'm picturing something spade shaped, which would sit across the top of all three FH boosters, since this would improve the aerodynamics/vibration environment during the boost phase.  If you put the heat shield on one face of the "spade" you get a much larger area across which you spread the energy of reentry, with recessed superdracos and legs to allow a belly landing. 

The idea would be to keep the Merlin Vac, RP1/Lox tankage exactly the same, but not simply have wings, but rather use the extra volume for superdraco tanks + landing apparatus, and potentially payload bays or RP1/Lox tanks to practice on-orbit refuelling.

PS: I made a crappy photoshop mockup to show it better.

I probably should have said it explicitly... The implication of this would be that they are planning single stick and triple stick variants of the ITS, like was shown in the "Falcon X" and "Falcon X heavy" diagrams that used to appear in SpaceX material.

The reason I think they might do this is because currently, the ITS "tanker" variant is supposed to do several trips in order to perform on orbit refuelling of the ITS "crew" variant that will go to Mars.  Perhaps instead, they could have a single stick ITS "crew", and a larger, triple stick ITS "tanker" (both reusable of course).

EDIT: the specific benefit is that the fuel which the tanker is taking to the ITS crew version is only required to stay cryogenic for the hours/days it would take to make a single trip up, refuel the crew craft, perform checks, then go for trans-Mars injection.

EDIT 2: So the practice exercise would be that an ordinary Falcon 9 takes up a multi ton probe - A Jupiter probe with Europa lander, or even a Uranus/Neptune probe - to LEO or higher, and then a Falcon Heavy launches soon afterwards with a "tanker" second stage.  This rendezvouses with the probe+upperstage, refuels it, which then allows a direct injection/fast transit to the outer planets for a fraction of the cost.
« Last Edit: 07/29/2017 07:43 AM by mikelepage »

Offline Robotbeat

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Musk has basically ruled out triple core ITS.
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Offline mikelepage

Musk has basically ruled out triple core ITS.

Yeah I know.  But those statements were only considering the rocket science/physics of the problem.

The pros behind going back to a slightly smaller core diameter, triple stick ITS would have nothing to do with the rocket science, and everything to do with the logistics/business case.  Say that 1) the process of building FH has given them confidence that they've solved the unique issues with the triple stick config, and 2) they found the business case/logistics for reusable triple core with smaller core diameter (7-9m) is actually a cheaper/easier way to get a large mass to orbit, and 3) being able to launch all the fuel for the Mars vehicle in one go solves the cryogenic-fuel-transfer-in-orbit problem.

I dunno (I'm not set on this), I just think Elon's shown previously that he'll make trade-offs like that (favouring the less efficient but logistically simpler solution).

Offline Lars-J

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F9 - S2 reusable modification as evolution steps to BFS(ITS)
« Reply #53 on: 07/30/2017 06:58 AM »
I dunno (I'm not set on this), I just think Elon's shown previously that he'll make trade-offs like that (favouring the less efficient but logistically simpler solution).

Whether or not you are set on it doesn't matter. A multi-core ITS isn't happening.

ITS is difficult enough as it is. The complexity difference between F9 and FH should tell you the rest.
« Last Edit: 07/30/2017 06:14 PM by Lars-J »

Offline MP99




I probably should have said it explicitly... The implication of this would be that they are planning single stick and triple stick variants of the ITS, like was shown in the "Falcon X" and "Falcon X heavy" diagrams that used to appear in SpaceX material.

The reason I think they might do this is because currently, the ITS "tanker" variant is supposed to do several trips in order to perform on orbit refuelling of the ITS "crew" variant that will go to Mars.  Perhaps instead, they could have a single stick ITS "crew", and a larger, triple stick ITS "tanker" (both reusable of course).

In addition to the other comments, a BFR centre stage would RTLS, not land on a barge like FH centre core. This would reduce the tanker payload to orbit.

Cheers, Martin

Offline Ronsmytheiii

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Still think the simplest/easiest way for a second stage reuse is going to be an evolution of NASA's HIAD program, ULA is trying to use it for Vulcan 1st stage engine recovery so you dont need a massive redesign to "bolt it on"

Plus will eventually be useful for landing large payloads on Mars.

https://www.nasa.gov/directorates/spacetech/game_changing_development/HIAD/index.html

Edit screenshot from https://www.nasa.gov/sites/default/files/atoms/files/gcd_industryday_hiad.pdf
« Last Edit: 07/30/2017 08:38 AM by Ronsmytheiii »
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Offline AncientU

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Still think the simplest/easiest way for a second stage reuse is going to be an evolution of NASA's HIAD program, ULA is trying to use it for Vulcan 1st stage engine recovery so you dont need a massive redesign to "bolt it on"

Plus will eventually be useful for landing large payloads on Mars.

https://www.nasa.gov/directorates/spacetech/game_changing_development/HIAD/index.html

Edit screenshot from https://www.nasa.gov/sites/default/files/atoms/files/gcd_industryday_hiad.pdf

Is there one stitch of evidence for SpaceX following this tech path, or just wishful thinking?
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Offline spacenut

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I would hope they would use a sub-scale Raptor vacuum in place of the Merlin Vacuum and maybe widen the upper stage.  This would get the return hardware on and still get the same sized payloads to orbit.  It could also be equipped with on orbit refueling to test landing use in space. 

The other way is to stretch the upper stage, but this may make it only for use with FH due to length and stresses on F9.  At least 3 cores would make it through the heavy atmosphere and keep each other from bending. 

Online wannamoonbase

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I would hope they would use a sub-scale Raptor vacuum in place of the Merlin Vacuum and maybe widen the upper stage.  This would get the return hardware on and still get the same sized payloads to orbit.  It could also be equipped with on orbit refueling to test landing use in space. 

The other way is to stretch the upper stage, but this may make it only for use with FH due to length and stresses on F9.  At least 3 cores would make it through the heavy atmosphere and keep each other from bending. 

I think that would be exciting too, but many here think that is unlikely, that SpaceX will move onto the next vehicle generation.  Lots of changes to the pad equipment, new process and procedure and likely still 2 years from having a Raptor engine qualified.

US reuse, even with the M-Vac, which would be interesting from a capability and technology perspective.  But even that may eat up resources that delays the next generation of vehicle. 

Edit: I'm leaning toward them not doing US reuse.  The business case for $ payback with the F9/FH vehicles is harder to close. 
« Last Edit: 07/31/2017 03:30 PM by wannamoonbase »
Excited to be finally into the first Falcon Heavy flow, we are getting so close!

Offline titusou

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This is old SpaceX video (which not available on SpaceX's own channel) which show what they were thinking long time ago.


Having stage2 nose covered with heatshield and rentry as nose-first, as video suggested, well... 180deg flip back to nose-up at supersonic speed for landing burn would be quite challenging I guess.

Some other concept (not by SpaceX) has SuperDraco side-mounted (facing up) on stage2 for landing burn, which means Hydrazine engine next to LOX tank... well... :)

My personal favor would be a ITS upper stage liked design. Heatsheild one side, reentry as side-first, and then 90deg flip for landing burn. ITS upper stage is basically symmetric at launch. The extended heatshield under the engines, were merged with stage1 to make it symmetric at launch, asymmetric after separation.


Whatever design it will be, I hardly see any chance to have asymmetric shaped stage2, as it will generate complex force during liftoff. The symmetric shaped stage2 will make thing much less complicated.

Titus

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