Author Topic: Where will BFR launch from?  (Read 156685 times)

Offline pobermanns

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Re: Where will BFR launch from?
« Reply #480 on: 07/10/2016 10:44 pm »
That's not only not fair, it's completely wrong. The Merlin and F9 were designed to be reusable from the start. Designers and engineers have been commenting on how much harder it was to make everything reusable from day one. You don't just modify things like turbopumps made for a single flight to last for 50. They have to be designed and built that way from the beginning.

IMO the most glaring fact that points to reusability from the beginning is the ratio between first and second stage. The second stage is designed to do much of the total work to get to orbit. The first stage stages early. So early by design, that it is easy to land. Other launch vehicles have their first stages go much faster at MECO.

Not sure about that Gucky.  I think perhaps the fuel and ISP of the engines may have more been the driver in that.  Kerolox GG engines.  Delta 4 and Atlas V have higher efficiency hydrolox or staged combustion engines, and so could burn their cores longer because the performance doesn't fall off like solid or GG kerolox does.  (plus they use relatively low thrust upper stage engines in single RL-10's...especially in their heavier variants, so if they stage too early, there could be gravity loss issues)
Reuse wasn't really a factor in the design of the S-1C and S-1B stages, and they had stages that staged more comparable (although a little sooner) to F9 v1.0.  They both had high thrust 2nd stages, so they could.  F9 has a relatively high thrust 2nd stage too.

If reuse was the driver from the start (at least the boost back and propulsive landing type of reuse), wouldn't they probably go the other way?  Have the booster stage even sooner, so it's not going as fast and not as far down range at staging?  And make the upper stage larger in ratio?  A little more expended aluminum alloy tankage and up powering the Merlin-vac (if necessary) in the 2nd stage seems like a pretty minor cost to get your booster back as easy as possible?
Seems that'd make getting the booster back easier than it is now.  All that speed and distance down range that helps you get the 2nd stage and payload going, now works against you to recover. 

But as always, I could be wrong.  :-)

And indeed, you certainly could be. Were you in attendance at the board meetings at SpaceX when they discussed reusability? No? Darn!

Many other members - much more experienced than I am - have commented on the incremental path that SpX has been following. In that path, my opinion is that they have been prudent while open to improvements which would allow more aggressive operations. Seems pretty sensible to me. Probably reusability was a dream from the onset, and the sucesses prompted them to push on to grander goals. WRT to the timing of staging, I expect that, if they had it all to do over again, they woulcl change things  like you suggest. But, they are in full-experimental mode  - see what works, then expand from that. It is unfair to require them to have had an Übersicht for their whole operation.

Offline guckyfan

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Re: Where will BFR launch from?
« Reply #481 on: 07/11/2016 06:06 am »
The Falcon 9 design had to work well both expendable and reusable and it does. I think for BFR/BFS their optimization will go that direction, chose a steep trajectory, let the first stage eat mostly the gravity losses, stage early for easy RTLS and let the second stage do most of the work of getting to orbital speed. Inefficient when expendable, but a good compromise for reusability, particularly for RTLS.

I know not everybody agrees on RTLS but I think they will go that way and optimize the whole design for it with lessons learned from Falcon 9.

Offline oldAtlas_Eguy

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Re: Where will BFR launch from?
« Reply #482 on: 07/11/2016 06:42 pm »
RTLS as a design requirement means the pad area has to have enough space for the BFR to both launch and then land. Now also at not to far a distance the assembly/refurbishment/manufacturing plant and you need quite a lot of real estate.
« Last Edit: 07/11/2016 06:42 pm by oldAtlas_Eguy »

Offline pobermanns

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Re: Where will BFR launch from?
« Reply #483 on: 07/11/2016 06:43 pm »
The Falcon 9 design had to work well both expendable and reusable and it does. I think for BFR/BFS their optimization will go that direction, chose a steep trajectory, let the first stage eat mostly the gravity losses, stage early for easy RTLS and let the second stage do most of the work of getting to orbital speed. Inefficient when expendable, but a good compromise for reusability, particularly for RTLS.

I know not everybody agrees on RTLS but I think they will go that way and optimize the whole design for it with lessons learned from Falcon 9.
Good points. Mostly makes sense to me.

And don't forget, they're the first ones to try reusability. Like any engineering project, there will be lots of pros and cons to trade off with each other, and eventually better combinations become apparent. I don't know if RTLS has been a prime directive for them - vs. landing on a droneship - but you may be right.

