What do you think of the SLS?

[KASASpace]

KASASpace, what evidence can you show me to demonstrate your claim is true. All you have done as of yet is throw unfounded assertions around which mostly have been refuted and you are starting to sound as I'd you have a very biased view of the SLS.

For one thing, they can accomplish the same payload if not achieve a payload better than SLS using an infallible technique in KSP. Asparagus Staging. It is in fact a cheaper alternative. If we get the Russians in on it we can get RD-180s from them, or perhaps RD-190s if 180s are too powerful. You can then use RP-1 and LOX, a much cheaper alternative than Liquid Hydrogen. Or, using some new engine design, we could use Liquid Propane. Commercially available gaseous propane is easily available, and you can make it a liquid by running a pipe through a vat of liquid nitrogen.

Propane gives you a better Isp than RP-1, has a similar bulk density, and is stored at a similar temperature as LOX. So, less insulation in the common bulkhead.

Either that or Ethylene. Even higher Isp than Propane, but I don't remember it's bulk density.

[sgt_flyer]

well, i don't know for other people, but your answer smell completely trollish ... or an answer of someone who should need to completely rethink how real life rocket works and actually make some real searches on rockets (hint : there's plenty of infos on the net). there's no magical fuel lines in real life for asparagus designs. you'll need fuel valves, turbopumps (driven by the stage's rocket engines - so you'll need more powerful gas generators to both pump to their rocket engines, and to the stage next to it) - all of this is : both heavy, and give additional points of possible failure to a rocket.

then, the other 'rocket' fuel you're talking about is Methane - and no engines of this class have already flown for the moment.

if you wish a superheavy launcher different from SLS with no SRB's, maybe try Vulkan... now that's a superheavy launcher - and surprise - it would also be built with existing components. (AKA : 8x Zenit rocket first stages (Lox-RP1) 1 Energia core stage (lox-LH2) and 1 Energia M upper Stage (lox-LH2) - 175 tons to LKO...

but please, stop saying such things real life don't work like KSP...

[KASASpace]

well, i don't know for other people, but your answer smell completely trollish ... or an answer of someone who should need to completely rethink how real life rocket works and actually make some real searches on rockets (hint : there's plenty of infos on the net). there's no magical fuel lines in real life for asparagus designs. you'll need fuel valves, turbopumps (driven by the stage's rocket engines - so you'll need more powerful gas generators to both pump to their rocket engines, and to the stage next to it) - all of this is : both heavy, and give additional points of possible failure to a rocket.

then, the other 'rocket' fuel you're talking about is Methane - and no engines of this class have already flown for the moment.

if you wish a superheavy launcher different from SLS with no SRB's, maybe try Vulkan... now that's a superheavy launcher - and surprise - it would also be built with existing components. (AKA : 8x Zenit rocket first stages (Lox-RP1) 1 Energia core stage (lox-LH2) and 1 Energia M upper Stage (lox-LH2) - 175 tons to LKO...

but please, stop saying such things real life don't work like KSP...

You could do what the Soviets did. Use one turbo pump for multiple engines (well, combustion chambers). Thus, you only use ONE turbopump for each stage. Even for the cross-feed fuel lines. And, on the note of cross-feed fuel lines, we could use existing hardware. Want to know how the fuel was transported from the Shuttle External Tank?

Asparagus cross-feeding techniques could have been done in the 1980s. Yes, the 1980s. Believe it or not.

And, if that was in any way trollish than you should look some things up.

[SargeRho]

The fuel was pumped from the ET by the SSME's turbopumps. It wasn't much different from a regular rocket, just two extra bends in the tubes, as far as I know.

[KASASpace]

The fuel was pumped from the ET by the SSME's turbopumps. It wasn't much different from a regular rocket, just two extra bends in the tubes, as far as I know.

But the piping leading from the Ext to the Shuttle's SSMEs was just as complicated as a Cross-feed design would need. They provided a similar function and worked well for 135 flights.

