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[krenshala]

Oh, I agree that would be better.

[KitemanSA]

"What are we trying to do here?"

Lowest reliable $/lb to LEO.

Correct, so the issue is, would having a special, throw way core for the uniquely heavy loads be cheaper than maintaining an inventory of ONE reusable core and 1 reusable NBAT.
You can have any color so long as its black.
You can have any core so long as its THE core.

[kunkmiester]

No, because mass production. You might strip off extraneous parts like the landing gear, but two different part numbers is more expensive.

[D Tibbets]

"What are we trying to do here?"

Lowest reliable $/lb to LEO.

Correct, so the issue is, would having a special, throw way core for the uniquely heavy loads be cheaper than maintaining an inventory of ONE reusable core and 1 reusable NBAT.
You can have any color so long as its black.
You can have any core so long as its THE core.

I think (again?) that I appreciate KitemanSA's point. Again the answer is complex. Having a neutral weight auxillary tank (included thrust always counterbalancing Earth/s gravity) with a standard reusable core would perhaps be advantageous to having a heavier core alone. Using this auxiliary tank only with the occasional heavy lift mission as opposed to a bigger more powerful one use core may save money. Using the same core with landing legs and guidance vanes stripped off and reserve fuel need negated for the heavy mission is a closer comparison.

All of this assumes that the core has enough initial thrust to maintain an adequate thrust to weight ratio, despite the auxiliary tank mass.This would always require more thrust on the core- larger heavier and more powerful engines. Thus more weight on the core.

Having excess thrust on the auxiliary tank mitigates this problem. This is the Falcon heavy, and to a lesser extent any rocket that has strap on solid boosters or the multiple stage launch design.

The confusion may be due to the difference between mass and weight. In space there is mass but no weight (that needs to be entered into the equation. By space I mean in free fall- orbit or outside of a significant gravitational well. Weight is a result of a mass interacting with a force- the gravity at Earth's surface in this case. You can cancel the weight by applying a thrust equal in intensity and opposite in direction to the mass. The weight (the mass sitting on a balance scale on the Earth's surface becomes zero, but the mass is unchanged. A zero weight auxiliary tank- engine thrust cancelling out Earth's gravity, goes nowhere. It doesn't climb. To climb additional thrust has to be applied. A core with enough thrust to provide a 1.5 T:W ratio means it can accelerate the rocket at 1.5 G. Or, overcome Earth's gravity and accelerate the rocket at 0.5 G. By having an asexually tank with thrust that cancel' Earth's gravity acting on the tank,still leaves only the 0.5 G of excess thrust from the core. This thrust is acting on more mass though, so actual acceleration will be less. An example- a rocket core and payload of 1 million pounds. An auxiliary tank with a mass of 1 million pounds (expressed in KG if you are a purist) with 1 million pounds of thrust balancing it's weight. The total mass of the system is now 2 million pounds and the thrust is 2.5 million pounds. The thrust to weight ratio is not 2.5/2 or 1.25 instead of the core alone ratio of 1.5.
Only 500 thousand pounds of thrust are actually available for acceleration. You burn up more fuel to achieve the same velocity. With the auxiliary fuel tank weight decreasing as it burns fuel and possibly transfers it to the core, thrust becomes positive against gravity and it contributes to acceleration of the space craft. Either that or you throttle back the tank rocket, which makes little since given this argument.

What all of this means is that an auxiliary tank can help if it's weight against gravity is cancelled, but it still drags down the acceleration, especially on the early stages of launch. As pointed out this early acceleration against the mass of the rocket is the most fuel costly stage of the launch. Having auxiliary tanks (or solid boosters) can perhaps help, but without significant excess thrust contributed by that auxiliary structure the gains are minimized. Any excess thrust you can add to the tank will help.

