Most efficeint way to get to the Mun

[Geonovast]

Eh?  Couldn't possibly not work, right?  I mean it's right there.

[HolidayTheLeek]

I feel stupid, it took me so long to get this...

Very funny, haha!

[Jas0n]

I mean.... you could... but it wouldn't be that efficient....

[Rocket Farmer]

@JasOn

Actually that's about perfect for an efficient mun intercept.

Doing a direct ascent and mun transfer in one shot saves you the delta v of doing your perapsis burn.  Probably saves about 50m/s of delta v over the traditional method of getting to orbit and then doing a separate burn for your mun intercept.

As well he also has the correct mun location to pull it off.  I like restarting career every new update.  Typically I like doing a Mun flyby very early (2-3 launch) typically with stacks of SRBs).  I don't ever have the guidance system up at that point so I dead reckon the Mun.  Fire about 865m/s pointed prograde just after the Mun clears the horizon will get you a nice intercept (old tip from before we had a guidance systems).

Lastly the guy is using a SSTO.

So how exactly will you do it more efficiently?  I'm sure a true pro (which he might already be) might shave another 5% off with a different vehicle configuration and maybe another 5% with an optimum ascent profile but he is very close to optimum in my books as he has his flight profile perfect.

[Xyphos]

8 hours ago, Rocket Farmer said:

Doing a direct ascent and mun transfer in one shot saves you the delta v of doing your perapsis burn

not quite correct, not quite incorrect...

doing a burn straight up costs more DV due to gravitational energy losses (See the second law of thermodynamics)
doing a horizontal burn would almost eliminate that energy loss, but you'd have to compensate for gravitational forces, so for this situation, it's a bit too late, missed the launch window, tho you could still apply course corrections mid-way.

 

OP: The word "efficient" can be loosely translated as many categories;
for fuel efficiency, a low-mass vessel with high delta-V would be best, for example, launching an ion chair from the cargobay of a space plane in LKO
for cost efficiency, your vessel would cost as little as possible to produce, and be fully reusable in order to recover production costs when the mission completes
for part efficiency, your vessel would use fewer parts and/or low-tech parts.

however, most people tend to go for fuel efficiency, because not having to haul unnecessary fuel mass would make the mission go more smoothly.

Edited August 6, 2017 by Xyphos

[OhioBob]

11 hours ago, Rocket Farmer said:

Doing a direct ascent and mun transfer in one shot saves you the delta v of doing your perapsis burn.  Probably saves about 50m/s of delta v over the traditional method of getting to orbit and then doing a separate burn for your mun intercept.

I assume you mean apoapsis burn, i.e. the burn performed at apoapsis to circularize the orbit on a typically launch and orbit insertion?  You don't actually save that delta-v.  All the apoapsis burn does is raise the periapsis on the other side of the planet, which you are still doing.  It's just that you're doing it all in one burn rather than pausing and then completing the burn when you reach apoapsis.  The only advantage to performing the Mun transfer in one shot is that you're performing the ejection burn while at a lower altitude.  The exact savings depend on the circumstances, but a quick back of the envelope calculation tells me you might save in the neighborhood of 30 m/s.  But then again, much of the Mun transfer will be performed in the upper atmosphere, so that added speed will produce greater drag loses.  Your actual savings might be less.
 

11 hours ago, Rocket Farmer said:

As well he also has the correct mun location to pull it off.  I like restarting career every new update.  Typically I like doing a Mun flyby very early (2-3 launch) typically with stacks of SRBs).  I don't ever have the guidance system up at that point so I dead reckon the Mun.  Fire about 865m/s pointed prograde just after the Mun clears the horizon will get you a nice intercept (old tip from before we had a guidance systems).

Mun it not in the perfect position you think it is.  The old rule of thumb it that you preform the transfer burn just as you see Mun rising over the horizon, but that's from orbit.  You have to realize that having Mun on the horizon when observed from the surface of the planet is not the same as having Mun on the horizon when in an 80 kilometer orbit.  When in orbit you're seeing far beyond the horizon that can be seen from the surface below.  When an observer at sea level sees Mun on the horizon, an observer 80 km overhead will see Mun 28 degrees above the horizon.  So if you launch when Mun is on the horizon at the launch pad, you've waited too long.
 

Edited August 6, 2017 by OhioBob

[OhioBob]

2 hours ago, Xyphos said:

not quite correct, not quite incorrect...

doing a burn straight up costs more DV due to gravitational energy losses (See the second law of thermodynamics)
doing a horizontal burn would almost eliminate that energy loss, but you'd have to compensate for gravitational forces, so for this situation, it's a bit too late, missed the launch window, tho you could still apply course corrections mid-way.

