New mission: 4K fuel in orbit

[MPDerksen]

New play through for me, so only a third of the tech tree unlocked.  The mission is to put a station around Minmus, single launch. Simple enough.  Room for 11, cupola, all the regular stuff. But it also has to have “4,000 units of liquid fuel”.  
Cupola, 2 hitchhikers and a lab.  Fine.  But 4K of fuel too?  With out the big tanks and engines?  Maybe someone can help me better figure out if this is really a mistake.  That seems like a LOT of tanks to strap onto the top of an early career rocket.

[Curveball Anders]

Does it state that it has to be launched with 4k fuel?

Or can it be launched empty an topped up once in orbit?

[AHHans]

3 hours ago, MPDerksen said:

4,000 units of liquid fuel

That's 10 Mk1 Liquid Fuel Fuselages. That doesn't sound too far fetched.
If you want a "cleaner" design, then you can make the fuel tanks detachable [Edit: nope, doesn't work!] or even launch then in a separate launch and dock them in orbit. (As long as all these launches happen after accepting the contract.) [End Edit]

3 hours ago, Curveball Anders said:

Does it state that it has to be launched with 4k fuel?

I haven't seen any "Build a Station" contract that required the station to be launched with the fuel.

Edited June 13, 2021 by AHHans

[MPDerksen]

https://steamcommunity.com/sharedfiles/filedetails/?id=2516119796

there is the screen shot for the mission.  Looks like I have to build it complete, but can bring the fuel later (hadn't thought of that).
anyone wanna try building an example and posting a pic?  I can build the lifter without issue.  But remember I have <1/2 the tree unlocked, and no fairings or big engines.

[AHHans]

1 hour ago, MPDerksen said:

Looks like I have to build it complete

Yupp, looks like that!

1 hour ago, MPDerksen said:

anyone wanna try building an example and posting a pic?

Well, it would be easier if I knew which parts of the tech tree you already unlocked...

[MPDerksen]

5 minutes ago, AHHans said:

Yupp, looks like that!

Well, it would be easier if I knew which parts of the tech tree you already unlocked...

https://steamcommunity.com/sharedfiles/filedetails/?id=2516184156

Great point.  Sorry.  Is this helpful?
I have a few missions to get a rescue off Minmus South Pole, and plant some science.  So if I need to unlock another node to make this more reasonable, I can do that.  (probably get another 500 on the next Minmus mission(s)).

[AHHans]

3 minutes ago, MPDerksen said:

Great point.  Sorry.  Is this helpful?

No! Now I actually have to do that!

Are you willing to wait until you have "Specialized Construction", so that you have the regular-sized docking port? (And not only the junior?)

[MPDerksen]

22 minutes ago, AHHans said:

No! Now I actually have to do that!

Are you willing to wait until you have "Specialized Construction", so that you have the regular-sized docking port? (And not only the junior?)

Sure. I’ll go do the other Minmus missions you see on the first screenshot first tonight and do the Station build tomorrow 

[bewing]

The way you are intended to fulfill this sort of mission is with ISRU. Go to Minmus, land, mine the ore, convert to fuel. Launch back to orbit.

Once it's orbiting Minmus, the contract will complete.

Then you can do what you want with the craft -- generally land it again and turn it into a refueling base on the surface.

Yes, I understand that you don't have drills or fuel converters yet, but the point is to wait to complete the contract until you have the tech.

 

Edited June 13, 2021 by bewing

[AHHans]

Something like that?

P.S. AHTech Industries management does not approve of late night rocket designs. So I won't upload that onto KerbalX, you'll have to wait till tomorrow if you want the craft file.

[MPDerksen]

35 minutes ago, bewing said:

The way you are intended to fulfill this sort of mission is with ISRU. Go to Minmus, land, mine the ore, convert to fuel. Launch back to orbit.

Once it's orbiting Minmus, the contract will complete.

Then you can do what you want with the craft -- generally land it again and turn it into a refueling base on the surface.

Yes, I understand that you don't have drills or fuel converters yet, but the point is to wait to complete the contract until you have the tech.

 

That hadn’t even occurred to me.  I like it!

7 minutes ago, AHHans said:

Something like that?

