Saturn V acceleration

[ka9q]

EMR = Engine mixture ratio.  The J-2 engine had a valve that could control the flow of LOX to the combustion chamber.

The interesting thing about the EMR shift is that even though it took the engines even further from stoichiometric (which would be an 8:1 oxidizer:fuel mass ratio) the specific impulse actually increased a few seconds. The amount of unburned hydrogen in the exhaust increased, decreasing its average molecular weight and the efficiency with which the engine converted combustion energy into the kinetic energy of the exhaust. That more than made up for the lower combustion power.

Why not just run with the richer mixture? Two reasons I can see. First, the EMR shift was, as Bob says, timed to cause the two propellants to deplete simultaneously; this time was calculated on the fly by the IU based on measured consumption rates.

Second, while high specific impulse is generally a good thing, high thrust is even more important during the early stages of flight when the rocket is pitched up and overcoming gravity losses. As it pitches down toward the horizontal, gravity losses decrease and thrust can be decreased in favor of specific impulse since the rocket is at that time building up horizontal velocity to make orbit. Also, decreasing thrust toward the end of a stage burn can help limit peak acceleration stresses.

[Allan F]

Impressive. I ask a simple question, which leads to a whole new line of information I didn't even considered existed. Layer upon layer of complexity.

Thank you for your information.

[AstroBrant]

In addition to what's already been said about the shuttle acceleration, it must also be due to the two-part firing of the engines. By the time the SRBs were lit up, much of the craft's weight was already canceled out by the orbiter's main engines.
(IMHO)

[Bob B.]

The interesting thing about the EMR shift is that even though it took the engines even further from stoichiometric (which would be an 8:1 oxidizer:fuel mass ratio) the specific impulse actually increased a few seconds. The amount of unburned hydrogen in the exhaust increased, decreasing its average molecular weight and the efficiency with which the engine converted combustion energy into the kinetic energy of the exhaust. That more than made up for the lower combustion power.

The mixture ratio of most engines is at or near that that produces the highest specific impulse.  Hydrogen burning engines are an exception.  Most operate at mixtures ratios between 5:1 and 6:1, while highest specific impulse occurs closer to 4:1 or even lower.  The reason for using the higher mixture ratio is to lower the overall density of the propellant and decrease the size of the propellant tanks.  For example the total volume occupied by the LOX+LH2 at 4:1 is 27% greater than at 6:1 for the same mass.  Smaller tanks mean less weight and less drag, though this comes at the expense of slightly lower specific impulse.  It works out the maximum delta-v is achieved with mixtures ratios in that 5:1 to 6:1 range.

Second, while high specific impulse is generally a good thing, high thrust is even more important during the early stages of flight when the rocket is pitched up and overcoming gravity losses. As it pitches down toward the horizontal, gravity losses decrease and thrust can be decreased in favor of specific impulse since the rocket is at that time building up horizontal velocity to make orbit. Also, decreasing thrust toward the end of a stage burn can help limit peak acceleration stresses.

While the S-II (second stage) operated mostly at the high-mixture ratio setting, the S-IVB (third stage) operated at the intermediate- and low-mixture ratio settings.  The S-IVB operated at a time when gravity loses were inconsequential, so they could sacrifice some thrust in exchange for higher specific impulse.

[Abaddon]

Just by eye, one thing I noticed in being an avid launch watcher, The Shuttles acceleration from the launch pad seemed noticeably quicker than the Saturn V. Is this just my eye or a fact? I should look back over the launch videos and time it.

I don't believe that's just your eye.  The shuttle fairly leaps off the pad and is generally going in excess of 140 km/h when it clears the tower, according to one of the press kits.

How much does the prevalence of playing back high speed camera footage at lower speed contribute to the impression of slow motion? It's pretty commonplace in documentaries and so forth to do so.

[Glom]

Sounds like engineering thought went into this. Most disturbing.

Immediate thought is that it seems weird that reducing the molecular weight of the exhaust would increase specific impulse. I would have thought it would be the other way round as higher molecular weight gives more impulse for kinetic energy. But maybe I actually need to review the maths.

[Bob B.]

Immediate thought is that it seems weird that reducing the molecular weight of the exhaust would increase specific impulse. I would have thought it would be the other way round as higher molecular weight gives more impulse for kinetic energy. But maybe I actually need to review the maths.

