What's the duration of the burn?
The stage final mass may be quite sensitive to the throttle setting, but then so is the corresponding initial propellant load -- and you have some better constraints on the initial propellant load, right?
[...] For what it's worth, here is the output from my Inmarsat-5 sim. The orbit at SECO1 is about 160 x 590 kms, and mass is 25.5mT. At SECO2, mass is 11.6mT. A 4° plane change from 28.5 to 24.5° would require an additional 540m/s, for a burn to depletion S2 total of 9.9mT. Subtracting 6.1mT for the satellite, gives 3.8mT S2 dry mass, a touch lower than I would have expected.
The plane change plus the GTO injection sum as vectors, not as scalars, and they are at right angles. From this orbit, straight GTO injection needs about 2400 m/s. Adding 540 m/s at right angles means a total burn of sqrt(2400^2 + 540^2) = 2460 m/s. So adding the plane changes needs only an additional 60 m/s. That should give a more realistic second stage mass.
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The 934 kN thrust has been listed since 2015 when v1.2 specs were first posted. I rather doubt they were looking forward to Block 5 way back then.Does the data fit better if this was not in fact a burn to depletion, and there was 2,000 kg (or any arbitrary number) of propellant left at the end of the second burn?
Quote from: envy887 on 06/22/2017 12:52 pmThe 934 kN thrust has been listed since 2015 when v1.2 specs were first posted. I rather doubt they were looking forward to Block 5 way back then.Does the data fit better if this was not in fact a burn to depletion, and there was 2,000 kg (or any arbitrary number) of propellant left at the end of the second burn?One would think that there would be reserve propellant in the second stage to make up for potential shortfalls in first stage performance, although maybe the flyback and landing propellant constitutes all the reserve.
We have seen that the second stage throttles down before. See the attachments to my post here:Quote from: Semmel on 08/26/2016 08:59 pm[...]PS: Sorry, still no time to do programming at home and advance the script.@edit: added the plot for convenience.
Quote from: Semmel on 06/22/2017 12:55 pmWe have seen that the second stage throttles down before. See the attachments to my post here:Quote from: Semmel on 08/26/2016 08:59 pm[...]PS: Sorry, still no time to do programming at home and advance the script.@edit: added the plot for convenience.The M1D throttle change is pretty small (perhaps 4-5%). Perhaps it's a mixture ratio switch. In Apollo, they ran the second stage with a "maximum thrust" mixture ratio for the first part of the burn (where gravity losses are most important), then switched to a lower-thrust, maximum ISP mixture ratio for the rest of the burn. ( Described here.) This would generate a profile just like that shown in the plot.
Quote from: LouScheffer on 06/23/2017 12:42 amQuote from: Semmel on 06/22/2017 12:55 pmWe have seen that the second stage throttles down before. See the attachments to my post here:Quote from: Semmel on 08/26/2016 08:59 pm[...]PS: Sorry, still no time to do programming at home and advance the script.@edit: added the plot for convenience.The M1D throttle change is pretty small (perhaps 4-5%). Perhaps it's a mixture ratio switch. In Apollo, they ran the second stage with a "maximum thrust" mixture ratio for the first part of the burn (where gravity losses are most important), then switched to a lower-thrust, maximum ISP mixture ratio for the rest of the burn. ( Described here.) This would generate a profile just like that shown in the plot.That also answers the question of why they would throttle there rather than later in the burn. ISP is generally highest near maximum throttle so ISTM they wouldn't throttle until there is an acceleration limit, which is nowhere near that point in the mission.