Or, maybe EM should have read this book at the beginning.
« Last Edit: 07/11/2016 07:02 pm by pobermanns »

Offline envy887

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Re: Where will BFR launch from?
« Reply #484 on: 07/11/2016 07:23 pm »
RTLS as a design requirement means the pad area has to have enough space for the BFR to both launch and then land. Now also at not to far a distance the assembly/refurbishment/manufacturing plant and you need quite a lot of real estate.

Depends on what "refurbishment" means. If they can get several (5? 10?) flights with just inspections and maybe minor maintenance, then refurbishment could be done at some distance from the pad. For example, manufacture, and refurbish in Brownsville (or somewhere else with barge access), but integrate, inspect, maintain, launch and land at Boca Chica and LC-39/LZ-1

Offline Lobo

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Re: Where will BFR launch from?
« Reply #485 on: 07/11/2016 10:18 pm »
The Falcon 9 design had to work well both expendable and reusable and it does. I think for BFR/BFS their optimization will go that direction, chose a steep trajectory, let the first stage eat mostly the gravity losses, stage early for easy RTLS and let the second stage do most of the work of getting to orbital speed. Inefficient when expendable, but a good compromise for reusability, particularly for RTLS.

Yup, exactly the point I was originally trying for, I just did it in a bit of a clumsy way.  ;-)

I know not everybody agrees on RTLS but I think they will go that way and optimize the whole design for it with lessons learned from Falcon 9.

Yes, this is a voice that keeps speaking in my head (which could be the whole problem!  heh)

I worked with Hyperion on the MCT design team, and I don't doubt the numbers he and Dimitri have come up with.  And we've talked about it on a few occasions.  But it seems like they keep assuming a Falcon 9-like ascent profile.  Which I think is more optimized for ELV, since they weren't planning to try propulsive recover of the booster originally.  (back to my original point) It was designed to stage at optimum times for the power and efficiency of the engines and mass and type of fuel, as well as being road transportable, etc. etc. 
But MCT will be designed from the start for propulsive landing.  And there's things you can do to help mitigate the penalties for booster recovery that weren't part of F9's design. 

Is it possible to factor all of that in and create a RHLV that can get the required mass to LEO and have the booster be RTLS?  We'll all have to wait to see...I can't say.   But I think they're at least trying very hard for it because it greatly simplifies operations and logistics.  Especially for a future robust flight rate.  It's nice not to have to wait for barges to go down range, and come back, or even need barge access at all (if the stages are built on site, and then can  return to that site to land) or deal with the various sea and weather conditions at the recovery zone which can scrub launches. 

I guess I look at it like this.  Theoretically, the booster could have a lofted enough ascent profile that it wouldn't need to do any boostback.  It'd be like SpaceShipOne and Two.  It'd just go up and down again.  And a little down range, and the rotation of the Earth will move the launch site right back under the descent trajectory.  This is probably too steep, but I'm just using it as an example.  This means the upper stage has to do a lot of the ascent, and would need big tanks and a lot of thrust.  But if MCT is integrated on the upper stage and is coming back from the surface Mars to Earth on one tank of fuel, it'll already need big tanks and a lot of thrust by default.  You can't make it too small optimized for Earth ascent, since it needs to get itself back. (Unless there's some other LMO refueling infrastructure MCT will have that we don't know about yet).  Even if MCT has a dedicated reusable 2nd stage that just inserts the spacecraft into LEO, it could still be made large enough to do this if they wanted.
So you could have a powerful booster but with relatively small tanks to break the gravity loss and gain altitude.  Then a large upperstage/spacecraft that will take it from there to LEO for refueling.  Perhaps something like STS, where the ET was essentially a very large ground-lit upper stage.  This would be air-lit after boost phase, but it's just for analogy purposes.  It wasn't the most efficient ascent profile for a ELV, but it wasn't designed to be an ELV. 
And as long as the booster gets the spacecraft/upperstage just into vacuum, it's vacuum engines will be much more efficient than sea level Raptors trying to push the stack in a vacuum.  So there can be some efficiency gain there, as long as there's sufficient thrust to overcome the gravity losses at "low and slow" staging.