I don't think @Rocket Farmer is talking about performing a straight up launch (correct be if I'm wrong).  I think he's suggesting using a normal, mostly horizontal ascent, but timing the launch so that launch and ejection can be performed all in one continuous burn; as opposed to burn-pause-circularize-pause-eject.  In theory, he's correct, one could save a small amount of delta-v by using that technique.  However, I think the potential savings is so small that's it's not worth it.  A flawless Mun transfer is a lot easier to execute from a parking orbit, but far be it from me to tell people how to have fun.
 

Edited August 6, 2017 by OhioBob

[Jas0n]

11 hours ago, Rocket Farmer said:

@JasOn

Actually that's about perfect for an efficient mun intercept.

Doing a direct ascent and mun transfer in one shot saves you the delta v of doing your perapsis burn.  Probably saves about 50m/s of delta v over the traditional method of getting to orbit and then doing a separate burn for your mun intercept.

As well he also has the correct mun location to pull it off.  I like restarting career every new update.  Typically I like doing a Mun flyby very early (2-3 launch) typically with stacks of SRBs).  I don't ever have the guidance system up at that point so I dead reckon the Mun.  Fire about 865m/s pointed prograde just after the Mun clears the horizon will get you a nice intercept (old tip from before we had a guidance systems).

Lastly the guy is using a SSTO.

So how exactly will you do it more efficiently?  I'm sure a true pro (which he might already be) might shave another 5% off with a different vehicle configuration and maybe another 5% with an optimum ascent profile but he is very close to optimum in my books as he has his flight profile perfect.

Ah my bad, I was thinking about going straight up to the mun, when it's directly over head.

[StupidAndy]

the most efficient way is hyperedit

[Rocket Farmer]

Ok

Im an arm chair rocket guy.  So I do call the burn to circularize my perapsis burn because I'm raising my perapsis but you are correct it should be called my aposis burn because it occurs at my aposis.

As well you are correct that I do raise my perapsis regardless during the Mun burn.

Also hats off to OhioBib.  I had not considered that the viewing angle from the surface was different than from orbit.  On the other hand the Mun is a very wide target so that likely wouldn't matter.

However there are a couple of asterisks to everybodies comments.

Size effieiency: This goes back to the air hog days (1.0-1.1) but at one point it was possible to intercept the Mun with a stock no clipping air hogged engine SSTO and a very small delta v outside the atmosphere.  You could almost hit the pre-requisite velocity (2200 orbit + 865 Mun transfer) near the top of the atmosphere using air breathing engines.  This made for a very efficient transfer fuel wise.  I haven't played with SSTOs much since then so I'm uncertain how much extra velocity you can now build but anything you manage is extra efficiency.

Price efficiency:  I use a more ballistic than SSTO but still one continuous burn for launching an SRB rocket early on tocthe Mun.  No unlike the famous video I have never landed onche Mun only using SRBs.

[blorgon]

1 hour ago, StupidAndy said:

the most efficient way is hyperedit

booooooooooooo

[HebaruSan]

46 minutes ago, Rocket Farmer said:

On the other hand the Mun is a very wide target so that likely wouldn't matter.

Is it? This looks like a lot less than 28° to me... Reddit says 0.955° to 1.005°.

Edited August 6, 2017 by HebaruSan

[NSEP]

I dont know about efficiency, but it's definitely really easy.

[adsii1970]

Hyperedit... the easiest way to get to where you want to go!

[regex]

2 hours ago, Rocket Farmer said:

 This goes back to the air hog days (1.0-1.1) 

Air hogging died as a valid technique when 1.0 arrived.

Edited August 6, 2017 by regex

[StupidAndy]

4 hours ago, regex said:

Air hogging died as a valid technique when 1.0 arrived.

whats air hogging?

[HebaruSan]

1 minute ago, StupidAndy said:

whats air hogging?

In pre-1.0 versions, you could use jet engines very high up in the atmosphere by adding lots of air intakes (3+ per engine was not uncommon).

[StupidAndy]

Just now, HebaruSan said:

In pre-1.0 versions, you could use jet engines very high up in the atmosphere by adding lots of air intakes (3+ per engine was not uncommon).

oh yeah, I saw that in pictures!

[regex]

1 minute ago, StupidAndy said:

whats air hogging?

Before we had a semi-realistic aerodynamic sim we had what was commonly referred to as the "souposphere" and it was generally terrible and eventually revealed to be a placeholder after being the single sorest gripe about pre-1.0 KSP.

Under such a simulation the jet engines had isps (I think) four times greater than they are now (they're still vastly superior to anything humans can produce insofar as fuel consumption is concerned) and their intakes produced intake air in an additive fashion, so you would stack as many intakes as looked good to feed your engine pretty much all the way to space. You could literally reach orbit with jet engines pre-1.0 by closing intakes and feeding off their stored intake air.

[OhioBob]

58 minutes ago, regex said:

Under such a simulation the jet engines had isps (I think) four times greater than they are now (they're still vastly superior to anything humans can produce insofar as fuel consumption is concerned) ...

