I thought the vacuum ISP was supposed to be 380 seconds.
Quote from: llanitedave on 01/21/2015 10:29 pmI thought the vacuum ISP was supposed to be 380 seconds.Maybe?as of http://www.nasaspaceflight.com/2014/03/spacex-advances-drive-mars-rocket-raptor-power/ it was 363/321 at 1Mlbf, but later..."A June 2014 talk by Mueller provided more specific engine performance target specifications indicating 6,900 kN (705 tonnes-force) of sea-level thrust, 8,200 kN (840 tonnes-force) of vacuum thrust, and a specific impulse of 380 s for a vacuum version.[1] Earlier information had estimated the design Isp under vacuum conditions as only 363 s.[2]"
MCT will have meaningfully higher specific impulse engines: 380 vs 345 vac Isp. For those unfamiliar, in the rocket world, that is a super gigantic difference for stages of roughly equivalent mass ratio (mass full to mass empty).
Quote from: Burninate on 01/21/2015 10:34 pmQuote from: llanitedave on 01/21/2015 10:29 pmI thought the vacuum ISP was supposed to be 380 seconds.Maybe?as of http://www.nasaspaceflight.com/2014/03/spacex-advances-drive-mars-rocket-raptor-power/ it was 363/321 at 1Mlbf, but later..."A June 2014 talk by Mueller provided more specific engine performance target specifications indicating 6,900 kN (705 tonnes-force) of sea-level thrust, 8,200 kN (840 tonnes-force) of vacuum thrust, and a specific impulse of 380 s for a vacuum version.[1] Earlier information had estimated the design Isp under vacuum conditions as only 363 s.[2]"From the Redditt AMA Elon Musk:QuoteMCT will have meaningfully higher specific impulse engines: 380 vs 345 vac Isp. For those unfamiliar, in the rocket world, that is a super gigantic difference for stages of roughly equivalent mass ratio (mass full to mass empty).
Okay, thanks, that mystery is solved - must have missed that comment. What nozzle diameter is required for a practical reusable FFSC CH4-LOX 500klbf engine, in vac and at SL?
Okay, thanks, that mystery is solved - must have missed that comment.
Quote from: Burninate on 01/21/2015 11:41 pmOkay, thanks, that mystery is solved - must have missed that comment. What nozzle diameter is required for a practical reusable FFSC CH4-LOX 500klbf engine, in vac and at SL?No idea. I don't think that anybody outside of SpaceX knows.As a crude estimate, the M1C had a SL thrust of 161klbf and a bell diameter of about 1m. The Raptor:M1D thrust ratio would be 3.1056:1, and given an equivalent thrust:area ratio the SL Raptor engine bell would be about 1.76m in diameter.
Quote from: malu5531 on 12/12/2013 12:37 pmWhile trying to understand Merlin 1D and in particular "Merlin 1D+"* in depth, I've iterated my calculations a few times and have reached internal coherence and good balance with reality using the following characteristics/specs. Merlin 1D..Merlin 1D Vac..Merlin 1D+..Merlin 1D+ VacNozzle diameter, m1.073.031.073.03I've tried to draw the Falcon 9 first stage with your diameter, but I can't get it to fit. A circle of 8 engines with a diameter of 1,07 m each is going to have an outer diameter of at least 3,8 m as drawn in my CAD program, and that's with the engine nozzles touching each other. But if you look at images of the launch, the engines do not protrude outside the first stage diameter. And there's a gap between the engines. If I limit the outer diameter of the 8 engines to 3,66 m and allow some spacing between them, the nozzle diameter is around 96,5 cm (my drawing was in 1:144). I've also tried measuring the diameter from the second photo. Ignoring the distortion, the space between the center engine and the outer engines is 0.452 times the diameter of the center engine. So the total diameter of the ring of 8 engines is (3 + (2*0.452)) times the diameter of one engine nozzle. If the total ring diameter is 366 cm, then one engine must be 93,5 cm in diameter.
