Surprising fuel flow

[Chik Sneadlov]

I haven't played for a couple of years and have just been getting back into it. I thought I'd build a frame in the shape of a huge plus sign and put long legs on it. Then I'd put some engines on the bottom of each leg. Fuel tanks would hang from the center of the plus sign. The lowest fuel tank would drop when its empty and then the next tank and the next. Asparagus staging. I'd run fuel lines from the tanks to the engines but the lines wouldn't stretch for the twenty feet. I was then surprised to see that somehow fuel was getting to the engines without any lines. I haven't found a way to feed the fuel to the engines one fuel tank at a time. Its always draining all the tanks which foils my asparagus staging plans. 

This is the main question. Is there a way to prevent the fuel from getting to the engines?

Is there a way for me to run fuel lines so that the engines do get fuel and they drain one tank at a time in the order that I want?

Is there a way to make a wire frame base on which I can attach parts like fuel tanks, engines, command modules? The work I did with the wire frame was very tedious and there are lots of limits on attachment points. I don't know why Kerbal puts so many limitations on their parts like fuel lines. Why are they limited to short distances? Why do a lot of wire frames set up with attachments points on the ends but not the other sides?

 

 

 

 

 

 

[Aegolius13]

Yes, I think there's a way to do this fairly easily.  KSP relatively recently came out with a feature called Fuel Flow Priority, which is great with things like this. 

First you will need to enable Advanced Tweakables in the options menu.

Once that is done, you can right click on tanks (in either the VAB/SPH or during a mission) and reassign a priority number.  Higher numbers burn first.  E.g., all tanks with priority 10 will burn out, then all tanks with priority 9, and so on.  The numbers only matter in this relative sense - so you can use 10, 9,8, or 100, 90, 80 and it will do the same thing.  You can also set negative numbers but, aside from being lower (and thus burning later) than positive numbers, these are not different.  The game tries to predict a fuel priority you might want based on its interpretation of your staging, but the results are spotty.  

The other element to getting this right is crossfeed rules.  I assume you have decouplers between each of the stages you're intending to drop. You will want to right click each of these decouplers in the VAB and select "enable crossfeed."  This will allow fuel to flow up from the bottom tank towards the top one, in the order specified by the priority.  

Finally, the fuel lines.  You will not need fuel lines from every tank to the engine, since fuel will be flowing through the decouplers to the top of the fuel stack.  If your frame has parts that do not allow fuel crossfeed (some girders, etc.), you may need one set of fuel lines, from the topmost tanks to the engine.  But if your frame has all parts that allow crossfeed, you may not even need those.  One of the best things about the fuel priority system is that it mostly eliminates the need for the weight, cost, mass, drag, ugliness, and hassle of fuel lines.  

When in flight, you will probably have to right click your outermost tank and watch it to figure out when it's empty and ready to stage.  The regular fuel indicator in the corner is not great about figuring this stuff out yet.

 

As for your other question on frames, I'm not sure exactly what you're looking for. A screenshot of the issue would be helpful. To add attachment points, you can always use a Cubic Octagonal or similar small strut (these don't need to attach TO a node, but provide an extra node).  The "not Rockomax micro-node" may  be of interest, since it adds a bunch of nodes at one.  You might also be able to find some helpful mods - for example, "Near Future Construction" has a lot of girders and similar parts.  .  

Edited December 9, 2017 by Aegolius13

[HvP]

There may be a couple of ways to do what you're suggesting, but first it's important to understand some basics of how KSP handles the fuel flow rules.

First, some parts will allow fuel to pass through them while others do not, and some parts will allow you to toggle fuel flow through them on and off. Modular girder segments and octagonal struts for example will always allow fuel to pass along them to other attached parts. In fact, most parts in KSP allow fuel to go through them with some exceptions. The I-beams and structural panels however will not allow fuel to pass through them so if you want engines attached to I-beams to get fuel you will have to run a fuel line across instead. All of the docking ports will pass fuel by default, but right-clicking them will give you the option to disable fuel flow. In contrast, decouplers and separators will block fuel flow by default, but you can enable fuel flow in their right-click menu.