But Hyperion and Dimitri are both way smarter than me, so I don't put enough stock in my gut feeling and speculation to do anything more than say IMO they're likely at least trying to figure out if there's any possible way they can build RTLS into MCT's design.  :-)


Offline Lobo

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Re: Where will BFR launch from?
« Reply #486 on: 07/11/2016 10:20 pm »

And indeed, you certainly could be. Were you in attendance at the board meetings at SpaceX when they discussed reusability? No? Darn!

Sorry, I had schedule conflicts and was unable to attend those board meetings.  ;-)


Many other members - much more experienced than I am - have commented on the incremental path that SpX has been following. In that path, my opinion is that they have been prudent while open to improvements which would allow more aggressive operations. Seems pretty sensible to me. Probably reusability was a dream from the onset, and the sucesses prompted them to push on to grander goals. WRT to the timing of staging, I expect that, if they had it all to do over again, they woulcl change things  like you suggest. But, they are in full-experimental mode  - see what works, then expand from that. It is unfair to require them to have had an Übersicht for their whole operation.

Yup, I agree fully with this.  :-)

Offline Stan-1967

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Re: Where will BFR launch from?
« Reply #487 on: 07/11/2016 10:42 pm »
The Falcon 9 design had to work well both expendable and reusable and it does. I think for BFR/BFS their optimization will go that direction, chose a steep trajectory, let the first stage eat mostly the gravity losses, stage early for easy RTLS and let the second stage do most of the work of getting to orbital speed. Inefficient when expendable, but a good compromise for reusability, particularly for RTLS.

Yup, exactly the point I was originally trying for, I just did it in a bit of a clumsy way.  ;-)

I know not everybody agrees on RTLS but I think they will go that way and optimize the whole design for it with lessons learned from Falcon 9.


But it seems like they keep assuming a Falcon 9-like ascent profile.  Which I think is more optimized for ELV, since they weren't planning to try propulsive recover of the booster originally.  (back to my original point) It was designed to stage at optimum times for the power and efficiency of the engines and mass and type of fuel, as well as being road transportable, etc. etc. 
 

I guess I look at it like this.  Theoretically, the booster could have a lofted enough ascent profile that it wouldn't need to do any boostback.  It'd be like SpaceShipOne and Two.  It'd just go up and down again.  And a little down range, and the rotation of the Earth will move the launch site right back under the descent trajectory....



And as long as the booster gets the spacecraft/upperstage just into vacuum, it's vacuum engines will be much more efficient than sea level Raptors trying to push the stack in a vacuum.  .....

It sounds like you have just described a BFR modeled on a "New Shepard" type of flight profile.  Is there thinking at SpaceX suggesting that path, while less technically impressive that DPL or a flyback RTLS, is it a better solution for re-use?

Blue Origin seems to be much further along to "gas & go" on the vehicle than SpaceX.  I wonder if that a demonstration of foresight of the BO team?  If so, perhaps Blue Origin is more on technical par with SpaceX if you look at barge landings as a dead end for reuse.

Offline envy887

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Re: Where will BFR launch from?
« Reply #488 on: 07/12/2016 12:42 am »
...
This means the upper stage has to do a lot of the ascent, and would need big tanks and a lot of thrust.  But if MCT is integrated on the upper stage and is coming back from the surface Mars to Earth on one tank of fuel, it'll already need big tanks and a lot of thrust by default. 
...
So you could have a powerful booster but with relatively small tanks to break the gravity loss and gain altitude.  Then a large upperstage/spacecraft that will take it from there to LEO for refueling.
...

The Falcon 9 upper stage is already pretty large and stages quite low. Staging to LEO is probably going to be the most demanding leg of the trip, so I don't know if it will be oversized for that, since it would be way oversized for the other legs.

Offline JamesH65

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Re: Where will BFR launch from?
« Reply #489 on: 07/12/2016 08:32 am »
It sounds like you have just described a BFR modeled on a "New Shepard" type of flight profile.  Is there thinking at SpaceX suggesting that path, while less technically impressive that DPL or a flyback RTLS, is it a better solution for re-use?

Blue Origin seems to be much further along to "gas & go" on the vehicle than SpaceX.  I wonder if that a demonstration of foresight of the BO team?  If so, perhaps Blue Origin is more on technical par with SpaceX if you look at barge landings as a dead end for reuse.

BO's dev path is different - they have gone down the small, suborbital, check out G&G route, move to orbital. SpaceX have gone another way, get to orbit first, then incrementally move to G&G.

