....The one thing that is not possible or practical is an escape system at either Mars or the moon. The reason for this is quite obvious. All escape systems depend on being rescued after the fact. This is possible just about any place on earth. Even the escape system for the B-58 Hustler could sustain somebody floating in the water are in the Arctic for up to 4 days. There is nobody on the moon or Mars to rescue anybody. Even if you successfully escaped the BFR you would still inevitably die. That’s the way it would be for the foreseeable future. Only after the BFR has made so many trips to those destinations to prove its reliability would rescue in those places be possible but then there would not be a need for a rescue system on the BFR.
Quote from: DAZ on 10/06/2017 11:28 pm....The one thing that is not possible or practical is an escape system at either Mars or the moon. The reason for this is quite obvious. All escape systems depend on being rescued after the fact. This is possible just about any place on earth. Even the escape system for the B-58 Hustler could sustain somebody floating in the water are in the Arctic for up to 4 days. There is nobody on the moon or Mars to rescue anybody. Even if you successfully escaped the BFR you would still inevitably die. That’s the way it would be for the foreseeable future. Only after the BFR has made so many trips to those destinations to prove its reliability would rescue in those places be possible but then there would not be a need for a rescue system on the BFR.That is not quite true. If there is a fueled BFS available on the Moon or Mars. A rescue mission could be mounted with the BFS on a ballistic hop to pick up the survivors.Yes, it is the equivalent of sending a cruse ship to do the job of a life boat.
ITS first stage PMF was given as 0.96. This rocket is going to be smaller, so I don't see how it could have a better ratio. Those 31 Raptor engines are going to weigh around 31 tonnes, likely more, all by themselves. First stage engine mass probably accounts for only 1/4th of the total stage dry mass. Those assumptions right there gets us close to 0.96. - Ed Kyle
Modern turbofans are operating at higher and higher temperatures in order to get higher and higher efficiency, and their turbine blades actually have to interact with this hot flow. But in a rocket, only the turbopump's blades have to do that (and it can be designed for lower combustion temperature).And there's one huge advantage for rocket engines over turbofans when it comes to reliability: turbofans will ingest anything in the air. Birds, insects, sand, volcanic ash, people, etc. That can and does cause catastrophic failure. Rocket engines bring their own air which can be carefully screened for contaminants, with actual screens being put in place to catch anything that might hurt the engine.
Quote from: Zed_Noir on 10/07/2017 08:22 amQuote from: DAZ on 10/06/2017 11:28 pm....The one thing that is not possible or practical is an escape system at either Mars or the moon. The reason for this is quite obvious. All escape systems depend on being rescued after the fact. This is possible just about any place on earth. Even the escape system for the B-58 Hustler could sustain somebody floating in the water are in the Arctic for up to 4 days. There is nobody on the moon or Mars to rescue anybody. Even if you successfully escaped the BFR you would still inevitably die. That’s the way it would be for the foreseeable future. Only after the BFR has made so many trips to those destinations to prove its reliability would rescue in those places be possible but then there would not be a need for a rescue system on the BFR.That is not quite true. If there is a fueled BFS available on the Moon or Mars. A rescue mission could be mounted with the BFS on a ballistic hop to pick up the survivors.Yes, it is the equivalent of sending a cruse ship to do the job of a life boat. How does the unpiloted bfs plot this hop? I would think this a tall ask in an emergency without some sort of GPS analog around the moon / Mars first?
Quote from: DJPledger on 10/06/2017 08:18 pmQuote from: fthomassy on 10/06/2017 08:10 pmQuote from: DJPledger on 10/06/2017 07:38 pmNo rocket will ever be as safe as an airliner so ...I understand the anxiety and lack of trust. But what is your basis for this being permanent state?Because rocket engines are running much closer to the limits of chemistry and materials than commercial turbofans. ......like some of your other statements, that isn't actually true.Modern turbofans are operating at higher and higher temperatures in order to get higher and higher efficiency, and their turbine blades actually have to interact with this hot flow. But in a rocket, only the turbopump's blades have to do that (and it can be designed for lower combustion temperature).And there's one huge advantage for rocket engines over turbofans when it comes to reliability: turbofans will ingest anything in the air. Birds, insects, sand, volcanic ash, people, etc. That can and does cause catastrophic failure. Rocket engines bring their own air which can be carefully screened for contaminants, with actual screens being put in place to catch anything that might hurt the engine.
