Quote from: guckyfan on 07/05/2015 01:56 pm......I don't think it is a very efficient architecture. It means several MCT with all their mass would need to be accelerated a significant part of TMI. Also you mention using half of their fuel. It would not be necessary to reserve half of the fuel for return. Injecting into a highly ellicptic orbit would give the Mars bound MCT much of the needed delta-v and brings the booster MCT back to earth basically free. Why do you propose to get them back to LEO? More efficient to land them for a new launch with payload.I think the most efficient way is giving MCT tanks large enough to do TMI burn and Mars EDL by themselves. Use tanker MCT to refuel in LEO either directly fuelling up an MCT or filling depots. They need that tankage and the delta-v to get back to earth from the Mars surface.You are right, it's not the most fuel efficient. But it might be a method for reaching Mars faster than the least-energy transfer orbit to reach Mars, perhaps in in 3-4 months rather than 6. Also a method that would represent a non-SEP architecture if Elon is serious about it. I am sure this has already been addressed somewhere and lies on someone's spreadsheet.
......I don't think it is a very efficient architecture. It means several MCT with all their mass would need to be accelerated a significant part of TMI. Also you mention using half of their fuel. It would not be necessary to reserve half of the fuel for return. Injecting into a highly ellicptic orbit would give the Mars bound MCT much of the needed delta-v and brings the booster MCT back to earth basically free. Why do you propose to get them back to LEO? More efficient to land them for a new launch with payload.I think the most efficient way is giving MCT tanks large enough to do TMI burn and Mars EDL by themselves. Use tanker MCT to refuel in LEO either directly fuelling up an MCT or filling depots. They need that tankage and the delta-v to get back to earth from the Mars surface.
......There have been several MCT designs that had large enough tanks to do the 4 month transfer direct from LEO. (I think the jump from 4 to 3 months is pretty insane most synods).If you think those tanks are too large, and want to transfer to a higher energy orbit before TMI, you should just use one extra MCT instead of 4. That will save hundreds of tons of propellant because you are moving less dry mass. A reusable SEP tug is another popular option.
One way to speculate about the BFR launcher for the MCT is to look at mass fraction to LEO efficiency. The F9 has a mass fraction of ~2.7% to LEO. Despite full re-usability most of us amazing peoples/girls here expect that SX will somehow improve on that with the fully re-useable BFR. The tables below 1st assume 180mT to LEO with the dry MCT massing 80mT and the 2nd table assumes a dry MCT at 100mT. Various optimistic mass fractions yield different BFR takeoff weights. A T/W ratio of 1.2 is assumed yielding 1st stage thrust and dividing by 500KLBs/Raptor, the # of Raptors needed.MCT + Payload = 180mT to LEO MASS FRACTION BFR BFR TAKEOFF TO LEO WEIGHT mT WEIGHT M LBS THRUST M LBS # RAPTORS @ 500KLB5.0% 3500 7.7 9.2 19 4.5% 3889 8.6 10.3 21 4.0% 4375 9.6 11.6 23 3.5% 5000 11.0 13.2 26 3.0% 5833 12.8 15 31 MCT + Payload = 200mT to LEO MASS FRACTION TO LEO BFR BFR TAKEOFF WEIGHT mT WEIGHT M LBS THRUST M LBS # RAPTORS5.0% 4000 8.8 10.6 21 4.5% 4444 9.8 11.7 24 4.0% 5000 11.0 13.2 26 3.5% 5714 12.6 15.1 30Assuming Raptor engine bells are ~1.6m wide, it's likely that 1st stage diameters of over 10m are preferred with 12.5m or even better 13.5m best to allow for max # of engines in case mass fraction drops. A smaller MCT dry weight really helps reduce BFR mass & # of engines as would be expected.
Quote from: philw1776 on 07/02/2015 08:01 pmOne way to speculate about the BFR launcher for the MCT is to look at mass fraction to LEO efficiency. The F9 has a mass fraction of ~2.7% to LEO. Despite full re-usability most of us amazing peoples/girls here expect that SX will somehow improve on that with the fully re-useable BFR. The tables below 1st assume 180mT to LEO with the dry MCT massing 80mT and the 2nd table assumes a dry MCT at 100mT. Various optimistic mass fractions yield different BFR takeoff weights. A T/W ratio of 1.2 is assumed yielding 1st stage thrust and dividing by 500KLBs/Raptor, the # of Raptors needed.MCT + Payload = 180mT to LEO MASS FRACTION BFR BFR TAKEOFF TO LEO WEIGHT mT WEIGHT M LBS THRUST M LBS # RAPTORS @ 500KLB5.0% 3500 7.7 9.2 19 4.5% 3889 8.6 10.3 21 4.0% 4375 9.6 11.6 23 3.5% 5000 11.0 13.2 26 3.0% 5833 12.8 15 31 MCT + Payload = 200mT to LEO MASS FRACTION TO LEO BFR BFR TAKEOFF WEIGHT mT WEIGHT M LBS THRUST M LBS # RAPTORS5.0% 4000 8.8 10.6 21 4.5% 4444 9.8 11.7 24 4.0% 5000 11.0 13.2 26 3.5% 5714 12.6 15.1 30Assuming Raptor engine bells are ~1.6m wide, it's likely that 1st stage diameters of over 10m are preferred with 12.5m or even better 13.5m best to allow for max # of engines in case mass fraction drops. A smaller MCT dry weight really helps reduce BFR mass & # of engines as would be expected.Are you counting the mass of MCT as payload, or as the stage itself? If it is it's own 2nd stage, then you should figure 180 or 200mt -gross- to LEO.