There may be an exception, where the thrust of the core is great enough, that the added mass is not too painful. An example. Core thrust 3 million pounds, throttlable downward for later portions of the launch so that you do not tear the spacecraft apart/ incapacitate the crew. Core and payload mass of 1 million pounds, and maxillary tank mass of 1 million pounds. Even without engines on the tank, the T:W ratio is 1.5. Here the tank could be disposable but cheap, or it might be recoverable. This is the approach of the Shuttle. The thrust of the main engines and boosters was so great, that the passive mass of the external fuel tank was tolerable, and miner in the initial challenging launch regime. Later, after booster separation some of the weight had been consumed and the down range velocity decreased the Earth's gravity contribution to the effective thrust versus acceleration picture. There are a lot of competing considerations that have to be balanced for the best solution for a rocket design.

Note that I mixed mass and weight units while arguing about weight versus mass. Hopefully this was not too confusing.

Dan Tibbets

[TDPerk]

Because some degree of Thrust over Weight is required, and is usually a T/W ratio of 1.333 at minimum, and frequently close to 2, I cannot conceive of a neutrally weighted booster/AuxTank cross feeding into the core as being useful, unless the T/W of the booster/AuxTank is at least equal to the T/W of the core.

As long as the T/W of the Core when fully fueled just after AuxTank departure is acceptably greater than 1 (and I suspect you want the AuxTanks dropped before MaxQ), I think you'd want the T/W of the assembly at launch to be greater than the T/W of the Core alone; because you are shedding weight you can no longer benefit from.

The relatively immense quantities of thrust required for liftoff are an embarassment that must be throttled down at MaxQ, quit hauling it up.

[krenshala]

I didn't bother flying them, as I don't think that would add to the discussion at all, however, here are some screenshots of various core set-ups and the delta-v provided by them. I did not attempt to match the F9, just the general setup SpaceX says they plan to be using.

Each screenshot shows the staging (icons on the lower right side), part and mass info (middle), and the delta-v, TWR, and burn time numbers (top). I've cut and pasted stuff around to reduce the images sizes, which is why they look a bit choppy. Delta-v numbers are "vacuum", and do not have the reduction due to being in atmosphere applied (the Isp of the engines improves as atmospheric pressure drops, plus atmospheric drag eats away some of the capability).

First, there is the payload. In this case, a single kerbal capsule (Gemini equivalent, though roomier, since kerbals are only a meter tall): http://koboldi.net/img/Payload.png. Mass is 4764kg, and it provides 1708 m/s of delta-v. This is the equivalent to the dragon capsule and second stage. I am not removing the delta-v provided by this stage from the numbers below, but keep in mind they all share those 1708m/s.

The single core version: http://koboldi.net/img/SingleCore.png. This has a total mass of 15,137kg, and 3920m/s of delta-v (gets to orbit and on an efficient ascent would have about 300m/s of delta-v left due to how small the planet Kerbin is in the game). Thrust to weight ratio is 1.45 at lift off.

The boosters-burn-first version: http://koboldi.net/img/BoostersFIrst.png. If we add to extra cores (with nose cones on them instead of the payload), the mass of the entire rocket goes up to 35,937kg and the thrust to weight ratio drops to 1.22 at launch. Total delta-v jumps to 5654m/s in this configuration as the boosters are adding 1733m/s to the single core numbers.

Then where all three cores burn at once: http://koboldi.net/img/ThreeAtOnce.png. If you burn all three engines at the same time, but do not do fuel cross feed, the overall delta-v drops to 4951m/s. The only difference between this and the previous version is the central core lights off with the boosters at launch. Thrust to weight ratio jumps up to 1.83 with this change, however, which means you fight atmospheric drag for a shorter time.

And finally, the version with fuel cross feed from the boosters to the central core: http://koboldi.net/img/CrossFeed.png. This configuration gives you 5647m/s of delta-v, which is a mere 7m/s less delta-v than the boosters first version, but you have a TWR at launch of 1.82. This gives you both the ascent profile advantages of booster first and the thrust to weight at lift-off advantages of all three cores at once. The mass of the entire stack does go up to 36037kg due to the fuel lines (the yellow tubes in the picture), and in real life would probably add more mass than as shown here, however, you can see it clearly makes up for its own extra mass in performance. In a larger vessel, the improvement would be amplified since the fuel cross feed hardware wouldn't have to change much (a wider pipe), assuming more fuel flow. (The game ignore that part when doing fuel transfers).

