Jump to content

My personal challenge: Offer advise if you like


togfox

Recommended Posts

Below is my second test flight. I\'ve attached a whole lot more LFT and LFE and reached a height over 100,000m. It is really stable but I used pretty much all the fuel to reach this height. I don\'t have enough for the Mun injection, de-orbit and return trip. I also noticed my \'real\' boosters used up fuel before my gimbals. This is not efficient so I\'ll think about that one as well. The vid shows the staging sequence and I\'m happy with it.

Am I ready for solids yet? :o

My space program needs to be lean and calculating and so too are my vids. I\'ll keep each one no more than 60 seconds so enjoy. :)

Looking good :)

The reason the gimboling engines lasted long is they simply burn less fuelper second than the non-gimboling ones. They\'re also a little less powerful (neatly keeping them to the same thrust to burn rate ratio). What you could do to ensure indentical timed burn out on them is link the tanks with fuel lines, non-gimbol being fed by gimboling.

The vanilla solids are great for a kick off the launch pad, not so good for gaining altitude. They will save fuel on your long burning liquid boosters however and that will get you more altitude and Delta-V.

Best of luck!

Link to comment
Share on other sites

What you need, my friend, is a clever use of fuel lines. I\'ve attached a picture showing you how to set it up so the outer booster sets\' engines will keep lit when there is still fuel left out there.

I would recommend getting rid of the winglets, as they can wobble your rocket around while using ASAS, or you could also throw in a SAS along with the ASAS to dampen it out and provide correctional torque (this is how I can make non-symmetric craft work).

Another suggestion I have is to build the lifter stage off of the center stack instead of attaching it to stack decouplers all the way out there. It would make the rocket easier to control due to less moment of inertia. You can tie down the lander to the lifter with struts.

Link to comment
Share on other sites

I agree with Kosmo-not about the winglets: they are not needed here.

To be sure i tested your design (Mun 1 of post #21) without winglets (but with 2 struts connecting the outer stacks to the center stack). One test with 3 gimballing engines, one with 3 standard engines. Both stable like a rock.

Link to comment
Share on other sites

What you need, my friend, is a clever use of fuel lines. I\'ve attached a picture showing you how to set it up so the outer booster sets\' engines will keep lit when there is still fuel left out there.

This is gold! First, until now I didn\'t know fuel lines could be connected directly to engines. Second, I\'ve been frustrated trying to figure out this arrangement. Thanks!

Another suggestion I have is to build the lifter stage off of the center stack instead of attaching it to stack decouplers all the way out there. It would make the rocket easier to control due to less moment of inertia. You can tie down the lander to the lifter with struts.

Another benefit to building this way: if the engines of the center stack are not blocked by decouplers, you can often use them during the ascent. This has a dramatic affect on efficiency. Pipe in fuel from the boosters so that your descent module still has full tanks after staging. It\'s a favorite design element that keeps showing up in my ships.

Link to comment
Share on other sites

That fuel line thing is good advise. I\'ve read about that before but didn\'t think to do it. My gimbal ratio is now different. I\'m now putting that as a smaller priority. Gimbals will burn that little bit longer - no biggy.

Here is version 3. The \'dangly\' bits on the end are gone. It looked a lot more Kerbal but if I\'m going for safety then a central stack is much smarter. You\'ll see the lifter now comprises eight engines with four tanks of fuel each. Two of those are gimballed. The winglets are gone (which makes me nervous!) but I\'m not sure four engines so close together is clever - heat wise. I might have to throttle down mid-flight and do an early controlled abort. >:(

The centre stack engine is intended to be for mun injection and mun orbit/de-orbit. I really have no idea on fuel efficiency until I reach orbit. If there are no words of warning then I\'ll launch this baby in about 2 hours! ;D

Edited by togfox
Link to comment
Share on other sites

Attached is my first orbit. It actually worked! It\'s slow and heavy but the autopilot got her up alright with a little bit to spare. A staging mishap meant I jettisoned my fully stocked mun-insertion rocket (centre gimbal) along with the empty lifer tanks. Oh well, that didn\'t impact the mission. I did a retro-burn with the lander so I\'d land in water. Watch right till the end of the clip - I jettison the rover ~1000m above sea level and open my chute. Watch the top right corner and see if you\'re as surprised as I was!!! Just to see what would happen, I switched to tracking centre to see if I could control the rover but it had disappeared. :( Ah well.

Mission successful. Casualties NIL.

Next flight - adding solids to the lifter. ???

Question - when in space (not orbit) does weight/mass affect the thrusters? Will jettisoning the empties make the mun-injection burn easier/quicker? How does zero gravity and all that affect weight/mass?

http://youtu.be/0gzUDL_YcB0

Link to comment
Share on other sites

Question - when in space (not orbit) does weight/mass affect the thrusters? Will jettisoning the empties make the mun-injection burn easier/quicker? How does zero gravity and all that affect weight/mass?

