But M1D has an underexpanded nozzle at altitude; isn't possible that throttling makes the nozzle less underexpanded (gaining some ISP).
Is the only difference between a Merlin 1D and a Merlin 1D Vac the nozzle extension on the latter?
Quote from: ClaytonBirchenough on 05/05/2013 04:55 pmIs the only difference between a Merlin 1D and a Merlin 1D Vac the nozzle extension on the latter? No there are other differences. For example the M1D gas generator exhaust has a separate nozzle and the M1D Vac gas generator exhausts into the main nozzle F-1 style.
It was probably relatively easy to convert Merlin 1D into Merlin 1D VAC though, right?
It's not a stupid question.. but the answer isn't obvious. There is some advantage to adding more thrust to an upper stage, due to the Oberth effect, but it is minor compared to improving ISP.. so, it's only really a sensible tradeoff between comparable engines. Improving ISP of an upper stage gives you an exponential improvement, which can be "spent" in making the stage more bulky in order to hold the more bulky propellants, for a net win.
Also, adding thrust means worsening your fmp (if you don't enlarge the whole stage). A worse fmp is just as critical as isp. And adding a second engine usually means a smaller nozzle.For purely orbital maneuvers, isp trumps thrust almost every time. As stated before, you have the Oberth Effect, and the more T/W the more your maneuver approaches an instant impulse, which might be more critical on certain application. But for uppers stages, the general rule is that thrust is not critical unless you are sub orbital. Thus, highly capable second stages, like the Centaur or DIVUS, could take the fmp hit of a second RL10, and improve their LEO performance, because of the gravity losses, but for higher energy orbits, the performance is worse with two engines, since the suborbital time is much less.In general, the Falcon 9 US has a lot of T/W, but that's the consequence of using a 9 to 1 engine relationship. And with the Merlin 1C, the Vac version had less thrust than the first stage version. Since the 1D is the same engine (turbopump, chamber, etc.) with a better nozzle expansion, the Vac version will be more powerful than the first stage version. This might worsen the high energy performance of the Falcon 9, but might be needed for the Falcon Heavy.
A second engine doubles the total thrust, which means you'll accelerate quicker, but it also doubles the rate you use the propellant, which means you can't fire the engines for as long. These effectively cancel out, but you're also carrying the additional weight of the extra engine, so performance is poorer.Because you have more thrust, you could extend the stage and carry more propellant, but this reduces your acceleration. Also, the first stage now has more mass to carry and this reduces that stage's performance.There's plenty of room on a Falcon 9 for additional engines, as the diameter of the two stages are the same. The fact that there is only one second stage engine, despite SpaceX's fondness for redundancy, tells you something.
Quote from: baldusi on 05/06/2013 01:16 pmAlso, adding thrust means worsening your fmp (if you don't enlarge the whole stage). A worse fmp is just as critical as isp. And adding a second engine usually means a smaller nozzle.For purely orbital maneuvers, isp trumps thrust almost every time. As stated before, you have the Oberth Effect, and the more T/W the more your maneuver approaches an instant impulse, which might be more critical on certain application. But for uppers stages, the general rule is that thrust is not critical unless you are sub orbital. Thus, highly capable second stages, like the Centaur or DIVUS, could take the fmp hit of a second RL10, and improve their LEO performance, because of the gravity losses, but for higher energy orbits, the performance is worse with two engines, since the suborbital time is much less.In general, the Falcon 9 US has a lot of T/W, but that's the consequence of using a 9 to 1 engine relationship. And with the Merlin 1C, the Vac version had less thrust than the first stage version. Since the 1D is the same engine (turbopump, chamber, etc.) with a better nozzle expansion, the Vac version will be more powerful than the first stage version. This might worsen the high energy performance of the Falcon 9, but might be needed for the Falcon Heavy.Sorry, you lost me in the first sentence: 'fmp'? Also the 2nd stage should be able to handle the width of 2 engines without nozzle diameter reduction - shouldn't it?Thanks.
There was some discussion on another thread about the difference between running engines dry and shutting down with some small abount of propellant in the lines and pumps, which got me to wondering.What is the likely failure mode if you run a kero-lox turbopump engine like the Merlin-1D dry? I've always assumed it would be the turbos going overspeed and self destructing, but I don't really know. Maybe it's something else? And, is RUD pretty much guaranteed, likely or just possible in this scenerio?