I also think that it’s cool that the flame is about 50% longer than the rocket is tall. I don’t know if this is typical, but it sure is impressive on something this big.

Probably related to the velocity of the expelled hot gasses from the engines. Higher velocity equals higher specific impulse IIRC and I think the Raptor engines have a pretty high specific impulse.

There are some scaling laws at work here.
The volume and thus loaded weight goes up with the 3rd power of dimension.
The base area of the rocket where you can mount engines and generate thrust goes up with the 2nd power of dimension. As the rocket grows there comes a point where you can't fit enough rocket under the vehicle to lift it. As scales much beyond Super Heavy the rocket will need to be shorter and stubbier.

The proposed Sea dragon was shorter in comparison to the length/diameter ratio of super heavy.

Strap-ons.

Yeah watching it emerge from the Tower and that tail just kept going and going and going.

MikeC are you saying it would be about 1.5G near sealevel if sitting in the Ship?
With propellent consumed at 40,000lbs per SECOND I bet that ramps up very fast.

I wish / hope that Spacex would dribble out some engineering video. Seems that has tightened up a bit lately. Maybe it is too ITARy

Not sure I follow that.

Volume (and thus roughly mass) of a cylindrical rocket is pi * (1/2 * diameter)^2 * length

Area of the base (and thus roughly available thrust) is pi * (1/2 * diameter)^2

Therefore, so long as each engine can overcome its own weight plus the aero forces exerted on the area required to accommodate it, the rockets can get bigger in diameter almost without bound (there are some limits as the strength of certain parts of the base have to be increased and therefore there is a practical limit but this is a theoretical discussion).

Thrust to weight ratio is roughly a ratio of the above which in the unitless simplifies to just length. Thus maximum length of a rocket is determined by the thrust/area of each engine.

Not sure I follow that.

Volume (and thus roughly mass) of a cylindrical rocket is pi * (1/2 * diameter)^2 * length

Area of the base (and thus roughly available thrust) is pi * (1/2 * diameter)^2

Therefore, so long as each engine can overcome its own weight plus the aero forces exerted on the area required to accommodate it, the rockets can get bigger in diameter almost without bound (there are some limits as the strength of certain parts of the base have to be increased and therefore there is a practical limit but this is a theoretical discussion).

Thrust to weight ratio is roughly a ratio of the above which in the unitless simplifies to just length. Thus maximum length of a rocket is determined by the thrust/area of each engine.

350 seconds for the sea level Raptor, 380 for the vacuum optimized version. Not exactly an RS-25, but there are 33 of them.

If the rocket has the same visual shape,
ie the same ratio of length to diameter.
The volume/loaded mass goes up with 3rd power
while the base area goes with 2nd power.

For constant engine pressure and isp the thrust is
proportional to exit area.
(Some other terms here, but linearly proportional)

So at some point as the rocket gets bigger you can no longer
lift it without making it stubbier.

Maybe they should try running it LOP?

They run it LOP _and_ ROP at the same time (one turbo pump runs LOP, the other runs ROP).

Yes, approximately.

17,000,000 lbs thrust / 11,000,00 lbs mass = 1.55 G.


11,000,000 lbs at lift off with 17,000,000 lbs thrust. They need to get down to about 8,000,000 lbs to get 2 G (assume some aero drag). That will take 75 seconds.

So the G force ramp up is not very fast, quite gentle really.

Mike C.

Its an interesting problem in rocket design. If you take off at very low acceleration, almost all of the thrust initially is fighting gravity, at 1.5g, only .5 of that is actually causing acceleratoin. Then as fuel burns off the acceleration increases - usually on the order of a few X for the first stage.

But...fuel tanks are less expensive than engines, so if you started with a rocket that took off at 3 Gs, adding 2X the total fuel would significantly increase your weight to orbit.


Yes, approximately.

17,000,000 lbs thrust / 11,000,00 lbs mass = 1.55 G.


11,000,000 lbs at lift off with 17,000,000 lbs thrust. They need to get down to about 8,000,000 lbs to get 2 G (assume some aero drag). That will take 75 seconds.

So the G force ramp up is not very fast, quite gentle really.

Mike C.

Only stubbier in length diameter ratio. Not shorter.

Say 1 engine can lift a 200 foot long rocket the diameter of the engine. 10 engines can lift 10 times the area, but still 200 feet long.

Next scheduled launch is Dec 10th at 7:00 PM.