While trying to understand Merlin 1D and in particular "Merlin 1D+"* in depth, I've iterated my calculations a few times and have reached internal coherence and good balance with reality using the following characteristics/specs. Merlin 1D..Merlin 1D Vac..Merlin 1D+..Merlin 1D+ VacNozzle diameter, m1.073.031.073.03
Quote from: malu5531 on 12/31/2013 01:39 amI fully agree, nozzle diameter should be something like ~93-94 cm. I'm not fully there with the model. I have a similar issue with the RD-0162, not sure yet if those are related or there is a more trivial problem with the 1D model (such as adjusting the chamber pressure a bit, since that info might be old). Quote from: malu5531 on 12/31/2013 01:39 amI fully agree, nozzle diameter should be something like ~93-94 cm. I'm not fully there with the model. I have a similar issue with the RD-0162, not sure yet if those are related or there is a more trivial problem with the 1D model (such as adjusting the chamber pressure a bit, since that info might be old).Chamber pressure need not enter into it In the 1D model*. Thrust is F = q ve + Ae (pe - pa) ,where pa is the ambient pressure. Therefore the difference between sea-level (pa = pSL = 1 atm) thrust and vacuum (pa = 0) thrust is Fvac - FSL = Ae pSL ,so Ae = (Fvac - FSL) / pSL .If we take the total thrust of the Falcon 9's first stage at sea level and in vacuo from the Falcon 9 web page and divide by nine, we get single-engine thrusts of 653.9 and 741.3 kN, respectively. For pSL = 101.325 kPa, we get Ae = 0.863 m2 and hence a diameter of 1.048 m, which agrees closely with the value you've calculated. On the other hand, if we look at the web page for the Merlin engine itself, we're told that the engine's vacuum thrust is just 716 kN. This lower thrust gives an exit area of 0.613 m2 and a diameter of just 0.883 m. This fits within the geometric limit found by Hobbes-22.Now, it could be that this simple analysis violates some constraints imposed by your more extensive model. I would think, though (and please correct me if I'm wrong), that the the other constraints have are pretty loose, given SpaceX's reluctance to give engineering specifics.* Actually, I suppose that's not strictly true. If we're going to assume that flow separation occurs once the pressure drops more than a certain amount below ambient, then the effective nozzle area would depend on chamber pressure. Thus far, though, we've been assuming there's no flow separation. As far as I know (which isn't very far), flow separation is usually avoided these days. (The sustainer of the classic Atlas was over-expanded at sea level to the point that flow separation did occur, but that was back when men were men .) Anyway, allowing for flow separation would tend to increase our estimate of the nozzle's size. Since we're pretty close to the size allowed by geometry already, this suggests that separation does not occur.
I fully agree, nozzle diameter should be something like ~93-94 cm. I'm not fully there with the model. I have a similar issue with the RD-0162, not sure yet if those are related or there is a more trivial problem with the 1D model (such as adjusting the chamber pressure a bit, since that info might be old). Quote from: malu5531 on 12/31/2013 01:39 amI fully agree, nozzle diameter should be something like ~93-94 cm. I'm not fully there with the model. I have a similar issue with the RD-0162, not sure yet if those are related or there is a more trivial problem with the 1D model (such as adjusting the chamber pressure a bit, since that info might be old).
I fully agree, nozzle diameter should be something like ~93-94 cm. I'm not fully there with the model. I have a similar issue with the RD-0162, not sure yet if those are related or there is a more trivial problem with the 1D model (such as adjusting the chamber pressure a bit, since that info might be old).
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?
As a crude estimate, the M1C had a SL thrust of 161klbf and a bell diameter of about 1m. The Raptor:M1D thrust ratio would be 3.1056:1, and given an equivalent thrust:area ratio the SL Raptor engine bell would be about 1.76m in diameter.
15m would be the limit
You need gimballing space:
Quote from: BobCarver on 01/22/2015 02:59 amYou need gimballing space:Do the outer 8 engines gimbal in 2 dimensions each, or only one?
Quote from: BobCarver on 01/22/2015 02:59 amYou need gimballing space:With that many engines, some could be fixed and others able to gimbal. With fast computers and advanced electronic accelerometers and gyros, all the thrust vectoring could be done by only a few of the engines.You also have the option of skirt fairings.