So not directly comparable - they are taking different routes to the same goal.

Will be interesting to see how the timescales match up when they converge to similar capabilities.

Offline Lobo

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Re: Where will BFR launch from?
« Reply #490 on: 07/12/2016 04:14 pm »
...
This means the upper stage has to do a lot of the ascent, and would need big tanks and a lot of thrust.  But if MCT is integrated on the upper stage and is coming back from the surface Mars to Earth on one tank of fuel, it'll already need big tanks and a lot of thrust by default. 
...
So you could have a powerful booster but with relatively small tanks to break the gravity loss and gain altitude.  Then a large upperstage/spacecraft that will take it from there to LEO for refueling.
...

The Falcon 9 upper stage is already pretty large and stages quite low. Staging to LEO is probably going to be the most demanding leg of the trip, so I don't know if it will be oversized for that, since it would be way oversized for the other legs.

It stages low and slow compared to some ELV's like Delta IV and Atlas V.  But it stages higher and faster than the Shuttle SRB's did, than the Titan SRB's did, and than the S-1B and S-1C GG kerolox stages did, as well as the Soyuz kerolox GG boosters.

I'm just saying if optimizing for propulsive landing of the booster...especially if trying to recover it at the launch site...it would be easier if the booster phase was more like these really low and slow boosters, rather than more like current EELV's.

In that case, I picture it on the pad looking closer to the booster to upper stage ratio of Saturn 1B or Saturn INT-21, than to Falcon 9. 

And when considering both stages have the same propellant unlike Saturn 1B, the ratio could be more similar to Saturn 1B but stage the booster even sooner than Saturn 1B did and the upper stage do even more than the S-IVB did.  More like SRB boost profiles, but a little more lofted ascent.  Almost like a "booster assisted SSTO", if you will.  MCT would be SSTO when taking off from Mars anyway.  Actually "Single Stage to TEI".  So it really should just need -some- extra help with Earth's deeper gravity well to get itself to LEO...it's not even going all the way to TMI like it is when it's coming back.
The Spacecraft will have to be quite a capable single stage vehicle in it's own right.  Just needs some JATO-like assist to get off Earth's runway with a full load...so to speak.  And it may be able to take it from there itself.

I actually picture such a stack looking quite like this Saturn 1B with PLF sitting on the pad, ironically enough.







« Last Edit: 07/12/2016 04:20 pm by Lobo »

Offline envy887

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Re: Where will BFR launch from?
« Reply #491 on: 07/12/2016 04:57 pm »
Yes, I think BFR will loft more and stage slower than Falcon, to help with RTLS.

But it will still have to do SOME boostback. I think you're overestimating the dV for both TEI and do boostback, and underestimating the dV to get to LEO. LEO is over 6 km/s from where F9 stages, and BFS has to do that leg with over 100t of payload. BFS will single-stage to TEI, but only with about 25t of payload and that's only 6.5 to 7 km/s from Mars surface.

Once it's big enough to put 100t through 6 km/s, making BFS bigger is not optimal. Put the same mass into the booster and do a boost-back burn. Boostback is only pushing the dry mass of the booster, so it takes very little fuel to get incredible dV. And the ISP difference in vacuum between the engines is only 4% (363 vs 380) so you're saving very little by deferring that dry mass to the upper stage, at the cost of sending that mass to Mars and back.


Offline Lobo

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Re: Where will BFR launch from?
« Reply #492 on: 07/12/2016 05:54 pm »
Yes, I think BFR will loft more and stage slower than Falcon, to help with RTLS.

But it will still have to do SOME boostback. I think you're overestimating the dV for both TEI and do boostback, and underestimating the dV to get to LEO. LEO is over 6 km/s from where F9 stages, and BFS has to do that leg with over 100t of payload. BFS will single-stage to TEI, but only with about 25t of payload and that's only 6.5 to 7 km/s from Mars surface.

Once it's big enough to put 100t through 6 km/s, making BFS bigger is not optimal. Put the same mass into the booster and do a boost-back burn. Boostback is only pushing the dry mass of the booster, so it takes very little fuel to get incredible dV. And the ISP difference in vacuum between the engines is only 4% (363 vs 380) so you're saving very little by deferring that dry mass to the upper stage, at the cost of sending that mass to Mars and back.

Yes, valid points.

