I've read in quite a few places now, that the I_{sp} of different propellant mixes scales with the density of the fuel. So, for example, RP-1 offers more ISP than methane etc. (All other things being equal.. consider some hypothetical engine that burns Metholox as well as it burns Kerolox).Why is this?

Specific impulse scales with density? That's a new one on me. Hydrolox has a notoriously low density yet a very high specific impulse.Specific impulses are usually quoted for a particular expansion ratio. In effect, specific impulses given for propellant combinations are actually for particular idealized engines.More complex molecules (in the exhaust products) will have more non-translational degrees of freedom. In principle this does make complex exhaust products less attractive, but for a reasonably large expansion ratio, the effect is pretty small.

Quote from: Hop_David on 01/18/2014 04:23 pmHowever earth's orbit about the sun is a much more leisurely degree per day. (360 degrees/365 days). In this case [of a spacecraft in heliocentric orbit] we'd remain in a 60 degree neighborhood of perihelion for two months. 100 micro-gees * 2 months = 5 km/s.For heliocentric orbits, low thrust, high ISP engines might be able to enjoy some Oberth benefit. Once out of planetary wells, it seems to me we can have our ion cake and eat it too.That's very well expressed. Thanks! And just to check the reverse works too? I.e. a spacecraft that has just managed to escape from Mars (but is still in essentially the same orbit around the Sun as Mars) has plenty of time at aphelion to perform an ion "burn" that drops its perihelion down to intersect Earth?

However earth's orbit about the sun is a much more leisurely degree per day. (360 degrees/365 days). In this case [of a spacecraft in heliocentric orbit] we'd remain in a 60 degree neighborhood of perihelion for two months. 100 micro-gees * 2 months = 5 km/s.For heliocentric orbits, low thrust, high ISP engines might be able to enjoy some Oberth benefit. Once out of planetary wells, it seems to me we can have our ion cake and eat it too.

Certain prop combinations have standout Isp vs density (vs usability).

Approximately what portion of delta v goes into atmospheric drag?

Actually, thrust also goes as area, too. That places an upper limit on rocket height given a certain chamber pressure and propellant bulk density.

This Isp expressed in seconds is somewhat physically meaningful—if an engine's thrust could be adjusted to equal the initial weight of its propellant (measured at one standard gravity), then Isp is the duration the propellant would last.

Quote from: Adaptation on 03/30/2014 04:59 amApproximately what portion of delta v goes into atmospheric drag?I do not know if you are referring to something in particular. However, from another post and, in turn, from this book:Ariane A-44L: Gravity Loss: 1576 m/s Drag Loss: 135 m/sAtlas I: Gravity Loss: 1395 m/s Drag Loss: 110 m/sDelta 7925: Gravity Loss: 1150 m/s Drag Loss: 136 m/sShuttle: Gravity Loss: 1222 m/s Drag Loss: 107 m/sSaturn V: Gravity Loss: 1534 m/s Drag Loss: 40 m/s (!!)Titan IV/Centaur: Gravity Loss: 1442 m/s Drag Loss: 156 m/s

I got a few questions about gravitational drag. snipLast but not least, why is the Saturn V value of the Drag Loss so drammatically low??? What made it so efficient - assuming this is not just a typo?

I got a few questions about gravitational drag. Frankly speaking, I haven't been able to fully understand how this drag depends on the vertical and horizontal components of the speed. Why GD (gravitational drag) isn't described by an analytic function of the trajectory(x,y), weight(t) and speed(t)?snip

Quote from: pagheca on 04/16/2014 03:10 amI got a few questions about gravitational drag. snipLast but not least, why is the Saturn V value of the Drag Loss so drammatically low??? What made it so efficient - assuming this is not just a typo?A lot of good information about the Saturn V here. http://www.braeunig.us/apollo/saturnV.htmBraeunig says that the low aerodynamic drag was because the Saturn V thrust to weight ratio was low resulting in lower velocity within the lower, thicker atmosphere. He also says that the drag coefficient at max q was about 0.51. I note that Cd= 0.51 is quite low for a rocket moving just above Mach 1, which is when max q happens.

Oh there is. It's equal to -g + ac where ac is centrifugal acceleration. You just have to integrate it along the trajectory you want to fly. Of course g is a function of altitude and ac is a function of horizontal velocity and altitude so the integration is a little complicated...