If it helps, here is how the single core with payload appears: http://koboldi.net/img/Scale.png

I have to admit, I was expecting more of a difference between the booster first and fuel cross feed versions (in favor of fuel cross feed) when I started getting these screenshots set up.

[KitemanSA]

"What are we trying to do here?"

Lowest reliable $/lb to LEO.

Correct, so the issue is, would having a special, throw way core for the uniquely heavy loads be cheaper than maintaining an inventory of ONE reusable core and 1 reusable NBAT.
You can have any color so long as its black.
You can have any core so long as its THE core.

I think (again?) that I appreciate KitemanSA's point. Again the answer is complex. Having a neutral weight auxillary tank (included thrust always counterbalancing Earth/s gravity)

Not quite. The aux tank thrust assuring it accelerates equally along side the core.

[KitemanSA]

All of this assumes that the core has enough initial thrust to maintain an adequate thrust to weight ratio, despite the auxiliary tank mass.This would always require more thrust on the core- larger heavier and more powerful engines. Thus more weight on the core.

This would not change anythng about the core except to allow it to thrust with the same trust for longer (feeding it external fuel). If there were a bigger mass it would accelerate at a lower G because of the constant thrust. But you can get to the same velocity because of the longer thrust.

[KitemanSA]

Because some degree of Thrust over Weight is required, and is usually a T/W ratio of 1.333 at minimum, and frequently close to 2, I cannot conceive of a neutrally weighted booster/AuxTank cross feeding into the core as being useful, unless the T/W of the booster/AuxTank is at least equal to the T/W of the core.

PRECISELY equal to the T/W of the core. That is what neutrally boosted means.

[ladajo]

ok, and again, if you can push more, why wouldn't you other than Max Q?

On another thought, what range of thrust and control would this aux tank booster need? It seems that the engine would need full throttle range 0 to 100%, where 100% is a full tank above it. As I know, there are lower limits to throttle down, dependent on engine design.

[paperburn1]

You would not, the longer your in a gravity well the more fuel you need, period.

[KitemanSA]

ok, and again, if you can push more, why wouldn't you other than Max Q?

On another thought, what range of thrust and control would this aux tank booster need? It seems that the engine would need full throttle range 0 to 100%, where 100% is a full tank above it. As I know, there are lower limits to throttle down, dependent on engine design.

In order to push more you have to add structure in the core to take that push which means that you carry dead weight every time you do NOT need the NBATs. The idea is to only have to have ONE core design and one NBAT design and have ALL of them flyback.

[paperburn1]

For the first time, SpaceX is due to launch an entire room to the International Space Station – a room that can go into orbit folded up, and then be expanded like an accordion once it’s hooked up to the station.

The 3,100-pound Bigelow Expandable Activity Module, or BEAM, is the primary payload for Friday’s cargo resupply mission. BEAM will be packed in the “trunk” of SpaceX’s uncrewed Dragon cargo capsule when it’s lofted into space by a Falcon 9 rocket.

Liftoff from Cape Canaveral Air Force Station in Florida is set for 4:43 p.m. ET (1:43 p.m. PT) Friday.

[ladajo]

For those who love NASCAR and Space X Barge attempts:

http://www.spacex.com/webcast

I normally run both the technical broadcast and the hosted on two different screens. They have a time gap, and differing views. Technical runs slightly ahead of the hosted.

Enjoy

[krenshala]

Which is the technical stream? I've always preferred to watch those, but for the last couple of launches I've only been able to find the 'talk show' version.

And for those that didn't have it, NASA TV stars showing the SpaceX launch stuff at 1430 today, though I'm not sure if that is Central or Eastern time.