Monoprop Oberth Assist: Good idea?

[JerseyChris]

I’ve got a tanker that needs to insert into low Moho orbit from Kerbin.  Simple craft.  Giant fuel tank or two, probe core, docking port, two nuclear engines, and an RCS system.  The craft has four O-10 Puff engines to assist with orbital rendezvous and docking.  My question is: when doing my insertion burn from Kerbin, is it worth it to include some extra monoprop for those engines and burn them at periapsis?  Is the added weight of a lower efficiency propellant worth the gain I get from being able to complete my burn in a shorter time?  For reference I’m trying to insert into a 40km orbit around Moho.  Note that the four monoprop engines are not added weight, they were already there.

[bewing]

Do you have any extra room for LF, or are your LF tanks topped up? 1kg of LF is worth more than 1kg of monoprop all the way through your trip.

[JerseyChris]

32 minutes ago, bewing said:

Do you have any extra room for LF, or are your LF tanks topped up? 1kg of LF is worth more than 1kg of monoprop all the way through your trip.

LF tanks topped up.  But I’m trying to deliver as much LF as possible to low Moho.  My two nuclear engines can only deliver so much thrust.  Is the mass cost of additional monoprop to increase my TWR at periapsis by burning the O-10s worth it?  Lighting up the O-10s at periapsis to assist with the capture would be terribly inefficient from an ISP standpoint but excellent from an Oberth effect standpoint.  My question is, at 40km above Moho, which effect rules?

Edited August 30, 2018 by JerseyChris

[Xd the great]

1 hour ago, JerseyChris said:

LF tanks topped up.  But I’m trying to deliver as much LF as possible to low Moho.  My two nuclear engines can only deliver so much thrust.  Is the mass cost of additional monoprop to increase my TWR at periapsis by burning the O-10s worth it?  Lighting up the O-10s at periapsis to assist with the capture would be terribly inefficient from an ISP standpoint but excellent from an Oberth effect standpoint.  My question is, at 40km above Moho, which effect rules?

Save some monoprop for docking. Use the rest for slowing down or speeding up. The less LF you burn, the more LF you have.

[bewing]

The place where the extra monoprop costs you more is when you are leaving Kerbin, because you are thrusting that much extra mass. Once you are at moho, you always want to burn your lowest Isp fuel "first" -- which means yes, burn as much of it as you conveniently can during your insertion burn. You've already spent the deltaV to get the extra fuel to moho. Burning it up to get a maximally efficient insertion is basically free at that point.

 

[Tyko]

16 hours ago, bewing said:

The place where the extra monoprop costs you more is when you are leaving Kerbin, because you are thrusting that much extra mass. Once you are at moho, you always want to burn your lowest Isp fuel "first" -- which means yes, burn as much of it as you conveniently can during your insertion burn. You've already spent the deltaV to get the extra fuel to moho. Burning it up to get a maximally efficient insertion is basically free at that point.

 

...or just pack less monoprop fuel in the first place. You'd be better off adding a small LFO tank than a small monoprop tank because the LFO mass translates into more DV than the monoprop mass will.

[Laie]

22 hours ago, JerseyChris said:

insert into low Moho orbit from Kerbin [...] Is the added weight of a lower efficiency propellant worth the gain I get from being able to complete my burn in a shorter time?

 

No. Carrying it out of the Kerbin system is a losing proposition to begin with.

[pincushionman]

If you already have the monoprop onboard AND you have enough time to make your burns AND you make sure you retain enough for your other maneuvering needs, there is no downside to it. Un-burned…un-shot…un-disassociated? Un-used monopropellant is dead weight.

A mixed LFO/MP burn is less efficient than an LFO-only burn of the same tonnage of fuel. However, you’ll have two things going in your favor. First, you will shed more mass during the burn, so all subsequent burns will be more efficient. Second, you’ll have more thrust, so the maneuver will take less time, meaning you’re closer to the theoretical instant maneuver and you’ll lose less to cosine loses. Of course, the efficiency thing is most true if the monoprop is used first, while the thrust part is only true if you burn both fuels sumultaneously.

How much will it help? That we can’t tell unless you give us more info on your craft. You’re transporting a load of fuel, which is a heavy payload. Unless you way over-budgeted your MP, I’m guessing your MP is actually a small fraction of your mass, so the actual efficiency gain will be minimal. Likewise, unless your LFO engines are small (less than, say, 200 kN of thrust), the extra thrust isn’t going to be all that helpful, either. But you’ve already got it there, so you might as well use it.