Once they've factored in enough dV to get the ship plus X-mass (25mt?) from the surface of Mars back to Earth, then that's really the long pole task the spacecraft will have to do all by itself, as you say.  Then one would assume they'd design to booster to provide all the additional necessary dV to get 100mt from the surface of the Earth to LEO.  That plus whatever is required for boostback (barge or RTLS) will be what determines booster size and design.
That's the scenario that Hyperion and Dimitry used to determine that RTLS won't be possible.  So trying to speculate on -some- way SpaceX might be able to make it possible, by possibly oversizing the tanks of BFS for Earth ascent.  As I think we can all agree RTLS is preferable if at all possible.

A little larger propellant tanks really don't add much mass to a stage/ship, by themselves.  So there could be some trade off there if doing so would allow booster RTLS, and not doing so means a barge landing is necessary.  So not making BFS mass optimal, but possibly optimal for logistics/operations if it tipped the scale to allow for RTLS, as long as it could still perform it's other functions, -could- be a way SpaceX could try to get RTLS.  (Hypothetical speculation, of course.) 
And although adding some more tankage to BFS may not add much mass itself, if that extra tankage also requires other things like additional reinforcing structure, TPS or possibly an additional engine for sufficient thrust at early staging, then in totality the additional mass could be too great to do.



« Last Edit: 07/12/2016 05:56 pm by Lobo »

Offline philw1776

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Re: Where will BFR launch from?
« Reply #493 on: 07/12/2016 08:13 pm »
Yes, I think BFR will loft more and stage slower than Falcon, to help with RTLS.

But it will still have to do SOME boostback. I think you're overestimating the dV for both TEI and do boostback, and underestimating the dV to get to LEO. LEO is over 6 km/s from where F9 stages, and BFS has to do that leg with over 100t of payload. BFS will single-stage to TEI, but only with about 25t of payload and that's only 6.5 to 7 km/s from Mars surface.

Once it's big enough to put 100t through 6 km/s, making BFS bigger is not optimal. Put the same mass into the booster and do a boost-back burn. Boostback is only pushing the dry mass of the booster, so it takes very little fuel to get incredible dV. And the ISP difference in vacuum between the engines is only 4% (363 vs 380) so you're saving very little by deferring that dry mass to the upper stage, at the cost of sending that mass to Mars and back.

Agree with these parameters so I took another cut at BFR/BFS with a heavy 100mT BFS and in attached spreadsheet a lighter 85mT with a look to RTLS.  Attached spreadsheet much easier to read.
Mid 20s # engines stage one, 5 Rvacs stage 2.  Stage 2 the BFS does the Km/sec work having been boosted to altitude and just a couple Km/sec by the BFR which heads home for rapid refurb and re-flight.  Upon reflection if Elon is consistent with his minimize cost mantra, the craft needs to RTLS.

   BFR & BFS MODELS: MCT as 2nd Stage with 100  Metric Tons CARGO      
      BLUE: Enter parameter variable   
Mars MCT Freight to Earth   25   Elon "1/4 payload return to Earth" +25mT landing propellant   
S1 Avg ISP Sea L to MECO   335      
S2 vac ISP   380      
Raptor sea level thrust KLB & mT   518   235   Elon "over 230mT"
Rvac thrust KLB & mT   642   291   Rvac Thrust same 124% ratio as with F9 FT+
Propellant Density   1.06   mT/m3   3.7:1   O2 to CH4
         