As I recall, the problem was that jet engine thrust and specific impulse were incorrectly based on the total mass flow of both fuel and air, while it should have been based on just the mass of liquid fuel.  The fact that only the fuel is counted is why real life jet engines have Isp so much greater than rocket engines (which must carry their own oxidizer).  I think the engines had a air-fuel mixture ratio of something like 16:1.  Let's say that based on the thrust and Isp it is calculated that the fuel flow rate of a jet engine is 1.7.  If done correctly, that would mean that 1.7 units of liquid fuel is consumed, and 1.7*16 = 27.2 units of air.  But the way Squad did it, they said that 0.1 units of fuel and 1.6 units of air were consumed.  So jet engines burned only 1/17th as much fuel as they should have.
 

Edited August 7, 2017 by OhioBob

[KerikBalm]

The ratio of fuel to air was 1:15, making the fuel to total mass ratio 1:16.

The stated Isp was actually lower than what we have now: 2500, but back then the intake air did not have the flag "ignore for Isp" or something like that (it could have been equally fixed by making intake air massless, as with ion engines and electric charge - but for adding electric fans by modding there needs to be something with mass). Essentially it got effective Isp and real Isp mixed up.

2500 real Isp at a 16:1 ratio was 40,000 effective Isp. If I recall, at high altitude, the Isp dropped to 1200, but that's still 19,200 effective Isp. There was also a velocity modifier which modified the thrust but not the fuel consumption, so at higher speeds when thrust started to drop off due to that curve, you might only be getting 1,920 effective Isp instead - which is still pretty high. The extra drag of the soup-o-sphere could be an excuse for having unrealistically high efficiency in some areas to offset the extra drag... but this doesn't justify an effective Isp >10,000 at 60km.

On top of that, the Turbojets (that's what they were called before the name changed to turboramjets) could produce thrust until one reached 2,400 *surface* velocity - so if you flew east, you could easily accelerate to nearly 2,600 m/s *orbital* velocity... which could give you an apoapsis that was quite high with a periapsis around 60 km. Using stored intake air (because you could store it and use it back then) at apoapsis would raise the PE out of the atmosphere, and get you into orbit.

I guess at 2399 surface velocity, the thrust modifier would be nearly 0, and the effective Isp would drop to nearly zero.... but even at 2300 surface velocity, the effective Isp would still be better than the LV-N -> I'm not sure how the velocity curve was shaped, but I seem to recall that the modifier was 1.0 around 1,000 m/s. If we assume it was a linear decrease from 1000 to 2,400, then at 2,300 m/s you'd get ~7.1% of 19200 Isp = 1,370 Isp -> keeping in mind that 2,300 surface velocity is ~2,500 orbital, you can imagine how one would easily get to orbit with engines that had a peak efficiency of 40,000 Isp and could take you the rest of the way to orbitwith their Isp never dropping below the vacuum Isp of a LV-N, while getting a far better TWR than a LV-N ever could

[wumpus]

15 hours ago, regex said:

Under such a simulation the jet engines had isps (I think) four times greater than they are now (they're still vastly superior to anything humans can produce insofar as fuel consumption is concerned) and their intakes produced intake air in an additive fashion, so you would stack as many intakes as looked good to feed your engine pretty much all the way to space. You could literally reach orbit with jet engines pre-1.0 by closing intakes and feeding off their stored intake air.

Judging by the "ksp parts wiki", they are probably still twice the Isp of "real" jet engines (Juno should match an extremely efficient high-bypass turbofan.  Just assume it goes on a 747 or something.  Kerbal planes have notoriously outrageous thrust and that would probably work).  The modern sins of KSP jet engines remain:

* The "speed of sound" is the same for Earth, but orbital velocity is much lower.  On Earth we need ~"mach 20" for LEO.  On KSP we need ~"mach 6" for LKO.  The record for experimental aircraft (sustained, but accelerated to there by a rocket) is mach 9, so building spaceplanes on Kerbal would be a possibility.  On Earth they can't go to orbit (but a few [rockets with wings, not jets] have flown passed the Karaman line, which needs about as much delta-v as getting to LKO).
* The things are impevious to supersonic shock waves.  Going supersonic should tear up engines not designed for it (there is a reason that air moves subsonic in supersonic engines).

[monstah]

Is it me, or after the firstsecond reply, everyone started analyzing seriously the engineering of what was a good joke?

Edited August 7, 2017 by monstah

[Geonovast]

8 minutes ago, monstah said:

Is it me, or after the firstsecond reply, everyone started analyzing seriously the engineering of what was a good joke?

Ssssshhh.  I was gonna make a comment about the joke going over their heads, but figured it would evolve into a discussion about geosynchronous orbit and circular orbits and whatnot.

I'm enjoying the discussion.

[monstah]

Just now, Geonovast said:

a comment about the joke going over their heads,

HAHAHAHAHAHAHAHAHA!