With those part connection rules in mind, all fuel tanks that have a connection to any engines within their same stage will all drain fuel together at the same rate by default. But this can be altered two different ways. If you have "Advanced Tweakables" turned on you will have the option to adjust fuel flow priority. "Advanced Tweakables" can be turned on from the main game settings menu. It adds an extra slider to any tank's right-click menu that shows the tank's fuel flow priority number. It will also turn on an option to show an overlay of the path that fuel is taking from each tank to the engines (or if you are right-clicking an engine it will tell you which tanks that engine is drawing from.)

A different fuel flow priority number can be set for each tank. Within each stage the fuel will always drain from the tanks with the highest priority number to the lowest number. Remember, for fuel flow priority a lower number drains later -  lower is later.

The other option is to run fuel lines from one tank (or multiple tanks) to another tank in the same stage. As long as all tanks in that stage have the same fuel flow priority, then the fuel lines will pump fuel from whichever tank you first attached it to into the tank the other end is connected to. Be aware that those fuel lines are directional. They move fuel from the first end you attach to the second end you attach. All that's left to do is make sure that the last tank that receives fuel has a fuel connection to your engines.

Edited December 9, 2017 by HvP

[ARS]

1. Open KSP

2. Go to settings in main menu

3. Enable advanced tweakables

4. Go to VAB

5. Now you can set a priority of fuel consumption value by right clicking fuel tank. Higher number consumed first

[Chik Sneadlov]

I did it. Enabled advanced tweaks. Was impressed that Squad would create such a thing that shows a fuel diagram and lets you choose how the tanks get drained. It worked but needs work. I did find that my rather huge ship was quite uncontrollable with SAS and RCS. I even added a second RCS but still at the end of the solid boosters it went out of control. I didn't lose the ship, but it was spinning wildly and very difficult and fuel costly to bring under control. Annoying because the last eighth of the SRB's is where you go from 750 m/sec to 1500 m/sec even with instability. 

I had added a lot of fins on the whole thing, both as low down on the SRBs and high too. This smoothed out the ascent tremendously but still at the end it went nuts. Maybe a longer rocket? Maybe more weight at the top. I'm surprised they don't have a way for you to see how the rocket changes as fuel burns. I'm guessing the SRBs burn from the bottom to the top and so the rocket becomes too light on bottom whereas when burning liquid fuel the rocket gets to light at the top. Also, at the end of the SRBs I'm leaning more. The COM moves up and away from the force pushing up and leverage takes over. Gravity pulls down but now with the rocket leaning, gravity pulls down on the fulcrum of  The COM of the whole thing shifts up a lot whereas the force pushing up stays low. When you go straight up, it doesn't matter that you're COM and COL are close together or far apart. Gravity pulls straight down to earth and straight through the center line of the rocket so it doesn't affect stability. when you lean the rocket the COM is the fulcrum and the Col is feet away and lower. The COG pulls down above the COM and this produces a lot of torque which is not very strong compared to the engines but the engines are not doing anything to correct this.

 I could put the engine on the side of the SRB half way to the top. The COM would stay more constant that way. My original idea was to put the engines near the top of the rocket so that they'd be pulling it into space instead of pushing it but I lost focus on that by getting caught up in other matters. Oh wait. No. I can't move the engines on an SRB.

Even with engines mounted high, the COM will still shift up and as the rocket is angled the COG will still pull down if it is not centered on the COM. What you need is an engine that shifts up and down as the COM shifts. That would make the rocket much more stable. Also, I should pay attention to where the COG is at least when I build. I never even looked. 

The other thing is that I don't really need a rocket this huge. I just want to build it, but I don't know how to make it maneuverable. I don't know if there is a way with Kerbal to make an engine that slides up and down as needed or if there is a way to control that. It'd be cool to invent a program that does that and sensors to monitor changes in the rocket. 

 

 

[HvP]

@Chik Sneadlov I'm glad to see that your fuel flow is sorted out.

Since you are trying to design a very unorthodox rocket it's difficult to guess what exactly might be causing your stability issues without seeing a screenshot of it. Rocket flight dynamics is not exactly the most intuitive field of study.

Based on your description of when and how you lost control I'm going to guess that you probably just have too much drag at the top of your rocket. All those structural parts can catch a lot of air and pull your rocket over. You already know that you need your center-of-mass closer to the nose and your drag at the tail. The easiest way to counteract a draggy nose is to put larger fins at the bottom.

