Why is it terrible? Russian engines are pretty reliable, aren't they?
I don't believe this for a second. MCT needs to get to orbit with crew or 100t payload. The operational complexity of transfering cargo, including possible large pieces of equipment would be huge. The only thing that will be transfered is fuel. That is my firm opinion. It does not need that huge a launch vehicle. With the second stage being MCT its weight is not fully part of what would usually be counted as payload. The engines and fuel tanks are "free".
I do realize that if you do come to a mid-air full-stop hover that your grid fins can no longer contribute to stability and control. Still, the ability to throttle down closer and closer to a hover as you approach the pad is helpful.
I keep wondering how deeply Raptor will throttle.
Quote from: TomH on 01/22/2015 08:10 pmI do realize that if you do come to a mid-air full-stop hover that your grid fins can no longer contribute to stability and control. Still, the ability to throttle down closer and closer to a hover as you approach the pad is helpful.Why? This seems to get repeated over several threads. A computer monitors all the inputs and can 1000 (or 10,000, or even 100,000) times a second have a model of exactly how far from centre its current course with current thrust takes it. It can that many times a second adjust its commands that control grid fins, gymballing, thrust, and RCS. If it stops and hovers it risks having to make larger and larger corrections (oscillating) to actually stay in place then slowly land. Hover and land slowly will be far more difficult to program, and will have lower tolerances for wind (in aviation it would be called cross wind, but for a rocket landing vertically all wind is cross wind).
Quote from: nadreck on 01/22/2015 08:22 pmQuote from: TomH on 01/22/2015 08:10 pmI do realize that if you do come to a mid-air full-stop hover that your grid fins can no longer contribute to stability and control. Still, the ability to throttle down closer and closer to a hover as you approach the pad is helpful.Why? This seems to get repeated over several threads. A computer monitors all the inputs and can 1000 (or 10,000, or even 100,000) times a second have a model of exactly how far from centre its current course with current thrust takes it. It can that many times a second adjust its commands that control grid fins, gymballing, thrust, and RCS. If it stops and hovers it risks having to make larger and larger corrections (oscillating) to actually stay in place then slowly land. Hover and land slowly will be far more difficult to program, and will have lower tolerances for wind (in aviation it would be called cross wind, but for a rocket landing vertically all wind is cross wind).What TomH is referring to is that when then stage is hovering there is no airflow through the grid fins. Without airflow the grid fins don't work.It's another reason not to hover, just come in and land.
What TomH is referring to is that when then stage is hovering there is no airflow through the grid fins. Without airflow the grid fins don't work.It's another reason not to hover, just come in and land.
Quote from: RonM on 01/22/2015 08:39 pmWhat TomH is referring to is that when then stage is hovering there is no airflow through the grid fins. Without airflow the grid fins don't work.It's another reason not to hover, just come in and land.Not at all. During the landing burn the engine will give all control authority needed. The big reason to come in and land is saving fuel.
Quote from: guckyfan on 01/22/2015 07:15 pmI don't believe this for a second. MCT needs to get to orbit with crew or 100t payload. The operational complexity of transfering cargo, including possible large pieces of equipment would be huge. The only thing that will be transfered is fuel. That is my firm opinion. It does not need that huge a launch vehicle. With the second stage being MCT its weight is not fully part of what would usually be counted as payload. The engines and fuel tanks are "free".Won't a single rocket capable of sending 100t to the surface of Mars have to lift like 500+t to LEO? Even with 15m diameter BFR, is it plausible? Sea Dragon was supposed to lift 550t and be 23m in diameter.
The Apollo LEM was about 10mt dry (descent stage), but could land around 6mt of "cargo" in a fully fueled ascent stage, astros, provisions, cargo and equipment. Could a dryer MCT weight about 50mt, and carry 100mt of cargo, and then another X tonnes of propellant when fully fueled? That would put the minimum required single BFR to loft about 150mt. I think it could be around Saturn V size and do that.
Quote from: Lobo on 01/22/2015 09:50 pmThe Apollo LEM was about 10mt dry (descent stage), but could land around 6mt of "cargo" in a fully fueled ascent stage, astros, provisions, cargo and equipment. Could a dryer MCT weight about 50mt, and carry 100mt of cargo, and then another X tonnes of propellant when fully fueled? That would put the minimum required single BFR to loft about 150mt. I think it could be around Saturn V size and do that.As a wild uneducated guess I thought of 60t for the empty MCT so we are in the same ballbpark. Yes 150 to 160t is needed with fuel to spare for first stage RTLS. This leaves no margin for a escape pod or escape engines. Likely at least one, maybe two engines extra for engine loss capability during the long flight and to add ability to separate from a failing first stage if not enough to speed away from a fireball. Early small crews may board separately. Later, when 100 colonists go to Mars there will be a long and hopefully positive history behind the vehicle that allows the confidence to launch them in MCT.
AlsoQuote from: llanitedave on 01/22/2015 02:33 amAt 1.76m engine bells, looks like you can fit 19 of them comfortably on a 10m diameter stage. Think that would be enough for a start?No By the way, what do you use for modelling?
At 1.76m engine bells, looks like you can fit 19 of them comfortably on a 10m diameter stage. Think that would be enough for a start?
In my thought, all missions would require staging in LEO with several of these "smaller" BFR's to fuel them up, and then some sort of LEO taxi, probably a modified MCT for that purpose, takes the crew up. This LEO taxi could have some sort of LAS system. The one going to Mars doesn't need it. The taxi would take up, up to 100 colonists (eventually) with their personal gear.
Is the fixation on Mars mission profiles making people lose sight that the most important thing necessary to transform space travel is cheap access to low earth orbit? Once cheap access to LEO becomes possible then many other possibilities open up, including slow shipment of non-human freight to Mars and other places by ion propulsion (how the payload gets there doesn't have to decided by SpaceX), not to mention the fact that cheap launches could inspire lower value payloads. Perhaps this kind of thinking is starting to affect the design of SpaceX's next vehicle.I'm sure this has been discussed many times, so could someone please point me at a thread on this forum about optimising cost to LEO, without any other consideration being involved?