If, on the other hand, you haven’t launched yet, you’d be better off putting in a smaller MP tank to begin with.

Edited August 31, 2018 by pincushionman

[JerseyChris]

On 8/30/2018 at 6:51 PM, Laie said:

 

No. Carrying it out of the Kerbin system is a losing proposition to begin with.

So, you’re saying the weight of the monoprop costs more than the benefit I’d get from having a higher TWR?  The quicker I can complete my burn at Moho periapsis, the more efficient it will be because of Oberth effect.  Additional monoprop would help me complete that burn more quickly.  And it weighs a lot less than strapping on an additional nuclear engine.

On 8/30/2018 at 4:58 PM, Tyko said:

...or just pack less monoprop fuel in the first place. You'd be better off adding a small LFO tank than a small monoprop tank because the LFO mass translates into more DV than the monoprop mass will.

Yes but we are ignoring the Oberth effect which is what generated the question.  I get that nukes are more efficient.  See my above reply to see what I mean.

On 8/30/2018 at 11:04 PM, pincushionman said:

If you already have the monoprop onboard AND you have enough time to make your burns AND you make sure you retain enough for your other maneuvering needs, there is no downside to it. Un-burned…un-shot…un-disassociated? Un-used monopropellant is dead weight.

A mixed LFO/MP burn is less efficient than an LFO-only burn of the same tonnage of fuel. However, you’ll have two things going in your favor. First, you will shed more mass during the burn, so all subsequent burns will be more efficient. Second, you’ll have more thrust, so the maneuver will take less time, meaning you’re closer to the theoretical instant maneuver and you’ll lose less to cosine loses. Of course, the efficiency thing is most true if the monoprop is used first, while the thrust part is only true if you burn both fuels sumultaneously.

How much will it help? That we can’t tell unless you give us more info on your craft. You’re transporting a load of fuel, which is a heavy payload. Unless you way over-budgeted your MP, I’m guessing your MP is actually a small fraction of your mass, so the actual efficiency gain will be minimal. Likewise, unless your LFO engines are small (less than, say, 200 kN of thrust), the extra thrust isn’t going to be all that helpful, either. But you’ve already got it there, so you might as well use it.

If, on the other hand, you haven’t launched yet, you’d be better off putting in a smaller MP tank to begin with.

It’s an Oberth effect question.  I get that’s it’s less efficient.  But does the increased Oberth effect from being able to complete my burn in a shorter period of time outweigh the decrease in efficiency and increased mass?  Remember I’m coming into Moho like a bat out of hell so TWR is important.  To put it another way, if you’ve got a 20 minute capture burn on a fast hyperbolic trajectory, that sucks from an efficiency standpoint.  You’re hardly doing any of that burn at periapsis.  A say 0.1 ton weight penalty or say a 5% hit to ISP would probably be well worth it to cut that burn time to 1 minute.  A 10 ton weight penalty or a 50% hit to ISP probably wouldn’t be worth it.  Where is that point in the middle is my question.

The craft is simple.  One large Mk3 fuel tank with adapters on either end.  Chock full of LF only, no ox.  Two nukes.  Probe core and docking port.  Associated normal RCS and four O-10s.

The craft hasn’t been launched.  Would adding monoprop to add to my TWR at a critical point (capture PE) be worth the weight penalty of additional low efficiency propellant?  Or should I add another heavy AF nuke?  Or do nothing?  Remember Moho has such a small SOI and the capture burn is so long that TWR actually matters here.  How much does it matter?

Edited September 3, 2018 by JerseyChris

[Kryxal]

Are you actually looking at using that monoprop near the END of your burn?  That's the only way Oberth is going to matter much.  If it's near the start, you might as well have brought fuel to do twice the braking though over a prolonged period.  Even so, Moho isn't exactly the largest gravity well for Oberth to work with.

[bewing]

2 hours ago, JerseyChris said:

Would adding monoprop to add to my TWR at a critical point (capture PE) be worth the weight penalty of additional low efficiency propellant?