F9R 1.1 can put approx 13.1mT to LEO with recovery of the first stage. That might improve for 1.2 but we dont have numbers on that yet.Second stage weights around 3.9mT while F9R weights about 505mT at launch. That makes a gross mass to orbit of about 3.36%.But that is with KeroLOX all the way. With MethaLOX, alone that number would increase significantly. So yes, I guess it is safe to assume that the mass to orbit fraction will be around 4 to 5 % as you listed in your table. If SpaceX has enough unicorn hair and ferry dust left from their Dragon production, it might even go higher than 5%, simply because there is no payload adapter, fairings or what have you necessary.
Another interesting discussion is how much of the total delta V budget of the total LV is going into the booster and how much is going into the MCT (second stage)?I think that there will be a big difference from EELV LV's where most of the fuel is in the booster with a small second stage. 1. You will not be able to land the booster on a barge, it has to come back to a well prepared and solid landing site. This means you cant get to far away (unless you have an island to land on).2. You will have to have more thrust capacity on the MCT (second stage). But you will need this anyway if you are going to get of Mars.3. The MCT (second stage) is going to need a high delta-V both for high energy transfer to Mars (3-4 month as stated by Elon) and to get from Mars (delta-v budget of 6-9 km/s). This means you can take advantage of this when staging to LEO.Is there any flaws in my reasoning or mayor points I have missed?
Quote from: symbios on 07/08/2015 11:33 amAnother interesting discussion is how much of the total delta V budget of the total LV is going into the booster and how much is going into the MCT (second stage)?I think that there will be a big difference from EELV LV's where most of the fuel is in the booster with a small second stage. 1. You will not be able to land the booster on a barge, it has to come back to a well prepared and solid landing site. This means you cant get to far away (unless you have an island to land on).2. You will have to have more thrust capacity on the MCT (second stage). But you will need this anyway if you are going to get of Mars.3. The MCT (second stage) is going to need a high delta-V both for high energy transfer to Mars (3-4 month as stated by Elon) and to get from Mars (delta-v budget of 6-9 km/s). This means you can take advantage of this when staging to LEO.Is there any flaws in my reasoning or mayor points I have missed?First, I am not an aerospace engineer (I'm a EE) so I'm not sure I know what I'm doing. I have reached the exact same conclusions you cite above. Unlike the F9, the BFR/MCT will probably have a 1st stage that stages "low & slow" making boostback to launch site a given. And yes, the 2nd stage, a.k.a. MCT will after orbital re-fueling need sufficient delta V to escape LEO & transit to Mars in a few months and will also later need sufficient delta v to launch from Mars (having refueled again on the surface) and return to Earth or HEO. All these requirements dictate a high delta v capability and consequently a larger 2nd stage to 1st stage weight & propellant capacity design point than required for a simple LEO/GEO launcher. My latest models have a 1st stage with 14-15 million LBS thrust & 7 Raptors (slight overkill) powering the 2nd stage. The Vacuum Raptors are assumed to have ~610 thousand pounds thrust following the same 1.22 vac/sea level thrust ratios of the Falcon 9.Think of the MCT as a near SSTO that fell short but gets enough boost from the stage one BFR such that it's good to go.
(I just hope that the 2nd stage doesn't have that explode just before staging feature thingy the F9R has)
Quote from: philw1776 on 07/08/2015 04:00 pm(I just hope that the 2nd stage doesn't have that explode just before staging feature thingy the F9R has)Assuming that event has something to do with the helium pressurization, it won't. Both methane and LOX tank will have self pressurization, no helium involved, I am sure.
Quote from: CyclerPilot on 07/07/2015 01:52 am......There have been several MCT designs that had large enough tanks to do the 4 month transfer direct from LEO. (I think the jump from 4 to 3 months is pretty insane most synods).If you think those tanks are too large, and want to transfer to a higher energy orbit before TMI, you should just use one extra MCT instead of 4. That will save hundreds of tons of propellant because you are moving less dry mass. A reusable SEP tug is another popular option.Excellent. I have seen a lot of discussion with the SEP option, but not the propellant only option. Do you know where I could find those designs -- on this forum?
I am not sure where to put this.Yesterday there was an interview with Hans Koenigsmann in german TV ZDF. He repeated the argument that rockets need to be reusable like airplanes. He added that planes fly for decades and rockets will not fly that much but it should be 100 flights. He did not specify if this would be the Falcon Family or the goal for BFR/MCT.
Do you happen to know the show he said that in? It might be possible to still see it in the zdf mediathek from within Germany.
Quote from: Semmel on 07/23/2015 10:49 amDo you happen to know the show he said that in? It might be possible to still see it in the zdf mediathek from within Germany.It is available.http://www.zdf.de/ZDFmediathek#/beitrag/video/2452860/ZDF-heute-journal-vom-21-Juli-2015The part with Hans Koenigsmann is near the end. Skip through most of it. Edit: It's at 21:40
For all non-German speakers: [...](just to elaborate on what was mentioned above)