Viable massive Delta-V return ships without nuclear?

[Rylant]

Hey all. So I am looking for a large Delta-V ship used for returning at least 3 Kerbals to Kerbin from long distance planets after landing on them. My problem is, that I am able to get to these distant planets, but I am notoriously wasteful with my Delta-V. I also REALLY dislike the horrible Thrust of the Nuclear engine.  I have gotten probes to Eeloo, Moho and Jool, but not Kerbals.

Are there viable 15k Delta-V designs out there that don’t use Nuclear? Or do I have to either, suck it up with nuclear, or figure out how to be more efficient with my burns?

 Rylant

[Stamp20]

The Wolfhound engine has very good efficiency, but it's locked behind Making History.

If it takes 15k DeltaV to get to other planets you either have a massive craft or are wasteful with your dV. To save dV, go interplanetary at transfer windows instead of going right after you launch. Kerbal Alarm Clock lets you set alarms for transfer windows, telling you the optimal time to launch. Transfer Window Planner is another mod that goes very well with KAC, which provides more details for planning interplanetary maneuvers, including showing the angles of the planets. The transfer window times for going from Kerbin to another planet also apply to returning to Kerbin.

[Rylant]

6 minutes ago, Stamp20 said:

The Wolfhound engine has very good efficiency, but it's locked behind Making History.

If it takes 15k DeltaV to get to other planets you either have a massive craft or are wasteful with your dV. To save dV, go interplanetary at transfer windows instead of going right after you launch. Kerbal Alarm Clock lets you set alarms for transfer windows, telling you the optimal time to launch. Transfer Window Planner is another mod that goes very well with KAC, which provides more details for planning interplanetary maneuvers, including showing the angles of the planets. The transfer window times for going from Kerbin to another planet also apply to returning to Kerbin.

Hey thanks for the reply. For things like Moho or Laythe, from the surface of Kerbin, land there, and return, you are looking at even more than 15 k, even is you are efficient, no?

 Rylant

[magnemoe]

15K dv is very high even with nuclear. 

this is 18 with an nuclear 8+1 asparagus 15 if you see the chemical upper stage as payload, as in a couple of tons. 

Chemical you will need so huge rockets your frame rate will drop to zero. 
 

[jimmymcgoochie]

The rocket equation: Delta-V =g0xISPxln(m0/mf), this tells you how far you can go;

g0 is standard gravity, 9.807m/s/s;

ISP is per engine, let’s assume 350s for the Poodle;

m0 is the wet mass of the craft including all the fuel;

mf is the empty mass, after all the fuel has been burnt; any excess propellant e.g. monopropellant counts as dry mass for this equation since you’re not burning it in the engine.

ln is natural log.

The mass ratio is m0/mf, or in simple terms how much of your rocket’s mass is fuel compared to not-fuel; in stock KSP this is limited by fuel tanks which have a mass ratio no greater than 9, so you can’t have a mass ratio above 9 even if your rocket is made of thousands of fuel tanks and one tiny engine.

ln(9) is 2.197, so let’s put that into the equation:

delta-V = 9.807 x 350 x 2.197 = 7541m/s. This is the fundamental limit for the Poodle engine, no matter what you do you cannot go higher than this on a single stage.

Swap to the Wolfhound with 380s, the maximum possible delta-V is 9.807 x 380 x 2.197 = 8187m/s.

If you use multiple stages you can get more delta-V overall, but just remember that for each stage, whatever it’s carrying on top is counted as dry mass, so your mass ratio will be even lower and so delta-V will be reduced on that stage.

If you actually want to get somewhere and do something useful when you’re there, your mass ratio will be worse than the ‘ideal’ value because of the extra weight of your payload and to get a usable thrust to weight ratio so you actually get there without sitting for hours and losing delta-V through cosine losses, which happen whenever you burn in a direction other than directly prograde or retrograde.

Now the NERV is a little bit different because pure liquid fuel tanks (which are all based on aircraft fuselage shapes- Mk1, Mk2 and Mk3) are less mass-efficient than LF/Ox tanks so the best mass ratio you can get is 8 (ln(8) = 2.079) but on the other hand, ISP is more than double what any LF/Ox engine can manage so the maximum delta-V is higher: 9.807 x 800 x 2.079 = 16314m/s.

The Dawn ion thruster is another level up from the NERV with a massive 4200s ISP, however xenon tanks have really poor mass ratios of 2.728 (ln(2.728) = 1.004) and the thrust is really low. Maximum delta-V with those is 9.807 x 4200 x 1.004 = 41620m/s!

And just for fun, monopropellant- the Puff engine has an ISP of 250s, slightly better than RCS thrusters, and the best RCS tank has a respectable mass ratio of 8.5 (ln(8.5) = 2.140); 9.807 x 250 x 2.140 = 5246m/s. Not nearly as good as even the least efficient LF/Ox engines which manage 290s ISP in vacuum.

TL;DR version- LF/Ox delta-V can’t go above about 7.5km/s on a single stage using the best stock engine (Poodle), or about 8.1km/s with the DLC Wolfhound engine; NERVs can get twice as much delta-V but their thrust is a lot less, ions can get over 5 times as much but their thrust is puny in comparison. NERVs are the best choice for long-range flights because of their superior delta-V compared to LF/Ox engines and their much greater thrust than ion engines, which also require electricity to function and producing/storing that power adds mass, reducing delta-V.

Getting to and from planets in the stock system isn’t all that difficult, as long as you travel at the right time- there are various tools and mods that can help you time your transfers to use as little fuel as possible, meaning you can carry a bigger payload, use a lighter rocket or go to more places. Orbiting Duna and Eve  with a crewed craft and returning is feasible with LF/Ox engines, though for larger vessels or trips further afield e.g. the moons of Jool, nuclear is the best option.