P.S. AHTech Industries management does not approve of late night rocket designs. So I won't upload that onto KerbalX, you'll have to wait till tomorrow if you want the craft file.

No craft file needed. I wanted a concept to pursue.  Exactly what I wanted. Thank you.

[MPDerksen]

4 hours ago, AHHans said:

so that you have the regular-sized docking port? (And not only the junior?)

Why does the size of the port matter?  

 

3 hours ago, bewing said:

generally land it again and turn it into a refueling base on the surface.

 

Refueling on the surface?  I admit I don't understand that, but I've been away from KSP for the last 2 years (playing Factorio and lately Valheim)

[AHHans]

6 hours ago, MPDerksen said:

Why does the size of the port matter?  

Not for the contract. But for actual usability of the station.

[Reactordrone]

You can save some weight by not bringing the 4000 fuel as fuel/oxidiser tanks. Bring the 4000 payload in aircraft fuel tanks and the station can be a refueling point for NERV powered ships in the future.

[MPDerksen]

8 hours ago, Reactordrone said:

You can save some weight by not bringing the 4000 fuel as fuel/oxidiser tanks. Bring the 4000 payload in aircraft fuel tanks and the station can be a refueling point for NERV powered ships in the future.

I was baffled by the usefulness of the fuel without oxidizer.  I had completely forgotten about the NERV engine.  And Minmus is a pretty reasonable spot to refuel.  So yeah, we'll get the science to put a standard docking port on the next mission, then do this one.  
I never really did much mining before, so perhaps on this play-through it would be a good thing to approach.  

[MPDerksen]

On 6/13/2021 at 2:55 PM, AHHans said:

 

 

https://steamcommunity.com/sharedfiles/filedetails/?id=2523344988

I used a Lab, instead of the extra Hitchhiker.  And now we have a real teaching moment.  The Station is 65 tons.  I could NOT get this thing into orbit.  First, my SRBs must be slightly off, since I get rotation at liftoff.  Second, as soon as I dumped them, I started flipping (thus the addition of fins).  Finally, I didn't have enough dV to actually even get to 80K, even though I have done it with other craft hundreds of times, so I'm not a rookie.

I typically just use trial/error (highlighting the "error" portion) instead of actually understanding what I'm building.  So I look for about 3,400 dV for orbit, and call it good.  What I've built is woefully below that, but I look at ISP=207 and TWR=1.27.  That SHOULD mean something to me, but doesn't.  And if I wanted to add "Moar Boosters" to the bottom, I would be randomly throwing parts on the craft, instead of intelligently planning it.

A little education here would be appreciated.  In previous playthroughs, I've done multiple landings on Duna/Eve etc. but this time my goals was to better understand my designs.

[AHHans]

19 hours ago, MPDerksen said:

What I've built is woefully below that, but I look at ISP=207 and TWR=1.27.  That SHOULD mean something to me, but doesn't.

Well, first the rocket I built back then had one of the worst performing boosters I built in a long time. Getting that into orbit meant using a very steep (and inefficient) ascent profile so that the top stage (with the poodle) had enough time to accelerate the station into orbit before it fell back into the atmosphere.

ISP aka specific impulse is the measure of how efficient a rocket- or jet-engine is. That's mostly important for the vacuum stages of your rocket(s), for the booster stage(s) the thrust is more important. (A high ISP engine, that doesn't have enough thrust to get you off the pad is no very useful.) And it is only actually relevant for choosing the appropriate engine for a stage, once you decided on the engine - e.g. because you have little choice - then it becomes largely irrelevant.

TWR ak thrust-to-weight ratio is the power of the engine(s) compared to the weight (== the force of gravity) of the rocket (or whatever). A TWR <= 1 in the first stage means that your rocket just won't leave the pad, because the engine(s) cannot overcome the force of gravity. And the higher the TWR the faster the craft accelerates. (And as a stage burns fuel the weight goes down and thus the TWR up.)