The more mass, the more energy needed to accelerate it. Yes, jettisoning the empty fuel tanks will allow you to accelerate faster and use less fuel.

Link to comment
Share on other sites

The more mass, the more energy needed to accelerate it. Yes, jettisoning the empty fuel tanks will allow you to accelerate faster and use less fuel.

Being in space removes air drag from the equation, if in orbit it also removes gravity drag (energy expended by \'standing on your rocket plume\'). Everything else works the same, really. Test firing a rocket on earth (a really big, massive spaceship for all intents and purposes) doesn\'t appreciably move it.

Link to comment
Share on other sites

Being in space removes air drag from the equation, if in orbit it also removes gravity drag (energy expended by \'standing on your rocket plume\'). Everything else works the same, really. Test firing a rocket on earth (a really big, massive spaceship for all intents and purposes) doesn\'t appreciably move it.

Dude, mass is mass. You must exert a force to accelerate mass, on earth, in space, anywhere. The more mass there is, the more energy you need to accelerate it to a certain speed. Do the test yourself: Make a ship with a bunch of fuel tanks on decouplers. Get it in orbit. Spend all the fuel in those tanks, leaving just one or two tanks in your core ship. Turn off engines. Press F5 to quicksave. Boost at full thrust until you run out of fuel. Write down your final speed. Now load the quicksave (F9), jettison the empty tanks and again boost at full thrust until you run out of fuel. Notice the difference in the speed?

*Edit* Just did the test myself: got this ship into orbit

screenxxx.png

Boosted until the outer fuel stacks ran out of fuel. My speed was 3920 m/s. Quicksaved. Boosted at full thrust with the empty fuel tanks on until I ran out of gas.

Final velocity: 5019 m/s

Quickloaded. Decoupled the empty fuel tanks. Boosted at full thrust until I ran out of gas.

Final velocity: 5923 m/s

Dragging the mass of those 8 empty fuel tanks and 2 engines cost me over 900 m/s of delta V....

Link to comment
Share on other sites

I would recommend putting all those outer stacks on radial decouplers so you can shed mass on your way to orbit. Also, reducing the fuel stack size to 3 each might help a bit. I would drop 4 of them at around 10km, and then 2 at a time as they run out.

Keeping all that mass and unneeded thrust with you all the way to orbit, like the current design, hurts your fuel efficiency.

Also, seeing how the cart survived that fall, you might want to test dropping it on land. If it survives that, you could probably just drop it on a direct descent path with the moon and accelerate back to orbit.

Link to comment
Share on other sites

ACK! A redesign and an objective to reach Mun orbit and a problem! The auto ascent pilot misfired my stages and I had to throttle down and do a controlled abort!!! No casualties. :) Here is my new squat ship design incorporating the idea\'s in this thread.

I\'ll do a mission relaunch and trigger manual staging and see how my fuel goes. :)

Edited by togfox
Link to comment
Share on other sites

About the fuel lines:

It looks like you have connected the lower tank of each fuel stack to the lower tank of the corresponding engine stacks. I think that causes fuel to be drawn from both stacks at the same time, which means the fuel stack will drain slower so that you have to carry that weight for longer then it would be if fuel would (at first) be drawn only from the fuel stacks.

I would change the fuel lines so that it connects the lower tank of the fuel stacks to the upper tank of the engine stacks.

A general remark about the amount of fuel per engine: it is what determines the thrust-to-weight-ratio of the rocket. For launching optimal TTWR is around 2 to 1 for each lifter stage.

To determine ttwr of a stage you have to take into account that it lifts not only itself but also the rest of the rocket.

That means normally the mass of the first stage must be significantly lower than half its max thrust. Max thrust of a standard engine is 20 Kerbal mass units (the '200' thrust in the description of the engine is a factor 10 larger than a Kerbal mass unit).

3 tanks + the mass of a standard engine is already close to half the maximum mass that one engine can lift (9.5 mass vs a thrust of 20).

Of course it is different when you have fuel stacks that you drop when empty.

Aside from the fuel lines your design looks fine, except that i would add 2 SRBs to the first stage for a total of 6.

Link to comment
Share on other sites

About the fuel lines:

It looks like you have connected the lower tank of each fuel stack to the lower tank of the corresponding engine stacks. I think that causes fuel to be drawn from both stacks at the same time, which means the fuel stack will drain slower so that you have to carry that weight for longer then it would be if fuel would (at first) be drawn only from the fuel stacks.

I would change the fuel lines so that it connects the lower tank of the fuel stacks to the upper tank of the engine stacks.

