Author Topic: Mini-BFR ship F9 Second Stage  (Read 42824 times)

Offline BeamRider

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Re: Mini-BFR ship F9 Second Stage
« Reply #260 on: 11/10/2018 03:15 PM »
It is my understanding that BFS Mars reentry from Earth-Mars transit will require a nose DOWN (but still positive AOA) attitude in order to generate a lift component towards Mars.  This is to prevent BFS from re-exiting the atmosphere, because the incoming velocity will be higher than LMO velocity. OTOH, I am assuming reentry from LMO (or LEO) will be done nose UP, ala STS. Are these assumptions correct?

Offline speedevil

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Re: Mini-BFR ship F9 Second Stage
« Reply #261 on: 11/10/2018 03:38 PM »
It is my understanding that BFS Mars reentry from Earth-Mars transit will require a nose DOWN (but still positive AOA) attitude in order to generate a lift component towards Mars.  This is to prevent BFS from re-exiting the atmosphere, because the incoming velocity will be higher than LMO velocity. OTOH, I am assuming reentry from LMO (or LEO) will be done nose UP, ala STS. Are these assumptions correct?

We strictly speaking have no information on the Mars entry of the most recent version.
However, if you look at the forces initially entering Mars atmosphere, if you're coming in at a tangent, as you pretty much must be if you don't want to decelerate at 40G, if you are going 7km/s, and your path is curving to remain in the atmosphere, you pretty much have to be experiencing around 1G 'down', meaning the lift vector has to be pointed down, meaning the vehicle has to be inverted.

The Dear Moon presentation showed an entry from LEO, in which case the vehicle is never inverted, and enters pretty much like STS.

Entry from >LEO will look pretty much like the martian atmospheric entry, beginning with the lift vector down, as you skim the very edge of the atmosphere generating downforce to keep you inside the atmosphere. Then you flip the craft over at around LEO speed, and begin a STS-like reentry.

Up until ~20km/mach 3 or so, when the shuttle begins 'aerodynamic' flight, and BFS continues to slow to a stop horizontally, and fall vertically, going subsonic 10km up, and slowing aerodynamically to 100m/s or so.
« Last Edit: 11/10/2018 03:39 PM by speedevil »

Offline Rocket Science

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Re: Mini-BFR ship F9 Second Stage
« Reply #262 on: 11/10/2018 04:14 PM »
A lifting entry vehicle can use negative lift to hold itself in the atmosphere at greater than orbital speeds while decelerating; it can therefore re-enter at a shallower angle.
« Last Edit: 11/10/2018 04:31 PM by Rocket Science »
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Offline dlapine

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Re: Mini-BFR ship F9 Second Stage
« Reply #263 on: 11/10/2018 04:25 PM »
If the initial testing on a modified F9 2nd stage goes reasonably well, would a FH be able to loft a 5M reusable version of a mini BFS that carried sufficient extra fuel to compensate for the re-usability but maintained a reasonable payload to LEO? To enable a cost effective method to deploy Starlink in the near future before BFS is ready?

I'm not suggesting that 5m 2nd stage would be built before preliminary tests were carried out. And yes, I'm ignoring the design requirements that would allow a mini bfs on a FH to land propulsively for rapid reuse.

Could a FH lift a wider 2nd stage carrying sufficient additional fuel to support the necessary re-use elements (TPS, clamshell, fins, legs and retropropulsion) in a mini BFS design?

Offline Hotblack Desiato

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Re: Mini-BFR ship F9 Second Stage
« Reply #264 on: 11/10/2018 05:41 PM »
This is a very interesting bit of news, and I have also seen the video from Scott Manley. I think, this could be one of the options for a practical use of this returnable second stage.

So the problem is, the structure will be bottom-heavy because the engine is there, and everything else is mostly an empty tank.

What they could do: F9-Second Stage + Dragon 2 on a longer trunk, and with a door that can be opened (in the Dragon 2 trunk). A heatshield over half the system, like it's done with BFS. For landing, the system could use the Dragon 2s super-dracos (maybe with extra super dracos in the botton with the merlin vac)

This spaceship could be used to bring cargo and possibly even crew to the ISS or other future stations, with two major advantages: low price and high safety for the crew. In case of a failure, the whole dragon 2 can split away, abandoning the rest of the craft. If everything works as intended, this could bring flight costs down to a few million $ per launch, for up to 8 crew. If there is a failure, it'll still just be as expensive as a dragon 2 flight, since the dragon 2 itself is the most expensive part of this craft.

Sure, it's might have only a limited time of usage, since BFR/BFS will be there soon. But still, SpaceX needs to learn a few things, and if they can learn while somebody else is paying for it... that'd be great.