See, the question still isn't specific enough. If you add monoprop before launch from kerbin, it's certainly not worth it -- every Kg of monoprop costs you 10 times that much fuel to get the monoprop to LKO, and monoprop is sucky fuel. If you add the monoprop in LKO from a tanker or station, it's still not worth it because you have to burn so much LF to push the monoprop out to Moho. If the monoprop appears by magic in your tanks on the way to Moho then yes it's totally worth it to burn it, because it's fuel and burning fuel gets you into orbit at Moho, and you want to burn the worst fuel first. So it's not really an oberth question, because no matter what -- you'd be burning the monoprop before you get anywhere near Moho Pe.

 

 

Edited September 3, 2018 by bewing

[BABLBAM]

Is this question really important?

[Vanamonde]

2 hours ago, BABLBAM said:

Is this question really important?

It is to the person who asked it. 

[stormdot5]

I think it has been established that adding monoprop at any point in the mission planning our on route would be inefficient.

Secondly, it you end up with more monoprop than you need, you might as well burn it, in any mission. The change in weight due to using monoprop will improve the delta v of your craft. You can factor this in when designing. 

 

In regards to the oberth effect, I think that yes, you could improve the efficiency of the liquid fuel engines by burning the monoprop at the same time. Even using RCS ports to burn the fuel. Calculate the increase in thrust, and therefore the reduction in burn time. This would allow you to burn for a shorter time and closer in to the planet, which provides the Oberth effect improvements. But as stated, it would have been more efficient to not have brought any extra fuel to begin with.

 

As for your designing, I think I would leave the monoprop and pack as much liquid fuel as possible. Better to have more delta v and burn it slightly inefficiently than try to make up TWR by burning monoprop. I don't know the numbers but it's possible packing oxygen instead and a detachable LFO engine might even be more efficient than monoprop if you need a short term TWR boost.

[JerseyChris]

“Better to have more delta v and burn it slightly inefficiently than try to make up TWR by burning monoprop.”

THIS is the answer I was looking for.

Also Kryxal is right, I didn’t really consider how small Moho’s gravity well is either.  Extra monoprop goes out the window then.  Thanks everyone.

I am still curious where the “tipping point” is though.  There has to be some point (which would of course vary by body, isp, TWR, etc) where a lower efficiency is compensated for by the TWR.  To put it another way, there’s a place for engines other than ions, nukes, and ants for certain capture burns, I am sure.  I just don’t know where it is.

20 hours ago, bewing said:

See, the question still isn't specific enough. If you add monoprop before launch from kerbin, it's certainly not worth it -- every Kg of monoprop costs you 10 times that much fuel to get the monoprop to LKO, and monoprop is sucky fuel. If you add the monoprop in LKO from a tanker or station, it's still not worth it because you have to burn so much LF to push the monoprop out to Moho. If the monoprop appears by magic in your tanks on the way to Moho then yes it's totally worth it to burn it, because it's fuel and burning fuel gets you into orbit at Moho, and you want to burn the worst fuel first. So it's not really an oberth question, because no matter what -- you'd be burning the monoprop before you get anywhere near Moho Pe.

 

 

The monoprop would be coming from Kerbin.  I don’t want to add another nuke engine because they’re so heavy.  The existing monoprop is budgeted for RCS maneuvers.  “Burn the worst fuel first” please explain.

[Kryxal]

As a rule, you burn the fuel with the lowest efficiency first, because that way you're not hauling the extra mass anymore.  It's what makes solid boosters good off the pad, but bad for upper stages, they're cheap but heavy for what they can do.

Also, if you really want to get extra thrust to deliver more fuel, the place to look is going to be your transfer burn out of Kerbin orbit, where you CAN effectively use low-ISP, high-TWR engines for an initial kick.  Once you're done with them, you stage them away and go the rest of the way on your nukes.

[Tyko]

On 9/2/2018 at 6:13 PM, JerseyChris said:

Yes but we are ignoring the Oberth effect which is what generated the question.  I get that nukes are more efficient.  See my above reply to see what I mean.

I'm not ignoring the Oberth effect. The benefits of the Oberth effect are relatively low around Kerbin with it's deep gravity well. Moho's gravity is a lot lower, so less Oberth benefit. Compare this to the ISP of the a Terrier (345) versus monoprop (240) - the LFO Terrier is almost 50% more efficient - you're getting almost half-again more DV for each unit of LFO over monoprop fuel. 