BFR DIA   15.0   m                                                   1. Enter BFR Rocket DIA
MCT BFS Dry Wt & Cargo   200   mT   metric tonnes   2. Enter BFS Total Mass
S2 Dry Mass   100   mT   
Propellant for S2 BFS Landing   20   mT   
Mass to LEO   220   mT   
1st Stage Propellant Tank Length   16.5   m                    3. Enter S1 Propellant Tank length
S1 Propellant Volume   2914   m3   volume drives S1 mass & DeltaV
Propellant Mass   3089   mT   (real tanks much longer, just computing vol)
S1 Dry Wt %    4.5%                                                4. Enter Dry Wt %
S1 DRY Weight   146   mT   
S1 Total Weight mT   3235   mT   
S! Dry Wt Delta V (No 2nd stage)   10.2   Km/sec Rocket Equation   
Stage One Full Load Delta V    3.08   Km/sec Rocket Equation   
RTxx Propellant   315   mT                                              5. Propellant boostbk, re-entry, land
RTxx Delta V @Minimum Load   3.78   Km/sec Rocket Equation   Return To Launch Site RTLS
Est S1 Gravity Loss    0.8   Km/sec                               6. est to get S1 Vel @ Burnout
Est S1 Velocity @ Burnout   2.28   Km/sec   Elon 8,000 Km/Hr (2.2 Km/s)
2nd Stage Propellant Tank Length   6.0   m               7. Enter 2nd stage Tank length
Propellant Volume   1,060   m3   drives S2 BFS mass & Delta V
Propellant Mass   1,123   mT   (real tanks much longer, just computing vol)
S2 Mass w/MCT   1,323   mT   
S2 Mass w/MCT   2.9   Million LBS   
Calc # Rvac Raptor Eng   4.95      1.09                            8. Enter T/W to get # S2 engines
# Rvac Raptor Eng   5      (starting T/W <1 ok as still 'falling up')
Stage 2 Thrust mT   1457      
Stage 2 Thrust   3.2   Million LBS   
Stage 2 Km/sec    6.68   Km/sec Rocket Equation   need >6.5 Km/sec LEO to Mars landing
Stage 2 Drt Wt %   8.2%      
S1 + S2  Total Delta V Capability   9.0   Km/sec    ~< 9 Km/sec to LEO w/grav loss
S2 Mars Return XXmT Cargo   8.6   Km/sec Rocket Equation    >8Km/sec return Mars surf to Earth
TOTAL WT mT   4,558   mT   
TOTAL WT LBS   10.0   Million LBS   
THRUST Needed LBS force   13.4   Million LBS   
THRUST Needed mT   6062   mT   
THRUST Needed MegaNewtons   59   MegaNewtons   
1st Stage T/W @ Takeoff   1.33                                  9. Enter >1.2 to get # S1 engines  (Old F9 T/W=1.19)   
# ENG Not Rounded Up   25.8      
1st STAGE # ENG    26      
LEO Mass Fract   4.8%    %   
LEO Wet to Dry Mass Ratio   21    F9 v1.1 25/1 Musk   http://exoscientist.blogspot.com/2013/11/the-coming-sstos-falcon-9-v11-first.html
MCT Cargo Hold length  swag   10   m   
MCT Cargo Vol m3   1766   m3   
   Eng 16+8+2=26      
   Outer Ring, Inner Ring, Central Engs      
NOTE 1:   S1 Km/sec + S2 Km/sec must ~< 9Km/sec for LEO with grav losses      
NOTE 2:   Rocket Equation      http://www.quantumg.net/rocketeq.html
NOTE 3:    LEO esc 3.2Km/s + Fast transit ~1.5 Km/s + aerocapture + Mars brake & landing 1.8Km/s = 6.5 Km/sec Delta V for 2nd stage MCT      
NOTE 4:    Mars Return with reduced freight must be ~ 8Km/sec  Since propellant capacity exceeds this, won't fully refuel on Mars      

FULL SEND!!!!

Offline darkenfast

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Re: Where will BFR launch from?
« Reply #494 on: 07/14/2016 05:22 am »
https://forum.nasaspaceflight.com/index.php?topic=35425.msg1559579#msg1559579

Over on the Texas Launch Site thread, Jim says he doesn't think the MCT will launch from Boca Chica because of safety reasons and notes the size of the exclusion zone around the Saturn V launches. 
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Offline hkultala

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Re: Where will BFR launch from?
« Reply #495 on: 07/14/2016 06:09 am »
The Falcon 9 design had to work well both expendable and reusable and it does. I think for BFR/BFS their optimization will go that direction, chose a steep trajectory, let the first stage eat mostly the gravity losses, stage early for easy RTLS and let the second stage do most of the work of getting to orbital speed. Inefficient when expendable, but a good compromise for reusability, particularly for RTLS.

Yup, exactly the point I was originally trying for, I just did it in a bit of a clumsy way.  ;-)

I know not everybody agrees on RTLS but I think they will go that way and optimize the whole design for it with lessons learned from Falcon 9.

Yes, this is a voice that keeps speaking in my head (which could be the whole problem!  heh)

I worked with Hyperion on the MCT design team, and I don't doubt the numbers he and Dimitri have come up with.  And we've talked about it on a few occasions.  But it seems like they keep assuming a Falcon 9-like ascent profile.  Which I think is more optimized for ELV, since they weren't planning to try propulsive recover of the booster originally.