[R0cketC0der]

@KASASpace

Asparagus in real life isn't as easy as it is in ksp. As sgt_flyer already mentioned, there are a lot of possible complications with the pumps, but aerodynamics are also a huge limiting factor. Try installing FAR and kerbal engineer/mechjeb in ksp and compare the delta-v you need to get into orbit in a streamlined vehicle to the dV needed for an asparagus pancake. Also if you don't go reusable, the rocket itself will always be more expensive by an order of magnitude than the fuel, so the fuel is the worst place to save money. And reusability is way harder than simply sticking parachutes on your booster because liquid fuel rocket engines don't tend to like salt water and most first stage engines aren't designed to be reused.

@stg_flyer

Actually there is a methane engine that has been flown, namely the Morpheus lander engine. I don't think this counts though as it only has been flown at less than 200 meters.

[SargeRho]

No, it pumped from one tank (per pipe) to a single engine assembly, and only the ammount of fuel needed by those engines. Asparagus staging requires you to pump insane ammounts of fuel around.

[Jouni]

What other reusable rockets can you point to that compares to the Falcon. The only other reusable system I can think of is the STS. Which had an amazing safety record, and the two incidents which did occur were against the advice of the engineers.

The STS is based on completely different technology than the Falcon. If there are no other examples of reusable rockets, it's just one more reason to believe that the refurbished Falcon is unreliable, until proven otherwise.

In general, the proof of burden is on the one making claims. If you are selling launch services with a refurbished rocket, and I have an expensive payload I want to launch to orbit, it's your job to prove that your rocket is reliable. I have no interest in proving that your rocket might be unreliable - I'm just taking my money to someone else, if you can't convince me.

[Dodgey]

@KASASpace In any case it's still a non sequitur. What evidence do you have for your other claims. If you have non then your argument is because it isn't using asparagus staging it is a worthless and horrible rocket. I don't know why you bought it up. Have you just abandoned those arguments? It sounded trollish because you avoided the question and didn't justify your arguments.

Jouni you are right about the burden of proof. If you are saying it is unreliable you must prove it. Just as if you are claiming out is reliable. All you can say is that you are not sure of how reliable it is. My point with the STS is that it shows that reliability is not affected by making the rocket reusable.

Edited April 28, 2014 by Dodgey

[metaphor]

There's a few reasons why asparagus staging isn't used in reality. The basic reason asparagus works well in KSP is that you can discard unneeded mass early and still keep thrust constant. But real rocket stages have much higher fuel mass fractions than KSP rockets, something like 20:1 instead of 8:1. That means you get a much lower advantage by discarding the empty stages earlier. And there's the mass of the additional pumps to take into account, which would lower the advantage of asparagus staging even further. Also, there are large torques on a rocket when pumping fuel from side tanks to other side tanks or to a center tank.

The Falcon Heavy would be the first rocket to ever use asparagus staging (also called crossfeed) on some flights, but it hasn't flown yet, and won't be using crossfeed for the first flights.

[cantab]

There's no net torque if you pump fuel from two or more side boosters into the core, assuming you don't do something silly like run fuel pipes obliquely into stuff.

[Skyler4856]

Asparagus seems horribly risky to be used IRL.

[Red Iron Crown]

There's no net torque in pumping from side tank to side tank, either. The fuel is accelerated clockwise and then decelerated counterclockwise (or vice versa), exactly balancing. Otherwise it would be a reactionless thruster (there's another thread or two for that).

[cantab]

No, it would be a reactionless thruster if there was net force.

If you're pumping fuel clockwise around a ring of tanks, it's true that the entire system will experience no torque, but the body of the rocket - which is what matters - will experience a counter-torque.

[Red Iron Crown]

No, it would be a reactionless thruster if there was net force.

If you're pumping fuel clockwise around a ring of tanks, it's true that the entire system will experience no torque, but the body of the rocket - which is what matters - will experience a counter-torque.

The fuel comes to a stop rotationally when it gets to the destination tank, counteracting the force generated by the pumping in the first place. It would have to expel the fuel to create a net torque.

Otherwise, pumping fuel down from an upper tank to a lower one would create net thrust, a reactionless thrust. This is similar to using a boat-mounted fan to drive a sail, where the fan is the fuel pump and the sail is the destination tank.