Sometimes using an engine with a high gimbal range can help, like the Vector engine. The vectoring can steer the craft back into the prograde direction and adds a torque to counteract the aero force. But it's a fine line to walk as you are just as likely to encounter oscillation if it over corrects and thrusting too hard will still cause flipping out.

[Chik Sneadlov]

I stabilized it some more by wrapping the lowest stage of the fuel tanks with six more huge fuel tanks and putting huge engines on each. They go off with the SRBs and I put in a lot of struts between the inner fuel tanks and the SRBs. Things went smoothly until seconds before the end of the SRBs, but it was no big problem. I have to try again with more aggressive turning because I ended up about 650Km out in space which is more than I need and took a lot of fuel to circularize.

I notice I'm not using the oxidizer in the tanks on any engines. I've got those nuclear engines that aren't really nuclear and I've got the huge orange engines. Why am I schlepping oxidizer all over the place if I'm not using it? Would those ramjet engines with the great ISP use oxidizer or do they have to have air forced in from the atmosphere?

 

[Eric S]

This was an issue a while ago, and I hadn't heard that they had fixed it, so it might still help.  Try turning off gimballing on engines above the center of mass, at one time they'd steer in the opposite direction because they didn't realize they were above the CoM.

 

[HvP]

12 hours ago, Chik Sneadlov said:

I notice I'm not using the oxidizer in the tanks on any engines. I've got those nuclear engines that aren't really nuclear and I've got the huge orange engines. Why am I schlepping oxidizer all over the place if I'm not using it? Would those ramjet engines with the great ISP use oxidizer or do they have to have air forced in from the atmosphere?

The nuclear engines don't need any oxidizer, they just use the liquid fuel portion of the tanks. They are only really meant for operation in a vacuum so I wouldn't recommend burning them at launch because they have little thrust and poor ISP efficiency in atmosphere. In the VAB you can drain the oxidizer out of your tanks by right-clicking them and dragging the oxidizer portion of the fuel sliders to zero. Alternatively, you can design the vessel to use the tanks designed to only hold liquid fuel since they have a better wet/dry fuel weight ratio. The Mk1, Mk2, and Mk3 size profiles all have liquid fuel tanks but sadly there are none for the 2.5 diameter tanks.

The Rockomax engines (the orange ones like the Skipper or Mainsail) will still need the oxidizer. Be sure to keep the oxidizer in the tanks you are using for the initial launch. You can drop them later.

The turbo ramjet engines are atmospheric jet engines only. They won't work in space not even with on-board oxidizer.

In the VAB parts menu be sure to right-click on the picture of the engines and you'll see a list of the fuel that engine requires. You'll also see the ISP (fuel efficiency) rating for atmosphere and vacuum along with how much thrust you can expect from them in those situations.

It sounds to me that part of your difficulties may be that your thrust is too high at launch. Remember, the SRBs will burn at full thrust until they burn out, but you can reduce their default thrust setting in the VAB before you launch the vessel. Try right-clicking them and turning down the thrust slider a little on those solid boosters before you launch next time. That will allow your nose to fall a little for a better gravity turn. It also reduces ram pressure through the atmosphere for less drag and allows you more steering control.

Edited December 10, 2017 by HvP

[Chik Sneadlov]

I never noticed that you can adjust the thrust of SRBs. That's a great little tidbit. 

 

 

[Curveball Anders]

2 hours ago, Chik Sneadlov said:

I never noticed that you can adjust the thrust of SRBs. That's a great little tidbit.

That was a god send for me when I found it.

[Chik Sneadlov]

If you have too much thrust another thing that works is to add a fairly large middle stage and then put full size airplane wings on the thrusters in place of fins. If your rocket gets tippy towards the end of the thrusters, put huge fins like airplane tail fins or airplane wings near the top of the rocket. You can mount them on decouplers so you can jettison the extra weight once you're in vacuum. 

A computer could model the change in center of mass and thrust as it happens. It could also tell you, with some calculations how the changes in COM and COL and ascent angle are affecting the flight. I'm surprised Kerbal doesn't offer this kind of gizmo to let you know when to jettison stages and fins to optimize control of your rocket. Its probably on engineer redux and I just don't know what I'm talking about.