Quote from: AncientU on 07/12/2016 04:34 pmOne interesting tidbit, though:QuoteBut a noise is that SpaceX would work on a Raptor 700 Tf !Not sure of rumor source for an F-1 class engine, but certainly would simplify first stage design if nine 1.5-1.6Mlbf engines were used on booster.Not sure how this jives with the news that the AirForce is paying for a raptor based upper for F9 and FH. 2 engines or rumor wrong?
One interesting tidbit, though:QuoteBut a noise is that SpaceX would work on a Raptor 700 Tf !Not sure of rumor source for an F-1 class engine, but certainly would simplify first stage design if nine 1.5-1.6Mlbf engines were used on booster.
But a noise is that SpaceX would work on a Raptor 700 Tf !
So, it wouldn't surprise me if September reveal shows more than one engine size. (Three is my guess.)
...Landing engines on Mars would require deep throttling of 550klbf engine....
Quote from: the_other_Doug on 07/12/2016 03:29 pmYes, but -- how wide would such a scaled-up Dragon capsule be at the base, if it's got to be designed to carry at least 100 people, along with all the stuff (like food, water and life support) they will need for three to six months?I think simply scaling up the dimensions of a Dragon to where it has the interior space available for the actual stated mission requirements would make the base diameter in the hundreds of meters. How ya gonna fit that on any booster, much less one with the most-commonly-speculated (see L2) width of the BFR?And if you change the basic shape, you inevitably have to go through a lot of design work to achieve similar lift/drag characteristics, etc. In other words, BFS is gonna have to be designed from scratch, it won't take advantage of Dragon's shape or flying characteristics as a starting point.Let's do at least a first order pass at actually calculating this. If BFR's diameter is 15m, BFS can easily be at least 15 to 16m. Dragon's OML including the nose but not the trunk encloses about 25 m^3 with a major radius of 1.85 meters. Volume increases with size as r^3, so a 15 meter Dragon would enclose about 1,875 m^3 and a 16m version about 2,050 m^2Estimates of necessary volume per person and propellant mass vary, but usually range somewhere from 5 to 15 m^2 per person and 300-1200 tonnes of propellant. Averaging those gets 10 m^3 per person and 750t propellant, or 1000 m^2 habitable volume and 915 m^3 tank volume (100 people, 820 kg/m^3 methalox). That totals 1915 m^3, or pretty close to what a 15 to 16m Dragon would enclose.Obviously, that's only enclosed volume and not all the space can be utilized efficiently, and engines etc. all take up volume. But volume shouldn't be a show-stopper considering that it's probably feasible to launch up to a 20m diameter (4,500 m^3 volume) object on top of BFR.
Yes, but -- how wide would such a scaled-up Dragon capsule be at the base, if it's got to be designed to carry at least 100 people, along with all the stuff (like food, water and life support) they will need for three to six months?I think simply scaling up the dimensions of a Dragon to where it has the interior space available for the actual stated mission requirements would make the base diameter in the hundreds of meters. How ya gonna fit that on any booster, much less one with the most-commonly-speculated (see L2) width of the BFR?And if you change the basic shape, you inevitably have to go through a lot of design work to achieve similar lift/drag characteristics, etc. In other words, BFS is gonna have to be designed from scratch, it won't take advantage of Dragon's shape or flying characteristics as a starting point.
Quote from: AncientU on 07/12/2016 05:10 pm...Landing engines on Mars would require deep throttling of 550klbf engine....Just to address this point: Merlin throttles to 40%, and MVac to 30%. That's more than deep enough to land 100t payload on Mars even using two 550 klbf engines.
Even if spacecraft dry weight is same as payload, T/W is still greater than 1 with a single engine.
It is clear Spacex is mastering first stage usability, but they indicated they are not finding practical bringing the second stages down to earth. ULA on their part are toying with ACES, a second stage spending extended time in orbit, but they have not been explaining how they plan to bring supplies to orbit without creating a glut of second stages up there... If it is not practical to bring back to earth the second stage, the only remaining solution for a reusable second stage is to park it in orbit and perform a suborbital load transfer from the first stage carrying the load and a second stage descending from orbit to pick up the load. Unquestionable the maneuver is tricky and must be performed quickly... Have you seen discussions on this topic somewhere?
... If it is not practical to bring back to earth the second stage, the only remaining solution for a reusable second stage is to park it in orbit and perform a suborbital load transfer from the first stage carrying the load and a second stage descending from orbit to pick up the load. Unquestionable the maneuver is tricky and must be performed quickly... Have you seen discussions on this topic somewhere?
Quote from: AncientU on 07/12/2016 09:30 pmEven if spacecraft dry weight is same as payload, T/W is still greater than 1 with a single engine.There is the all important difference between weight and mass. It is the weight that would need to be countered for a hover. But they don't want to hover. It is the mass that needs to be decelerated. That's mostly the same effort as on earth for landing, just with less gravity losses.
My expectation is that the main propulsion will 'hover slam' to a full stop at a height of a few dozen meters above the ground and then cut out. Then the vehicle will touch down on orbital maneuvering engines located higher on the vehicle and canted outward, where they will not impinge on the ground, this will only require the equivalent of a dozen Super Draco engines.