Quote from: guckyfan on 07/07/2016 05:38 pmQuote from: Nomadd on 07/07/2016 05:23 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. :-)
Quote from: Nomadd on 07/07/2016 05:23 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.
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.
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.
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.
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.
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.
Quote from: guckyfan on 07/11/2016 06:06 amThe 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. ;-)Quote from: guckyfan on 07/11/2016 06:06 amI 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. .....
...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. ...
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.
Quote from: Lobo on 07/11/2016 10:18 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.
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.
Quote from: guckyfan on 07/11/2016 06:06 amThe 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. ;-)Quote from: guckyfan on 07/11/2016 06:06 amI 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.
https://forum.nasaspaceflight.com/index.php?topic=35425.msg1559579#msg1559579Over 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.
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 profile2) Expendable 1.1 ascent profile (second stage lenghtened much more than first stage, stages considerably earlier)3) Barge landing 1.1/FT ascent profile4) 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.
Quote from: darkenfast on 07/14/2016 05:22 amhttps://forum.nasaspaceflight.com/index.php?topic=35425.msg1559579#msg1559579Over 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.
...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....