Quote from: fthomassy on 10/06/2017 08:10 pmQuote from: DJPledger on 10/06/2017 07:38 pmNo rocket will ever be as safe as an airliner so ...I understand the anxiety and lack of trust. But what is your basis for this being permanent state?Because rocket engines are running much closer to the limits of chemistry and materials than commercial turbofans. ...
Quote from: DJPledger on 10/06/2017 07:38 pmNo rocket will ever be as safe as an airliner so ...I understand the anxiety and lack of trust. But what is your basis for this being permanent state?
No rocket will ever be as safe as an airliner so ...
Are rocket engines designed to withstand FOD damage? I think not. Takes just a 1mm size speck of metal entering a TP to destroy an RD-171 engine. EM should look closely at this and design Raptor to withstand FOD such as stray small pieces of metal etc. Giving Raptor FOD tolerance like modern turbofans have should make it and BFR much more reliable.
Quote from: DJPledger on 10/07/2017 11:37 amAre rocket engines designed to withstand FOD damage? I think not. Takes just a 1mm size speck of metal entering a TP to destroy an RD-171 engine. EM should look closely at this and design Raptor to withstand FOD such as stray small pieces of metal etc. Giving Raptor FOD tolerance like modern turbofans have should make it and BFR much more reliable.This is a rocket engine. A foreign object would have to come from one of the tanks. This different from an air breathing engine in a plane.
Quote from: jpo234 on 10/07/2017 12:59 pmQuote from: DJPledger on 10/07/2017 11:37 amAre rocket engines designed to withstand FOD damage? I think not. Takes just a 1mm size speck of metal entering a TP to destroy an RD-171 engine. EM should look closely at this and design Raptor to withstand FOD such as stray small pieces of metal etc. Giving Raptor FOD tolerance like modern turbofans have should make it and BFR much more reliable.This is a rocket engine. A foreign object would have to come from one of the tanks. This different from an air breathing engine in a plane. I'm unable to find the quote but I'm sure I remember reading about an engine where part of the design brief was to be able to ingest a loose nut into the turbopump without failure. Anyone else recall this?(Sorry for slight thread drift...)
QuoteMeanwhile, I've found a solution for the bounded problem (150 t LEO/20 t GTO for reuse, 250 t LEO for expendable version). The solution requires that second stage dry mass be roughly 45 tonnes, much less than the 85 tonnes mentioned in the presentation. With PMF ~ 0.96 for both stages, the numbers work out if something like 6-7% propellant fraction is assumed to be required for RTLS, landing, etc. I have S1 at 3278 t/131 t GLOW/Dry and S2 at 1122 t/45 t. - Ed KyleUsing Ed's cargo solution numbers... Where do we end up on the Tanker version?How many tonnes of off loadable prop to LEO... the 220 tonnes (1/5 full) hinted in the 2017 presentation?...And are the tanks likely a stretched 1250 tonnes prop volume as some have opinion'd?...Or something else?With no need to support a payload in front of them... could the tanks be a lighter, less beefy version?It's also thought the nose section is as light as possible with no openings beyond maybe a maintenance access hatch...
Meanwhile, I've found a solution for the bounded problem (150 t LEO/20 t GTO for reuse, 250 t LEO for expendable version). The solution requires that second stage dry mass be roughly 45 tonnes, much less than the 85 tonnes mentioned in the presentation. With PMF ~ 0.96 for both stages, the numbers work out if something like 6-7% propellant fraction is assumed to be required for RTLS, landing, etc. I have S1 at 3278 t/131 t GLOW/Dry and S2 at 1122 t/45 t. - Ed Kyle
That gives a SL Raptor T/W ratio of about 174:1. That sounds aggressive, until you discover the RD270 (only other complete FFSC actually built) had a T/W of 189:1. [EDIT. However that was from a Russian site. Astronautix give T/W as 153.25, but running the numbers gives more like 143:1, although I think the Russians sometimes leave off the TVC mass, maximum +/-12deg gimbal) ]
Quote from: Kaputnik on 10/07/2017 01:21 pmQuote from: jpo234 on 10/07/2017 12:59 pmQuote from: DJPledger on 10/07/2017 11:37 amAre rocket engines designed to withstand FOD damage? I think not. Takes just a 1mm size speck of metal entering a TP to destroy an RD-171 engine. EM should look closely at this and design Raptor to withstand FOD such as stray small pieces of metal etc. Giving Raptor FOD tolerance like modern turbofans have should make it and BFR much more reliable.This is a rocket engine. A foreign object would have to come from one of the tanks. This different from an air breathing engine in a plane. I'm unable to find the quote but I'm sure I remember reading about an engine where part of the design brief was to be able to ingest a loose nut into the turbopump without failure. Anyone else recall this?(Sorry for slight thread drift...)Ok found it- it was the Merlin!https://www.airspacemag.com/space/is-spacex-changing-the-rocket-equation-132285884/?no-ist=&page=2Although IMHO I'm wary of the accuracy of this account and would prefer a better source. It just sounds very unlikely to me that part of qualification testing involves chucking nuts and bolts into the fuel tanks...