Online FishInferno

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Re: Mini-BFR ship F9 Second Stage
« Reply #265 on: 11/10/2018 05:50 PM »
Sure, it's might have only a limited time of usage, since BFR/BFS will be there soon. But still, SpaceX needs to learn a few things, and if they can learn while somebody else is paying for it... that'd be great.

And SpaceX would be subject to a whole new round of NASA paperwork... BFR would be retired by the time that thing took crew to the ISS.
Comparing SpaceX and SLS is like comparing paying people to plant fruit trees with merely digging holes and filling them.  - Robotbeat

Offline Hotblack Desiato

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Re: Mini-BFR ship F9 Second Stage
« Reply #266 on: 11/10/2018 07:13 PM »
Sure, it's might have only a limited time of usage, since BFR/BFS will be there soon. But still, SpaceX needs to learn a few things, and if they can learn while somebody else is paying for it... that'd be great.

And SpaceX would be subject to a whole new round of NASA paperwork... BFR would be retired by the time that thing took crew to the ISS.

Yes, it would be an insane amount of paperwork, even though, it's a dragon 2 that already has passed all the paperwork.

Online Robotbeat

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Re: Mini-BFR ship F9 Second Stage
« Reply #267 on: 11/10/2018 08:10 PM »
I seriously doubt any customers would be happy for a completely different and untested 2nd stage to be used for their very expensive payload launch. Especially one with aero surfaces poking out....
The first flight of Falcon 9 with legs folded up occurred on CRS-5 with Dragon.

This is almost the same kind of thing, depending on how exactly they design the recovery system.


No, it really isn't. Taking musk at his word this is like a mini bfs. A major change to the current second stage. With fins and an entirely different payload deployment mechanism. ...
Now you're putting words in his mouth. He never mentioned a different payload deployment mechanism. "like a mini-BFR ship" is too vague to know exactly what features of the full BFS will be present. For instance, in a later tweet he clarified no propulsive landing. And the first tweet mentioned it's an upgrade of the Falcon 9 upper stage.

So it's easily within the realm of probability that it'll be essentially a Falcon 9 upper stage with TPS and aerosurfaces and a reentry like the BFS but without propulsive landing. Not too different than what SpaceX did with the first stage (i.e. adding grid fins and legs).

So please don't concern troll about how customers would never want to fly on the first flight of it until you actually know the extent of the upgrades.
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Offline Roy_H

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Re: Mini-BFR ship F9 Second Stage
« Reply #268 on: 11/10/2018 08:32 PM »
I have to back-track and say that my earlier argument for Raptor powered carbon fibre S2 has now been debunked with confirmation that it will be Merlin VAC standard S2. So I now agree this will be a minimum modification of a F9 S2 with a carbon fibre outer shell in the form of the BFS complete with canards, 3 fins (2 articulated) TPS and no attempt at payload. No 5m fairing. Used strictly for research.

Edit: I believe to be an effective simulation for BFS that the density (at least surface area to mass) has to be the same as proposed BFS. Also COG same. Not sure how they will deal with the mVac bell sticking out the rear, maybe longer skirt of the new shell covering most of it? Is the existing S2, no fuel, higher density than BFS, if so then I think the shell will be sized large enough to make density the same.
« Last Edit: 11/10/2018 08:42 PM by Roy_H »
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Offline John-H

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Re: Mini-BFR ship F9 Second Stage
« Reply #269 on: 11/10/2018 09:52 PM »
No, I don't mean de-orbit burn.  I mean use engine during re-entry to follow a trajectory that put less stress on TPS.
No, it will use the RCS and fins to pitch, bank, and roll to manage the energy and heating until the velocity is bled off for controlled free fall...
Then pretty much a Shuttle style re-entry - hopefully lighter.

John

Offline nacnud

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Re: Mini-BFR ship F9 Second Stage
« Reply #270 on: 11/10/2018 09:55 PM »
No it's a totally different concept to shuttle.

Offline TomH

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Re: Mini-BFR ship F9 Second Stage
« Reply #271 on: 11/10/2018 10:27 PM »
So the problem is, the structure will be bottom-heavy because the engine is there, and everything else is mostly an empty tank.

Rather than staging, the original Atlas jettisoned its outboard engines during flight. How difficult would it be to jettison the M-vac? OTOH, would this make the stage's density so much lower than BFS as to make the entire exercise non-comparable?

Offline Rocket Science

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Re: Mini-BFR ship F9 Second Stage
« Reply #272 on: 11/10/2018 10:33 PM »
Different than Shuttle with a lunar return velocity greater than 11 km/s and about twice the kinetic energy to dissipate. It could be a combination of direct entry and skip-entry to keep the g's at a reasonable level for the vehicle and crew...