There is no single "F9 ascent profile".

There are about four different styles of F9 ascent profiles, even for same destination orbit:

1) Original 1.0 ascent profile
2) Expendable 1.1 ascent profile (second stage lenghtened much more than first stage, stages considerably earlier)
3) Barge landing 1.1/FT ascent profile
4) RTLS ascent profile.

So when they were making the 1.1 version, they already considered the reusing of first stage and increased the size of the second stage considereably because of that.

« Last Edit: 07/14/2016 06:11 am by hkultala »

Offline BobHk

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Re: Where will BFR launch from?
« Reply #496 on: 07/14/2016 05:11 pm »
https://forum.nasaspaceflight.com/index.php?topic=35425.msg1559579#msg1559579

Over on the Texas Launch Site thread, Jim says he doesn't think the MCT will launch from Boca Chica because of safety reasons and notes the size of the exclusion zone around the Saturn V launches.

Ask yourself where are you going to get the LNG/Methane for all those Mars launches if not south Texas?  With any site named please indicate the proximity of cheap ergtons of LNG nearby, thanks.

Offline Lobo

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Re: Where will BFR launch from?
« Reply #497 on: 07/14/2016 05:16 pm »

There is no single "F9 ascent profile".

There are about four different styles of F9 ascent profiles, even for same destination orbit:

1) Original 1.0 ascent profile
2) Expendable 1.1 ascent profile (second stage lenghtened much more than first stage, stages considerably earlier)
3) Barge landing 1.1/FT ascent profile
4) RTLS ascent profile.

So when they were making the 1.1 version, they already considered the reusing of first stage and increased the size of the second stage considereably because of that.

Very good clarification.
I'm speaking more generally of F9 still has a more EELV like ascent profile.  It still has a large booster compared to it's upper stage, more like EELV's do.    It's booster is still moving pretty fast and is pretty far down range at staging.  Which is why it still has a pretty big penalty for RTLS.  (Although the upper stage stretch and Merlin 1-vac upgrades do help with that)

A clean sheet design would likely shorten and loft the booster ascent profile more, to mitigate that penalty for easier RTLS.  And either have a reusable upper stage, or a sufficiently powerful/economical expendable upper stage to make up the dV from the shorter boost phase.
If that was possible with the F9 hardware, F9-FT would probably be able to launch most typical GTO payloads and still be able to RTLS rather than still need barge landings.  Because isn't that the butter-zone for the commercial launch industry?  But if you are dancing with the girl that brought you, you have to work within it's limits.  And F9-FT seems to be about the best they can do with that.
« Last Edit: 07/14/2016 05:28 pm by Lobo »

Offline Robotbeat

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Re: Where will BFR launch from?
« Reply #498 on: 07/14/2016 05:44 pm »
https://forum.nasaspaceflight.com/index.php?topic=35425.msg1559579#msg1559579

Over on the Texas Launch Site thread, Jim says he doesn't think the MCT will launch from Boca Chica because of safety reasons and notes the size of the exclusion zone around the Saturn V launches.

Ask yourself where are you going to get the LNG/Methane for all those Mars launches if not south Texas?  With any site named please indicate the proximity of cheap ergtons of LNG nearby, thanks.
Any major city in the US has access to super cheap natural gas (super cheap in this context). And just like oxygen, SpaceX can liquify it themselves.
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Offline envy887

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Re: Where will BFR launch from?
« Reply #499 on: 07/14/2016 07:04 pm »
...
A clean sheet design would likely shorten and loft the booster ascent profile more, to mitigate that penalty for easier RTLS.  And either have a reusable upper stage, or a sufficiently powerful/economical expendable upper stage to make up the dV from the shorter boost phase.
...

These are two different design optima that suggest having different vehicles: a lander capable of transporting crew and cargo to the surface of Mars and back, and a large but light (and not expendable) upper stage that can launch extremely large LEO payloads, do direct GSO insertions, and most importantly launch all the fuel a Mars mission in less than 3 shots while still doing RTLS.

The BFS could stage at 2100 m/s but at only 900 tonnes with 4 Raptors, giving BFR more overhead for RTLS.
The Tanker could stage at 1400 m/s but mass 1600 tonnes with 6 RVacs, and the lower/slower staging point would allow the same BFR booster to lift the heavier stack and still RTLS. By my numbers (see attached spreadsheet) both should be under 12 mlbf at liftoff.

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