If Rocket Designers Could Drain their Tanks from the Bottom Up, Would They?

[Sarxis]

I find myself often having my multi-tank stages drain from the bottom tanks first, and then successively upwards until the end of the stage.  My thought is that this gives my gimballed engines more leverage to work worth as the center of gravity moves further forward during the stage  (lawn darts).   As long as the aerodynamics of the rocket hold stable, this seems to be working out fine.  But is this the correct approach?  Should I just allow for even drain rates for all of my tanks, or drain the stage from the top down?

 

I'm interested to know what your thoughts are on this and if there's ever been serious talk in real world design about this concept. 

[Laie]

With a low CoM, you need to be more gentle and careful with the steering, that's all. In KSP, being gentle isn't always easy, especially not on WSAD controls.

In real life, work on improved guidance systems has so far been considered more promising than trying to devise a tank that drains from the bottom up. Asymmetric rockets are a thing now, and tailfins have become scarce.

As for myself, I don't like to drain my tanks from the bottom. But if it's either that, or scratching the mission? I know my priorities.

[Rhomphaia]

When you start thinking of KSP tanks as real world they are somewhat strange.

The only way I could think something like them would work is if they had a cylindrical chamber for one component inside the hole of a toroidal chamber holding the other.

In the real world fuel and oxidizer tanks are usually arranged vertically, most commonly with the denser component on the bottom, although there are exceptions such as the Shuttle and the lower stages of th N1 which have the denser component on top, and the Proton which has the oxidizer in the middle and several fuel tanks arranged around it... somewhat similar to how KSP tanks work?

Edited May 17, 2020 by Rhomphaia

[Superfluous J]

In the real world they don't stack 15 tanks on top of each other for a single engine.

[AHHans]

1 hour ago, Rhomphaia said:

In the real world fuel and oxidizer tanks are usually arranged vertically, most commonly with the denser component on the bottom,

What makes you say that? Which rockets have the denser component on the bottom? The only one that I know of are the 2nd and 3rd stages of the Staurn V. (Where I believe they wanted  the lower weight on the common bulkhead.)

But back to the original question: having the CoM farther in the front makes the rocket more aerodynamically stable. So everything else being the same then yes, rocket engineers would have their tanks drain from the bottom up. But this is not enough of an issue to compromise anything else. As @Laie mentined: active steering and the right choice of flight path (pointing the nose prograde, straight into the wind) allow you to get even fairly unstable rockets into space. (In KSP and I believe also in RL.)

[Rhomphaia]

16 minutes ago, AHHans said:

What makes you say that? Which rockets have the denser component on the bottom? The only one that I know of are the 2nd and 3rd stages of the Staurn V. (Where I believe they wanted  the lower weight on the common bulkhead.)

 

A poor choice of words and failure to account for stoichiometry.  should have said lower volume

[pincushionman]

Having a second stage that’s a significant fraction of the total vessel mass naturally causes this without any extra work on the designers’ part, since the mass of the upper stages don’t begin to change until after the first stage is jettisoned.

And once that happens, you’re typically outside the bulk of the atmosphere and the location of the CoM is far less important due to the greatly reduced drag.

[Bill Phil]

This is mostly an issue for aerodynamics. 

Aerodynamic forces and moments are a function of dynamic pressure Q. In order to minimize aerodynamic moments rockets fly a trajectory where the flight angle of attack (the angle between thrust and velocity relative to air) is kept at zero. This is for the gravity turn part of the flight, which begins after a vertical liftoff and a turnover. Usually the turnover is done until a certain dynamic pressure or mach number is reached. Then the rocket turns until the angle of attack is zero and continues until first stage burnout.

The idea being to minimize aerodynamic moments so the changing CoM doesn’t cause the rocket to rotate in the airflow, especially around max Q.

[Geonovast]

Since this appears to be about real rockets, I've moved it to Science & Spaceflight.

[sevenperforce]

Sometimes when I am building a clone of a real-world rocket and have too much dV, I will do two tank parts, designating the upper tank as the oxidizer tank and the lower as the prop tank, which feels sorta cool. I really wish we could designate all tanks as either oxidizer, fuel, or monoprop.

One reason Starship has the LOX tank on the bottom is to help push the CoM toward the tail on entry and landing.

On 5/17/2020 at 11:36 AM, pincushionman said:

And once that happens, you’re typically outside the bulk of the atmosphere and the location of the CoM is far less important due to the greatly reduced drag.

You still want your CoM farther forward so you need less gimbal for the same amount of control.

[magnemoe]

On 5/21/2020 at 9:36 PM, sevenperforce said:

Sometimes when I am building a clone of a real-world rocket and have too much dV, I will do two tank parts, designating the upper tank as the oxidizer tank and the lower as the prop tank, which feels sorta cool. I really wish we could designate all tanks as either oxidizer, fuel, or monoprop.