Quote from: Robotbeat on 10/07/2017 01:19 pmMerlin was designed to ingest a nut.Merlins have proven to be excellent engines, but we have to remember that Raptor will operate at higher pressures and will have full-flow preburners, different propellants, etc. No guarantee that Raptor will end up as reliable as Merlin. - Ed Kyle
Merlin was designed to ingest a nut.
They didnt design the merlin to injest a nut because they expected it to ingest a nut. They designed it that way so that merlin would have suffucet margins for reuse. Raptor has the same design requirment, though perhaps not so vividly described as "eating a nut."
Quote from: rakaydos on 10/07/2017 02:57 pmThey didnt design the merlin to injest a nut because they expected it to ingest a nut. They designed it that way so that merlin would have suffucet margins for reuse. Raptor has the same design requirment, though perhaps not so vividly described as "eating a nut."I think it is so even worst case engine out does not become a RUD for the whole vehicle. With the nut the engine is no longer operational but won't destroy neighbouring engines or the tank.
Quote from: AncientU on 10/06/2017 05:13 pmBy the way, this payload corresponds to a payload mass fraction of 5.68% (250/4400t). Saturn V was 3.88%; Energia was 3.96%; F9 FT is 4.15% IIRC. (!)I've been trying to rocket-equation this, with little success. Here are the "knowns".GLOW 4400 tThrust Liftoff 5400 t, ISP = 330/356 secShip dry mass 85 tShip Mp 1100 tShip Thrust 775 t (4 engines) ISP 375 secShip Thrust 347 t (2 SL engines) ISP 330/356 secThese imply a first stage mass = 4400 t - 1185 t = 3215 tUnknown is first stage propellant mass fraction. When I plug the known numbers into the rocket equation, I get a first stage PMF required to be 0.97938 to get 250 tonnes to 9,200 m/s ideal delta-v (LEO). That's unrealistic because the first stage ends up with 20 tonnes lighter dry mass than the second stage "Ship". With PMF1 a more "reasonable" 0.96, I get total ideal delta-v = 9061 m/s, not usually good enough for LEO, but it depends on the details of the ascent. To get 9200 m/s with PMF1 = 0.96, payload maximum is 235 tonnes.S1: 3215 t > 128.6 t, ISP 347.4 sec, delta-v = 3734 m/sS2: 1185 t > 85 t, ISP 375 sec, delta-v = 5479 m/sPL: 235 t, delta-v total = 9217 m/sWhen I try to model the reusable alternative, assuming 10% propellant saved for first stage flyback landing and 6% for second stage retro and landing, I get only 105 tonnes of LEO payload, as follows.S1: 3215 t > 437 t, ISP 347.4 sec, delta-v (ascent) = 3265 m/sS2: 1185 t > 151 t, ISP 375 sec, delta-v (ascent) = 5446 m/sPL: 105 t, delta-v total = 9211 m/s Rough guesses, obviously, but I've yet to match the SpaceX charts. When I try to model the 20 tonne GTO mass, the numbers don't converge at all. I get no payload to GTO. - Ed Kyle
By the way, this payload corresponds to a payload mass fraction of 5.68% (250/4400t). Saturn V was 3.88%; Energia was 3.96%; F9 FT is 4.15% IIRC. (!)