The first stage of your rocket has a rather low (starting) TWR (my design has 1.53), and the second stage is even worse: my design has a starting TWR on the second stage of 0.67 (as displayed in the VAB) but yours uses the same engine with a lot more weight to push around. The low TWR on my second stage is the main reason it needs such a steep ascent profile, it actually looses speed at the start of the first stage but it still manages to get to orbit because it can push the AP far enough out so that it can gather enough velocity in the end. Your design will have a harder time to push out the AP (and "time outside atmosphere") because of the lower TWR on the second stage but will need even longer to reach orbital velocity because of the lower TWR on the third stage.

To have a rocket that is easy to get into orbit I aim to have a TWR of around 1.7 on the first stage (say between 1.5 and 2.0) and above 1.0 on the other pre-orbit stages. If the TWR raises above 2.0 while still low in the atmosphere (say below 20 km) then I usually throttle back to keep the TWR around 2.0 in order to avoid excessive drag.

[MPDerksen]

3 hours ago, AHHans said:

Well, first the rocket I built back then had one of the worst performing boosters I built in a long time. Getting that into orbit meant using a very steep (and inefficient) ascent profile so that the top stage (with the poodle) had enough time to accelerate the station into orbit before it fell back into the atmosphere.

ISP aka specific impulse is the measure of how efficient a rocket- or jet-engine is. That's mostly important for the vacuum stages of your rocket(s), for the booster stage(s) the thrust is more important. (A high ISP engine, that doesn't have enough thrust to get you off the pad is no very useful.) And it is only actually relevant for choosing the appropriate engine for a stage, once you decided on the engine - e.g. because you have little choice - then it becomes largely irrelevant.

TWR ak thrust-to-weight ratio is the power of the engine(s) compared to the weight (== the force of gravity) of the rocket (or whatever). A TWR <= 1 in the first stage means that your rocket just won't leave the pad, because the engine(s) cannot overcome the force of gravity. And the higher the TWR the faster the craft accelerates. (And as a stage burns fuel the weight goes down and thus the TWR up.)

The first stage of your rocket has a rather low (starting) TWR (my design has 1.53), and the second stage is even worse: my design has a starting TWR on the second stage of 0.67 (as displayed in the VAB) but yours uses the same engine with a lot more weight to push around. The low TWR on my second stage is the main reason it needs such a steep ascent profile, it actually looses speed at the start of the first stage but it still manages to get to orbit because it can push the AP far enough out so that it can gather enough velocity in the end. Your design will have a harder time to push out the AP (and "time outside atmosphere") because of the lower TWR on the second stage but will need even longer to reach orbital velocity because of the lower TWR on the third stage.

To have a rocket that is easy to get into orbit I aim to have a TWR of around 1.7 on the first stage (say between 1.5 and 2.0) and above 1.0 on the other pre-orbit stages. If the TWR raises above 2.0 while still low in the atmosphere (say below 20 km) then I usually throttle back to keep the TWR around 2.0 in order to avoid excessive drag.

super helpful!  Thanks again.  I didn't realize/check my TWR when I added the extra weight of the Lab.  Addressing just that part, and assuming I'll be engaging in vacuum, or nearly so, can I simply put a few engines on the side of that stage?  I also need to circularize at Minmus, so I need to keep that in mind too.  

Plan B:  move from 4 SRBs to 6.  Add some side engines (can't recall what they are called) to the upper stage.  Nearly circularize at Minmus, but drop the extra engines into the surface, THEN raise the Pe to a solid 30-40K.  Reasonable?

[AHHans]

3 hours ago, MPDerksen said:

Plan B:  move from 4 SRBs to 6.  Add some side engines (can't recall what they are called) to the upper stage.  Nearly circularize at Minmus, but drop the extra engines into the surface, THEN raise the Pe to a solid 30-40K.  Reasonable?

Errrr.... what did you say?

I don't really understand what you are planning, but adding more SRBs and adding more thrust to the final stage may work. But if you add more SRBs then I'd recommend to check the TWR of the first stage if it is high enough so that you can reduce the thrust limiter on the SRBs (to have them burn longer).  It might also be an idea to make it two groups of SRBs: one with full thrust that gets dropped first and another one with reduced thrust that burns longer and gets dropped second.

[MPDerksen]

56 minutes ago, AHHans said:

Errrr.... what did you say?