One reason Starship has the LOX tank on the bottom is to help push the CoM toward the tail on entry and landing.

You still want your CoM farther forward so you need less gimbal for the same amount of control.

Who they counter by putting the LOX header tank in the nose. 
Think its more about LOX tank is larger but you can pipe each engine to the bottom of tank. Engine design makes this easy too. more important for super heavy engine spam. 

COM matter less for real world rockets than in KSP as they are flown by advanced flight software. See no reason why you could not fly an aerodynamic unstable rocket, modern fighter jets are unstable. 
But yes you want to minimize load on max-Q as high load require an more sturdy and heavier rocket pretty sure they aim very strait at max-Q levels as in no gravity turn now. 
 

[Arugela]

Couldn't they just put the exit on the top of the tank and have it use pipes down to the bottom and use either pumps or other forces to pull it out? I would imagine the only reason not to is something to do with efficiencies in weight or other factors.

Maybe a separate system pushing a plate up to minimize or remove gaps. A second system pushes air in the bottom and the sealed plate pushes the fuel out along with pumps etc.

If it's light enough to be pushed by airpressure it can't be that heavy can it? If not put pumps on the bottom to push the plate instead of sucking out. Probably need mini chambers to hold it until release though as to not get air gaps. Or is that easy to deal with?

Edited May 29, 2020 by Arugela

[natsirt721]

On 5/29/2020 at 6:23 PM, Arugela said:

Couldn't they just put the exit on the top of the tank and have it use pipes down to the bottom and use either pumps or other forces to pull it out? I would imagine the only reason not to is something to do with efficiencies in weight or other factors.

Maybe a separate system pushing a plate up to minimize or remove gaps. A second system pushes air in the bottom and the sealed plate pushes the fuel out along with pumps etc.

If it's light enough to be pushed by airpressure it can't be that heavy can it? If not put pumps on the bottom to push the plate instead of sucking out. Probably need mini chambers to hold it until release though as to not get air gaps. Or is that easy to deal with?

That would be a plumbing nightmare. Not to mention the engineering challenges associated with developing a cryogenic gasket to keep the propellants from leaking back into the empty space as the tanks drained. Instead of a plate, you would use a blowdown bag, think a plastic bag inside of a thermos. As the propellant drains from the plastic bag, inert gas is pumped into the space between the bag and the thermos walls, keeping the edges of the bag taut.

But, as has been said before, all these schemes add weight, and *say it with me folks* every gram counts. It simply isn't worth the weight, because given enough man-hours of engineering effort, a computer can be taught to fly pretty much anything, stable or not. 

Edited May 31, 2020 by natsirt721

[YNM]

I'm fairly sure this is a modification of the old "engines on top of rockets" idea - Goddard's rocket used it - the answer, unsurprisingly, is that it has more to do with the thrust vector wrt the CoM rather than the location of engines or CoM. If there's enough gimbal, and the PID loop works correctly, and the controls works well, you should have no problem.

Aerodynamic forces are more of a concern, but that only exists pretty much on the lower part of the trajectory (and if you have a really janky controls), plus you can always just spin the rocket (more of a problem with human spaceflight but if you still have that problem when trying to make a human-rated launcher I suggest trying a different way entirely).

[cubinator]

I would not drain the tanks in reverse. That sounds really hard, and the engines are at the bottom anyway. Much easier to let gravity and a bit of pressure do the work for you.

[KerikBalm]

If they could? I'm pretty sure they can if they really really wanted to. It would give stability advantages, but you have to consider what sort of tradeoffs would be needed to make such a system work. I think any such system would add a lot more mass and cost than its worth.

They would do it if it was cheap, easy. and didn't increase mass. Since it would be hard, expensive, and would add a lot of extra mass, they don't.

[Nightside]

@Sarxis, isn’t staging essentially draining your tanks from the bottom, with the additional benefit of getting rid of the mass of tanks and sea level engines?

[Sarxis]

On 6/2/2020 at 4:58 AM, Nightside said:

@Sarxis, isn’t staging essentially draining your tanks from the bottom, with the additional benefit of getting rid of the mass of tanks and sea level engines?

Well yeah!  But as you know, until you get the tech for larger tanks, sometimes you end up stacking smaller tanks to make a whole stage (of course). 

The primary reason I was wondering about draining the lowest tanks first was for aerodynamic stability - shifting the center of mass forward while having control surfaces and gimballed engines in the rear.  I suspected this gives the ship a 'lawn dart' effect, thus increasing the chance a ship will survive max Q when flying.  

 

Game wise, it doesn't really matter though?  If I have serious aero problems, I just add more fins usually lol  

 

MOAR FINS!!!          

Edited July 16, 2020 by Sarxis