I'm sure one of our local math wizards could calculate the maximum DV benefit the Oberth Effect will give you around Moho, but I guarantee it's not going to give you the 50% more efficiency that the LFO fuel is offering you.

[JerseyChris]

10 hours ago, Tyko said:

I'm not ignoring the Oberth effect. The benefits of the Oberth effect are relatively low around Kerbin with it's deep gravity well. Moho's gravity is a lot lower, so less Oberth benefit. Compare this to the ISP of the a Terrier (345) versus monoprop (240) - the LFO Terrier is almost 50% more efficient - you're getting almost half-again more DV for each unit of LFO over monoprop fuel. 

I'm sure one of our local math wizards could calculate the maximum DV benefit the Oberth Effect will give you around Moho, but I guarantee it's not going to give you the 50% more efficiency that the LFO fuel is offering you.

 

23 hours ago, Kryxal said:

As a rule, you burn the fuel with the lowest efficiency first, because that way you're not hauling the extra mass anymore.  It's what makes solid boosters good off the pad, but bad for upper stages, they're cheap but heavy for what they can do.

Also, if you really want to get extra thrust to deliver more fuel, the place to look is going to be your transfer burn out of Kerbin orbit, where you CAN effectively use low-ISP, high-TWR engines for an initial kick.  Once you're done with them, you stage them away and go the rest of the way on your nukes.

Gotcha.  I’m convinced now.  Thanks for the explanation and your time.  Happy travels, I’ll leave the monoprop at home and off we go to rescue the poor guy and gal (at least I hope there’s only two of them, who knows, they’ve been unattended for a while) who have been orbiting Moho for a year and a half or so.

[mystifeid]

13 minutes ago, JerseyChris said:

I’ll leave the monoprop at home and off we go to rescue the poor guy and gal

Think about sending a station/base/lander and setting up a mining operation. It's easy to profitably bring fuel or ore back from Moho's surface to an orbiting station. This probably won't be the last rescue contract and it's a lot easier when fuel is already waiting there. Also, lucrative contracts to mine ore should become available.

[JerseyChris]

1 minute ago, mystifeid said:

Think about sending a station/base/lander and setting up a mining operation. It's easy to profitably bring fuel or ore back from Moho's surface to an orbiting station. This probably won't be the last rescue contract and it's a lot easier when fuel is already waiting there. Also, lucrative contracts to mine ore should become available.

I actually thought about that but I run parallel missions and don’t use any mods (even alarm clock) and my Jool station flotilla is arriving right about now so I’ve got a bit too much on my plate to launch another station project.  After the Jool station gets put together I’ll probably do that.  Right now I’m running 5 separate major missions composed of maybe 20 individual spacecraft so... yeah.  I’m like, “OH!  Right!  The Duna Rover is entering atmosphere today!  I forgot all about that!”

[Tyko]

11 hours ago, JerseyChris said:

I actually thought about that but I run parallel missions and don’t use any mods (even alarm clock) and my Jool station flotilla is arriving right about now so I’ve got a bit too much on my plate to launch another station project.  After the Jool station gets put together I’ll probably do that.  Right now I’m running 5 separate major missions composed of maybe 20 individual spacecraft so... yeah.  I’m like, “OH!  Right!  The Duna Rover is entering atmosphere today!  I forgot all about that!”

Wow....that's a challenge   best of luck!

[JerseyChris]

12 hours ago, Tyko said:

Wow....that's a challenge   best of luck!

Made much harder by the fact that I’ve got to squeeze KSP in with two kids and a third on the way, a full time job, and all the problems that come along with maintaining a house.  I’m lucky if I can get in two hours a week.  And given my current mission schedule most of THAT is loading screens and correction burns...

[JerseyChris]

An update for those who helped (I always like it when people do that):  tankers rendezvoused with the capsule.  One with extra monoprop which was used at first opportunity (thanks Kryxal for the “burn the worst fuel first”) and one without extra monoprop launched after this thread (thanks everyone).  Tankers linked up with each other approx half full and one was deorbited, the other was topped up with LF and docked with the capsule, and everyone is on the way home with plenty of dV.  My first return from Moho.  My plan for this mission was basically one of “launch them first and figure out how to get them home later.”  Second part accomplished.  Fun fact: This was briefly considered as an Apollo mission mode.  VERY BRIEFLY.