Edited May 14, 2021 by jimmymcgoochie

[Spaceman.Spiff]

14 hours ago, Rylant said:

Are there viable 15k Delta-V designs

Ion engines 

(I kid, those are even worse thrust than nukes)

[king of nowhere]

i made a return capsule with 15 km/s, but it uses ion engines, which have even worse thrust than nuclear.

as for the rest, @jimmymcgoochie already explained the rocket equation, and why this stops you from having too much deltaV without a ridiculous mass.

basically, if you want to run more complex missions, you must learn to be less wasteful.

[Lt_Duckweed]

3 hours ago, jimmymcgoochie said:

Now the NERV is a little bit different because pure liquid fuel tanks (which are all based on aircraft fuselage shapes- Mk1, Mk2 and Mk3) are less mass-efficient than LF/Ox tanks so the best mass ratio you can get is 8 (ln(8) = 2.079) but on the other hand, ISP is more than double what any LF/Ox engine can manage so the maximum delta-V is higher: 9.807 x 800 x 2.079 = 16314m/s.

Small nitpick here.  Mk1 liquid fuel tanks have the same mass ratio as rocket fuel tanks: 9.  And Mk0 tanks are actually the best in the game, with a mass ratio of 11, which gives a maximum possible delta v of 18,812 m/s

[jimmymcgoochie]

4 minutes ago, Lt_Duckweed said:

Small nitpick here.  Mk1 liquid fuel tanks have the same mass ratio as rocket fuel tanks: 9.  And Mk0 tanks are actually the best in the game, with a mass ratio of 11, which gives a maximum possible delta v of 18,812 m/s

I must have missed that, or my data was wrong. Either way it makes NERVs look even better.

[Fraktal]

Maybe, but are you really going to be that cheapskate to use Mk0 tanks for NERVs?

Or more to the point, just what kind of eldritch part count that 18k figure is going to take with only Mk0 tanks?

[Lt_Duckweed]

4 hours ago, Fraktal said:

Maybe, but are you really going to be that cheapskate to use Mk0 tanks for NERVs?

Or more to the point, just what kind of eldritch part count that 18k figure is going to take with only Mk0 tanks?

1000 Mk0's plus a single nerv would do the trick lol.

[Lt_Duckweed]

That is well into diminishing returns though, for a more realistic case, 100 mk0s and a single nerv gives you a starting twr of .2, and a delta v of 13,438 m/s

Doing the same thing with mk3 tanks (ie max dv at 0.2 twr) gets you 12,204 m/s so there is a pretty nice upside to using the mk0s

 

More realistically, lets ask, "What cargo fraction could you carry with a single stage craft having at least 2 m/s^2 of acceleration, and at least 5,000m/s dv (enough to, assuming plane changes are bundled into capture/ejection, capture into orbit of any body on a direct transfer from Kerbin, and vice versa).  At 2 m/s^2, the ejection from an elliptical staging orbit to Moho and Eeloo would take 11min, Jool would take 9 min, Dres would take 6.5 min, and Duna and Eve would take about 2 min

For Nervs with Mk3 tanks: 10% engine mass, 6.7% tankage mass, 47.3% propellant mass, 35.9% payload mass.

For Nervs with Mk0 tanks: 10% engine mass, 4.7% tankage mass, 47.3% propellant mass, 37.9% payload mass.  (A payload to payload improvement of 5.57%)

For the Wolfhound: 1.76% engine mass, 9.23% tankage mass,  73.87% propellant mass, 15.14% payload mass. (A payload to payload decrease vs Mk3 Nerv of 42.17%)

 

[jimmymcgoochie]

11 hours ago, Lt_Duckweed said:

1000 Mk0's plus a single nerv would do the trick lol.

If the trick was to create Kerbal Slideshow Program!

[Lt_Duckweed]

I had done more extensive math and a longer comment, but I ended up closing the browser out not realizing I hadn't posted it (oops lol).

The tldr is that 5000 m/s is enough to get from low Kerbin orbit to low orbit of any other body with no gravity assists.  If you assume a starting twr of .2 (with an initial kick or series of kicks worth 800m/s to an elliptical orbit this is about the lowest you can go and still get a reasonably efficient burn for a direct transfer to Eeloo), then Nervs with mk3s will get you about 80% more payload per overall starting mass than wolfhounds will.  Using mk0s will extend that to about 85%, so only worth it for extreme missions, or smaller scale crafts (with just one nerv you are looking at about 58 mk0 tanks, vs using just one mk3 short + one mk1).

Edited May 16, 2021 by Lt_Duckweed

[Lt_Duckweed]

Incidentally, these numbers make for a perfect transfer vehicle made from a cluster of 7 nervs, and 2 mk3 long tanks.  Dry mass of the tanks and engines is 35.28 tons.  If we assume a further 4.72 tons of structural, power, and control mass, this gives a nice round 140 ton wet, 40 ton dry mass.

This gives us the following dv benchmarks for this very simple craft, in terms of attainable acceleration and payload at these dv numbers:
2000 m/s:  0.95 m/sec², 304 tons
3000 m/s:  1.34 m/sec², 174 tons
4000 m/s:  1.68 m/sec², 110 tons
5000 m/s: 1.98 m/sec², 72 tons
6000 m/s:  2.25 m/sec², 47 tons
7000 m/s:  2.49 m/sec², 29 tons
8000 m/s:  2.69 m/sec², 16 tons
9000 m/s:  2.88 m/sec², 6 tons