I don't really understand what you are planning, but adding more SRBs and adding more thrust to the final stage may work. But if you add more SRBs then I'd recommend to check the TWR of the first stage if it is high enough so that you can reduce the thrust limiter on the SRBs (to have them burn longer).  It might also be an idea to make it two groups of SRBs: one with full thrust that gets dropped first and another one with reduced thrust that burns longer and gets dropped second.

Sorry.  First stage:  if I add a few more SRBs, my TWR goes from 1.27 to 1.65.  Helps me overcome initial gravity losses?  I did some other reading that 1.35 is a sweet spot for some?  Dialing back a few for longer burn is a trick I hadn't thought of.

For the upper stage, assuming I had enough fuel, and that those engines would engage in vacuum, I was thinking I could put a few MK-55 Thud engines around it to help with the TWR for creating my orbit, and for the circulation at Minmus.

The part where I might have lost you is what I've seen Matt Lowne do in a few videos.  He specifically releases parts on a collision course, then corrects.  That way, junk isn't left in orbit.  I was thinking that I could put the MK-55s on decouplers, and remove them once nearly in orbit, to keep the design clean.  But now I'm just being silly and making a mess of the mission, so let's ignore all that.

[AHHans]

10 hours ago, MPDerksen said:

Sorry.  First stage:  if I add a few more SRBs, my TWR goes from 1.27 to 1.65.  Helps me overcome initial gravity losses?

Yes, and it adds some more dV and thrust in general.

10 hours ago, MPDerksen said:

For the upper stage, assuming I had enough fuel, and that those engines would engage in vacuum, I was thinking I could put a few MK-55 Thud engines around it to help with the TWR for creating my orbit, and for the circulation at Minmus.

Well, my confusion is: why do you think you'll need more thrust for circularizing at Minmus? I can understand wanting more thrust to circularize your orbit around Kerbin, so that doesn't take that long. But why Minmus? You are already in an orbit when you get there, so you are not in a terrible hurry and can do it with a low TWR.

10 hours ago, MPDerksen said:

The part where I might have lost you is what I've seen Matt Lowne do in a few videos.  He specifically releases parts on a collision course, then corrects.  That way, junk isn't left in orbit. 

No, that's standard practice for AHTech Spaceflights. But I would drop the extra mass onto Kerbin, thus I don't understand why you mention Minmus there?

P.S. In case it wasn't obvious: once you get into the SOI of Minmus then the TWR that is displayed will be based on Minmus' low gravity, thus the number will be much higher but the acceleration of the craft will still be low.

[MPDerksen]

7 hours ago, AHHans said:

Yes, and it adds some more dV and thrust in general.

Well, my confusion is: why do you think you'll need more thrust for circularizing at Minmus? I can understand wanting more thrust to circularize your orbit around Kerbin, so that doesn't take that long. But why Minmus? You are already in an orbit when you get there, so you are not in a terrible hurry and can do it with a low TWR.

No, that's standard practice for AHTech Spaceflights. But I would drop the extra mass onto Kerbin, thus I don't understand why you mention Minmus there?

P.S. In case it wasn't obvious: once you get into the SOI of Minmus then the TWR that is displayed will be based on Minmus' low gravity, thus the number will be much higher but the acceleration of the craft will still be low.

that's helpful.  Thanks again.  I guess I thought reducing the time to circularize at Minmus would be helpful.  But perhaps not.  Even a burn of several minutes is irrelevant, provided I have the fuel for it.

[AHHans]

4 hours ago, MPDerksen said:

I guess I thought reducing the time to circularize at Minmus would be helpful.  But perhaps not.  Even a burn of several minutes is irrelevant, provided I have the fuel for it.

Welllll... The short answer is that for maneuvers from one orbit to another orbit (around a different CB or not) the TWR is a lot less relevant than for landing or getting into orbit. (That's why ion-engines are useful after all.) The somewhat longer answer is that really long burns (larger than a significant fraction of the orbit) become less efficient, but usually you can mitigate that by e.g. splitting a really long burn over several orbits. And the immediately relevant answer is that Minmus' gravity is so low that the capture burn to get into a circular orbit around Minmus is so small that you don't have to worry about TWR there.