Here a a description of the Shuttle entry phases:
« Last Edit: 11/10/2018 10:45 PM by Rocket Science »
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Offline groundbound

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Re: Mini-BFR ship F9 Second Stage
« Reply #273 on: 11/11/2018 12:12 AM »
No, I don't mean de-orbit burn.  I mean use engine during re-entry to follow a trajectory that put less stress on TPS.
No, it will use the RCS and fins to pitch, bank, and roll to manage the energy and heating until the velocity is bled off for controlled free fall...
Then pretty much a Shuttle style re-entry - hopefully lighter.

John

As others have said, very different. To start with, STS had a L/D ratio that in very round numbers was 1 (ish). This beast will have a L/D that is close to a much rounder number (zero.)  :)

Offline speedevil

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Re: Mini-BFR ship F9 Second Stage
« Reply #274 on: 11/11/2018 12:31 AM »
No, I don't mean de-orbit burn.  I mean use engine during re-entry to follow a trajectory that put less stress on TPS.
No, it will use the RCS and fins to pitch, bank, and roll to manage the energy and heating until the velocity is bled off for controlled free fall...
Then pretty much a Shuttle style re-entry - hopefully lighter.

John

As others have said, very different. To start with, STS had a L/D ratio that in very round numbers was 1 (ish). This beast will have a L/D that is close to a much rounder number (zero.)  :)

If you look at the entry trajectories, they are remarkably similar before 20km, just compressed a little in time, as in order to get comparable lift, you need more drag.

Note that no manned reentry since before Apollo has been designed to normally not fly a trajectory with some lift.
Dragon has a lift/drag of about 1/5, similar to Apollo, and Soyuz.
BFS will have a rather higher lift, though smaller than shuttle.

This is required to reduce entry G from some 8G down to 2-3G.


Offline Rocket Science

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Re: Mini-BFR ship F9 Second Stage
« Reply #275 on: 11/11/2018 02:08 PM »
Don't get too fixated on L/D, you can get creative with entry profiles such as Apollo IV "double dip'" entry. I'm sure SpaceX is bound to do some surprising with this unique vehicle...

https://infinitehistory.mit.edu/video/apollo-4-mission-1967
https://www.youtube.com/watch?time_continue=833&v=9dXdDXX4QFA
« Last Edit: 11/11/2018 05:04 PM by Rocket Science »
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Offline guckyfan

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Re: Mini-BFR ship F9 Second Stage
« Reply #276 on: 11/11/2018 02:17 PM »
So the problem is, the structure will be bottom-heavy because the engine is there, and everything else is mostly an empty tank.

BFS needs to enter fully loaded, on Mars, partly loaded when returning from Mars or point to point, or empty returning from Mars or as a tanker.

They can simulate all those even on commercial flights or Starlink flights by having varying extra mass in the payload adapter.

Offline speedevil

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Re: Mini-BFR ship F9 Second Stage
« Reply #277 on: 11/11/2018 02:30 PM »
Don't get too fixated on L/D, you can get creative with entry profiles such as Apollo IV "double dip'" entry. I'm sure SpaceX is bound to do some surprising with this unique vehicle...
Quite. However, people seem fixated on 'belly flop' - and assuming it has absolutely no lift, which is absolutely false.
The potential entries from over low orbital speed are varied.
The potential entries that keep under 6G or so all require some degree of lift. (or of course propulsion to provide that lift, which is not part of the baseline BFR)

Offline Rocket Science

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Re: Mini-BFR ship F9 Second Stage
« Reply #278 on: 11/11/2018 02:49 PM »
Don't get too fixated on L/D, you can get creative with entry profiles such as Apollo IV "double dip'" entry. I'm sure SpaceX is bound to do some surprising with this unique vehicle...
Quite. However, people seem fixated on 'belly flop' - and assuming it has absolutely no lift, which is absolutely false.
The potential entries from over low orbital speed are varied.
The potential entries that keep under 6G or so all require some degree of lift. (or of course propulsion to provide that lift, which is not part of the baseline BFR)
Which people? All that is needed is to "manage" the energy via the controls...
« Last Edit: 11/11/2018 03:44 PM by Rocket Science »
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Offline wannamoonbase

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Re: Mini-BFR ship F9 Second Stage
« Reply #279 on: 11/11/2018 04:17 PM »
Why can't this be a different shape and configuration as long as its capable of testing the objectives?

Given the stated date of June 2019 for a flight and the goals of testing control surfaces and ultra light heat shield, it seems this may not have to mean an subscale model of the BFS.

F9 US with some extra 'bits'.  Connecting the objective of US recovery and testing technologies for BFR. 
Needing a copy of 'Tales of Suspense #39'

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