Quote from: octavo on 10/07/2017 09:02 amQuote from: Zed_Noir on 10/07/2017 08:22 amQuote from: DAZ on 10/06/2017 11:28 pm....The one thing that is not possible or practical is an escape system at either Mars or the moon. The reason for this is quite obvious. All escape systems depend on being rescued after the fact. This is possible just about any place on earth. Even the escape system for the B-58 Hustler could sustain somebody floating in the water are in the Arctic for up to 4 days. There is nobody on the moon or Mars to rescue anybody. Even if you successfully escaped the BFR you would still inevitably die. That’s the way it would be for the foreseeable future. Only after the BFR has made so many trips to those destinations to prove its reliability would rescue in those places be possible but then there would not be a need for a rescue system on the BFR.That is not quite true. If there is a fueled BFS available on the Moon or Mars. A rescue mission could be mounted with the BFS on a ballistic hop to pick up the survivors.Yes, it is the equivalent of sending a cruse ship to do the job of a life boat. How does the unpiloted bfs plot this hop? I would think this a tall ask in an emergency without some sort of GPS analog around the moon / Mars first?There should be SX Starlink satellites around if the BFS is there at the Moon or Mars.Distress beacons and reflective panels from the survivors's means of escape from a doomed BFS to tracked the trajectory to the landing site from orbital and static observation asserts. Since both the Moon and Mars are extensively survey by orbital satellites. The BFS doing the rescue can use a terrain matching navigation system to go to the survivors once they are located by orbital asserts.
Quote from: edkyle99 on 10/06/2017 06:31 pmQuote from: AncientU on 10/06/2017 05:13 pmBy the way, this payload corresponds to a payload mass fraction of 5.68% (250/4400t). Saturn V was 3.88%; Energia was 3.96%; F9 FT is 4.15% IIRC. (!)I've been trying to rocket-equation this, with little success. Here are the "knowns".GLOW 4400 tThrust Liftoff 5400 t, ISP = 330/356 secShip dry mass 85 tShip Mp 1100 tShip Thrust 775 t (4 engines) ISP 375 secShip Thrust 347 t (2 SL engines) ISP 330/356 secThese imply a first stage mass = 4400 t - 1185 t = 3215 tUnknown is first stage propellant mass fraction. When I plug the known numbers into the rocket equation, I get a first stage PMF required to be 0.97938 to get 250 tonnes to 9,200 m/s ideal delta-v (LEO). That's unrealistic because the first stage ends up with 20 tonnes lighter dry mass than the second stage "Ship". With PMF1 a more "reasonable" 0.96, I get total ideal delta-v = 9061 m/s, not usually good enough for LEO, but it depends on the details of the ascent. To get 9200 m/s with PMF1 = 0.96, payload maximum is 235 tonnes.S1: 3215 t > 128.6 t, ISP 347.4 sec, delta-v = 3734 m/sS2: 1185 t > 85 t, ISP 375 sec, delta-v = 5479 m/sPL: 235 t, delta-v total = 9217 m/sWhen I try to model the reusable alternative, assuming 10% propellant saved for first stage flyback landing and 6% for second stage retro and landing, I get only 105 tonnes of LEO payload, as follows.S1: 3215 t > 437 t, ISP 347.4 sec, delta-v (ascent) = 3265 m/sS2: 1185 t > 151 t, ISP 375 sec, delta-v (ascent) = 5446 m/sPL: 105 t, delta-v total = 9211 m/s Rough guesses, obviously, but I've yet to match the SpaceX charts. When I try to model the 20 tonne GTO mass, the numbers don't converge at all. I get no payload to GTO. - Ed KyleA big incorrect assumption here: that 9.2km/s is required. Technically the absolute minimum energy for a 200km orbit (i.e. Including the potential energy from 200km altitude) above the equator (so using Earth's spin to maximum effect) is just 7.5-7.6km/s.BFR doesn't use hydrogen, so it should get lower aero losses and higher averaged thrust to weight ratio (since your tanks empty sooner). Therefore the benchmark 9.2km/s need not apply, and I am nearly certain I've seen a legitimate estimate for BFR that shows a trajectory of 8.9something km/s to orbit.If you use the exact same mass fraction as last year's ITS booster (slightly better than 96%) and your numbers for Isp and stage mass, then you get 250 tons expendable to LEO at about 8.99km/s. That fits perfectly with all the rest of the info we have.No mystery. If you try to mess with SpaceX's numbers to fit more conservative assumptions, then of course you'll not be able to recreate their figures. That's not a mystery, either.