Quote from: CuddlyRocket on 10/15/2017 12:00 pmIf they're using the BFS in SSTO mode for testing then they could replace some or all of the Vac engines with SL ones.No, they won't be doing that:Quote from: Elon MuskWill be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.(my bold)https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/
If they're using the BFS in SSTO mode for testing then they could replace some or all of the Vac engines with SL ones.
Will be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.
Quote from: LucR on 10/15/2017 12:11 pmQuote from: CuddlyRocket on 10/15/2017 12:00 pmIf they're using the BFS in SSTO mode for testing then they could replace some or all of the Vac engines with SL ones.No, they won't be doing that:Quote from: Elon MuskWill be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.(my bold)https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/The deep space Raptors, which your quote shows they don't need, *are* the Vac engines.
Quote from: Kaputnik on 10/15/2017 12:18 pmQuote from: LucR on 10/15/2017 12:11 pmQuote from: CuddlyRocket on 10/15/2017 12:00 pmIf they're using the BFS in SSTO mode for testing then they could replace some or all of the Vac engines with SL ones.No, they won't be doing that:Quote from: Elon MuskWill be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.(my bold)https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/The deep space Raptors, which your quote shows they don't need, *are* the Vac engines.Yes, that was my point, he didn't say anything about *replacing* them, just not using them...
The "vacuum" or high area ratio Raptors can operate at full thrust at sea level. Not recommended.
Quote from: LucR on 10/15/2017 12:47 pmQuote from: Kaputnik on 10/15/2017 12:18 pmQuote from: LucR on 10/15/2017 12:11 pmQuote from: CuddlyRocket on 10/15/2017 12:00 pmIf they're using the BFS in SSTO mode for testing then they could replace some or all of the Vac engines with SL ones.No, they won't be doing that:Quote from: Elon MuskWill be starting with a full-scale Ship doing short hops of a few hundred kilometers altitude and lateral distance. Those are fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep space Raptor engines.(my bold)https://www.reddit.com/r/space/comments/76e79c/i_am_elon_musk_ask_me_anything_about_bfr/dodec8l/The deep space Raptors, which your quote shows they don't need, *are* the Vac engines.Yes, that was my point, he didn't say anything about *replacing* them, just not using them...BFS with 3 engines does not have enough thrust to carry the fuel to fly hundreds kilometers. Elon's comment is clearly meant to communicate the use of sea level capable engines.
My take is that an SSTO variant may become more relevant again in a few years, once the Raptor engine has (inevitably) improved over the current version and once SpaceX has learned more about their composite tanks. Looking at the huge strides that the Merlin 1D has made since the very first version and assuming only a fraction of that improvement for the Raptor, an ITS/Tanker derived SSTO with a large enough payload to LEO seems feasible. I think that for LEO payloads an SSTO with a F9 sized payload would be cheaper to operate than the BFR/BFS- two stage system. Since it would be smaller, it would probably also offer some operational advantages.
Missions beyond LEO might require on orbit refueling and then it is a question whether two stages are better than two SSTOs...