A general remark about the amount of fuel per engine: it is what determines the thrust-to-weight-ratio of the rocket. For launching optimal TTWR is around 2 to 1 for each lifter stage.

To determine ttwr of a stage you have to take into account that it lifts not only itself but also the rest of the rocket.

That means normally the mass of the first stage must be significantly lower than half its max thrust. Max thrust of a standard engine is 20 Kerbal mass units (the '200' thrust in the description of the engine is a factor 10 larger than a Kerbal mass unit).

3 tanks + the mass of a standard engine is already close to half the maximum mass that one engine can lift (9.5 mass vs a thrust of 20).

Of course it is different when you have fuel stacks that you drop when empty.

Aside from the fuel lines your design looks fine, except that i would add 2 SRBs to the first stage for a total of 6.

The engine looks for fuel starting from the engine and moving upward. If a fuel line is encountered, it draws from the fuel line first before drawing from its own stack. So, fuel lines get priority. You should try this stuff. Fuel lines are a very powerful tool if you know how it works.

Thrust is the amount of thrust specified in the part\'s description. Mass is the same way. TWR is thrust divided by weight. So, for 200 thrust and a mass of 10, TWR is 2.04 at the surface of Kerbin.

@ togfox

Something you should keep in mind is that each of the big engines weighs as much as 6.66 empty fuel tanks. It\'s very beneficial to get rid some engines when the thrust isn\'t needed.

Link to comment
Share on other sites

fuel lines get priority.

That\'s good to know.

I do use fuel lines and droptanks, but never tried that because arranging them the way i described also works. 'If it ain\'t broke,...'.

Now that i know i\'m sure it\'ll come in handy.

TWR is thrust divided by weight. So, for 200 thrust and a mass of 10, TWR is 2.04 at the surface of Kerbin.

Right, so you can\'t just devide the numbers.

The tricky part that you don\'t mention is factoring in a gravitational constant of 9.81, which i rounded to 10 because it\'s accurate enough for this purpose.

While scientifically more accurate to express thrust in Newtons, it is not uncommon to express it in units that correspond to the same units used to express mass: 'kilogram-force', which makes calculating ttwr a little more intuitive.

KSP itself is a bit tricky here in that it does not specify units of mass and thrust. That\'s forgiven because it\'s in alpha, but we definately do not all know that thrust is most properly expressed in Newtons, and i think that\'s not unexpected.

Link to comment
Share on other sites

Thanks for the input guys - it has been an educational and fun journey. :)

Posted here is a video my last and final flight in this challenge. I won\'t tell you if the challenge has been a success. I\'ll let you watch for yourself. The movie is 6 minutes long so I apologise for it\'s length. I could have easily made it longer!!! SOUND IS ESSENTIAL. This is not your usual vid of ships floating silently in space. For those familiar with Mun landing procedures, this visual will be easy to follow. I do hope you enjoy the vid and check the credits at the end. :)

http://youtu.be/5hm005rUkoI

Link to comment
Share on other sites

Pretty awesome and a big sigh of relief once I\'d touched down. There were two potential failure points on this mission:

- if you watch the vid after I touch down, I release the rover as per mission plan. You see the rover move away from the lander? Well, to my shock and horror, it is actually ROLLING away from the lander. I did not drive that! I had not foreseen the slope I parked on would make the rover move uncontrolled. I\'ve no idea what would happen if that had rolled into my tanks or landing legs. Maybe not much but that was a close one.

- my second goof was pilot error. The vid doesn\'t show it (I didn\'t capture it as I was panicing!) but as I was descending to Kerbin, relieved the mission was almost over, I executed all my stages. This should have dumped the lander and opened the chute on my capsule. WRONG!!! I\'d forgotten that the emergency escape decoupler does not fire with the space bar - it activates on B. What this meant was that the parachute opened but the lander did NOT jettison. The speed of the fall plus the weight of the engines could have easily wripped the chute right off! :o Panic!! :o I quickly released what happened and jettisoned the capsule and chute for a nice landing over water (didn\'t risk a terrain landing).

Just a note on the solids, I have four dangling off the sides in an awkward fashion. These are an after-thought and I couldn\'t make them symmetrical because the tower would interfer with that side of the craft. I didn\'t really need them but a rocket is not a rocket without solids. :)

On the vid, I was disappointed I didn\'t capture the landing in motion - my FPS dropped to 2 so I had to turn off FRAPS and press F1 a lot of times instead. :(

Whilst this mission is over I\'m going to keep the craft as it is a proven rover deployer. I shall tune her and make her more efficient. I\'ll take out tanks I don\'t need, add solids for extra height, put fuel lines in more strategic places, remove the escape decoupler and things like that. I shan\'t bore this thread with those details however. :)

Link to comment
Share on other sites

Guest
This topic is now closed to further replies.
×
×
  • Create New...