Lightweight molecules can be accelerated to higher velocity.  Heavy molecules have more momentum per molecule, but light molecules have more momentum per unit mass.

[gwiz]

Temperature of the gas in the combustion chamber is a measure of the average kinetic energy of the molecules, proportional to mass times velocity squared.  For a given maximum temperature that you can get your rocket to withstand, low mass molecules have greater velocity than high mass ones.

[cjameshuff]

Sounds like engineering thought went into this. Most disturbing.

Immediate thought is that it seems weird that reducing the molecular weight of the exhaust would increase specific impulse. I would have thought it would be the other way round as higher molecular weight gives more impulse for kinetic energy. But maybe I actually need to review the maths.

More impulse for a given amount of kinetic energy, but specific impulse is a measure of impulse per fuel mass. Specific impulse is a measure of mass efficiency, not energy efficiency: the faster you eject a given mass, the greater the total impulse you get from it, at the cost of energy requirements increasing with the square of exhaust velocity.

[JayUtah]

This is one of the reasons why solid fuel rocketry is still a thing.  While solid fuels generally have lower specific impulse, they have much greater mass density and invoke the tradeoffs involved with building the rocket casings and fuselages.  Aside from the qualitative advantages of solid fuels in some application, you can cram more propellant mass into a smaller fuselage that way.  Fuselage and casing structural requirements scale generally according to the cube of the rocket size, affecting the mass ratio.

[AstroBrant]

This is one of the reasons why solid fuel rocketry is still a thing.  While solid fuels generally have lower specific impulse, they have much greater mass density and invoke the tradeoffs involved with building the rocket casings and fuselages.  Aside from the qualitative advantages of solid fuels in some application, you can cram more propellant mass into a smaller fuselage that way.  Fuselage and casing structural requirements scale generally according to the cube of the rocket size, affecting the mass ratio.

I would think another effective advantage of solid fuel is that it eliminates the need for weighty fuel/oxidizer pumping systems.

[Allan F]

This is one of the reasons why solid fuel rocketry is still a thing.  While solid fuels generally have lower specific impulse, they have much greater mass density and invoke the tradeoffs involved with building the rocket casings and fuselages.  Aside from the qualitative advantages of solid fuels in some application, you can cram more propellant mass into a smaller fuselage that way.  Fuselage and casing structural requirements scale generally according to the cube of the rocket size, affecting the mass ratio.

I would think another effective advantage of solid fuel is that it eliminates the need for weighty fuel/oxidizer pumping systems.

In order to get maximum take-off thrust, you need to have a complex hollow shape in the middle of your solid fueled rocket, otherwise you'd get a very slow start, and an undesireable power ramp-up till burn-out. So a lot of your solid fueled rocket will be empty space.

[Echnaton]

Do this elimination of certain problems lead to a cost advantage?  Or does it just introduce other problems that offset the advantages and eat up cost savings elsewhere?  I remember reading around here in the past that solid boosters introduced a significant vibration due to uneven levels of thrust. 

I would think that the transportation costs for the "fueled" solid rockets needed for a heavy lift booster would start to offset many other advantages.  Vs Lox and H2 produced on site at the KSC. 

[Bob B.]

In order to get maximum take-off thrust, you need to have a complex hollow shape in the middle of your solid fueled rocket, otherwise you'd get a very slow start, and an undesireable power ramp-up till burn-out. So a lot of your solid fueled rocket will be empty space.

From photos and diagrams I've seen, the center channel of at least some fuel blocks looks to be about 1/3 of the diameter.  That would make the block about 1/9th empty space.

It should also be noted that it's possible to control the thrust profile by using center channels with different shapes.  For example, a smooth cylindrical channel results in a progressively greater thrust because the exposed surface area of the block increases as the fuel burns away.  With a larger burn surface the fuel burns at a greater rate, producing more gas and more thrust.  Another common type is a "star" shaped channel.  This produces relatively constant thrust because the surface area doesn't changed much as the fuel burns away.  It's possible to produce many different thrust profiles simply by changing the solid fuel geometry.

[Bob B.]

I would think that the transportation costs for the "fueled" solid rockets needed for a heavy lift booster would start to offset many other advantages.  Vs Lox and H2 produced on site at the KSC.

I believe the Space Shuttle SRBs where shipped to KSC via barge from Louisiana.   It's may understanding that water is the least expansive mode of transportation.