Is there some flaw in my understanding of this?

[Dkmdlb]

Yes there is.

[Red Iron Crown]

Yes there is.

Care to elaborate? "You're wrong" doesn't alleviate my apparent ignorance.

[Dkmdlb]

If the fuel is moving in a counterclockwise direction, there is a counter force on the body of the rocket in a clockwise direction. The way to stop that rotation is to stop the fuel from rotating counterclockwise, but that never happens as long as fuel lines are running. Since fuel makes up almost all of the mass of a rocket, the body will rotate much faster than the fuel.

[cantab]

Perhaps the flaw is in my (mis)use of the term "net torque".

To take a terrestrial example, if we stand on a turntable and both start walking clockwise, the turntable will turn anticlockwise. The whole system - us and the turntable - does not rotate, but the parts of it rotate in opposite directions. We don't need to leave the turntable for that rotation to occur. If we stop the turntable will stop turning, but it will not revert to its original orientation.

Likewise in a rocket, pumping fuel between tanks in a clockwise direction will make the rocket body turn anticlockwise, even if the fuel isn't expelled. In practice, of course, the fuel is expelled.

(And no you can't fully cancel it out by pumping the fuel one way and the oxidiser the other, because the fuel and oxidiser do not generally have a 1:1 mass ratio.)

[Deathsoul097]

I think the SLS is a glorious, beautiful and uniquely insane waste of money. Until it proves it's mettle and can convince me otherwise I stand by that.

[Red Iron Crown]

Perhaps the flaw is in my (mis)use of the term "net torque".

To take a terrestrial example, if we stand on a turntable and both start walking clockwise, the turntable will turn anticlockwise. The whole system - us and the turntable - does not rotate, but the parts of it rotate in opposite directions. We don't need to leave the turntable for that rotation to occur. If we stop the turntable will stop turning, but it will not revert to its original orientation.

Likewise in a rocket, pumping fuel between tanks in a clockwise direction will make the rocket body turn anticlockwise, even if the fuel isn't expelled. In practice, of course, the fuel is expelled.

(And no you can't fully cancel it out by pumping the fuel one way and the oxidiser the other, because the fuel and oxidiser do not generally have a 1:1 mass ratio.)

I understood your use of net torque correctly, I think. If I'm not mistaken, you're ignoring the force imparted by the fuel coming to a stop in the rotational direction when it arrives in the destination tank.

Using your analogy, if we start walking on the turntable and stop, will it still be turning? If we time it so that I'm starting just as you're stopping (and we mass the same), would it move at all?

Or to simplify further, if we remove the two connected tanks from the core stage and pump fuel from one to the other, will the whole thing move (other than adjusting for the center of mass moving)?

[Dkmdlb]

The fuel coming to a stop stops the torquing, but it doesn't reverse it. The fuel moving counterclockwise will cause a rotation in the clockwise direction on the body of the rocket. This rotation will stop once the fuel stops moving, but while the fuel is pumping there will be a force on the rocket (which is the problem).

Edited April 29, 2014 by Dkmdlb

[Red Iron Crown]

The fuel coming to a stop stops the torquing, but it doesn't reverse it. The fuel moving counterclockwise will cause a rotation in the clockwise direction on the body of the rocket. This rotation will stop once the fuel stops moving, but while the fuel is pumping there will be a force on the rocket (which is the problem).

Won't the fuel be continuously stopping in the destination tank while the pump is running?

I.e. the forces look something like this ridiculously well-drawn diagram:

Where the force of the fuel pump against the tank is exactly equal and opposite to the force of the fuel stopping. Do you think the arrangement in the diagram would have a constant force to the right as long as the pump is running?

[Dkmdlb]

If the fuel is pumping from the right to the left, the body will move from left to right.

You have two balanced forces in blue there, and an unbalanced force in red. That will balance by moving the body of the rocket. That's the problem.

[SargeRho]

Only if the fuel was staying there, but it isn't. It's going out the engine. So not all of its energy is transferred back to the rocket, producing a net torque.