Asparagus set-up?

[strider3]

Does an asparagus setup have the same benefits in a vacuum as it does as a booster stage launching from Kerbin? My inclination is to say...maybe? Mass is still mass no matter the location of the ship but I would think the amount of gravity will have some effect on an asparagus's usefulness.

[Entropian]

Yes, the asparagus concept still works in a vacuum.  Same concept, same understanding, just different environments.  Gravity would affect the design of the rocket but the asparagus design still would work.

Edited July 4, 2020 by Entropian

[StrandedonEarth]

Yep, I use asparagus in vacuum, but the drop tanks don't usually need engines since TWR is less of an issue.

[strider3]

Thank you.

[Zhetaan]

3 hours ago, strider3 said:

Does an asparagus setup have the same benefits in a vacuum as it does as a booster stage launching from Kerbin? My inclination is to say...maybe? Mass is still mass no matter the location of the ship but I would think the amount of gravity will have some effect on an asparagus's usefulness.

Gravity has nothing to do with it.  Asparagus works because it removes dry mass and thereby keeps your rocket's mass ratio high--in exactly the same fashion as other staging.  The advantage of asparagus is that it allows you to use multiple engines but feed them from a common propellant tank (or tanks, since symmetry is important).  You get the advantages of high thrust thereby, but without needing to sacrifice the efficiency of maintaining a high mass ratio.

One reason that you might be concerned about gravity is because one of the key advantages of asparagus--high thrust--is important when fighting gravity.  Using more engines for more thrust lets you escape the gravity more quickly, so asparagus definitely has an application there.  In space, it is somewhat less useful, but it works quite well with drop tanks arranged around a core engine or cluster.

Of course, once in space, you have the advantage of being able to use resource transfer while doing something other than trying not to flip or crash, so setting up the fancy asparagus ducting and all may add more mass than you actually need to get the same effect.

 

Edited July 4, 2020 by Zhetaan

[StrandedonEarth]

2 hours ago, strider3 said:

Thank you.

As an example, this asparagussed two-kerbal deep-space rescue craft has just over 8400m/s of dV. One set of tanks have extra LV-Ns to keep the TWR up (and early burn times down), while the other set of tanks (4 total, can drop a pair at a time thanks to the inline decoupler) don't really need the extra thrust.

I built it to rescue Make from an orbit past Dres, and then was able to run the Eve-3 rally (Eve-Duna-Minmus, not necessarily in that order) on the way

home.

Edited July 4, 2020 by StrandedonEarth
pic would help, eh

[magnemoe]

8400m/s of dV, hold my bear

This thing was used to rescue an kerbal in retrograde orbit outside of Dress. 
It did not go as planned, well I captured the kerbal and his ship, however I had not thought life support trough so so I had to reach Kerbin in 270 days, I was also a bit low on dV but that was a bit irelevant. 
Solution was to launch an second of this things and do an 6400 km/s burn to intercept it grab the capsule and then do an braking burn, I thought I needed more than 13 km/s but I saved a bit doing the intercept around Minmus orbit. 
Note that dV is shown with the lowest stage almost burned out, I refueled that in  LEO, had I knew I would just dropped it. 
And yes its 8+1 asparagus, with an booster and an upper stage since LV-N is so heavy. 

Edited July 5, 2020 by magnemoe

[Vanamonde]

Asparagus works better in vacuum because the additional drag caused by side-stacks is no longer a factor. 

[StrandedonEarth]

14 hours ago, magnemoe said:

8400m/s of dV, hold my bear

Nice, although I wasn't trying for a record setter. It was just an example of a compact, kinda light, high-dV manned craft. I could have gone for more or longer pairs if I thought I needed it. And that doesn't include the kick stage to get it out of LKO. And if it was just a probe...

[Grogs]

What asparagus staging (or any staging really) allows you to do is "cheat" the tyranny of the rocket equation a bit. The equation is

dV = g0 * ISP * LN(m_wet / m_dry), where g0 is the standard gravity (~9.81 m/s), m_wet is the mass of the ship with fuel, and m_dry is the mass of the ship with no fuel.

In KSP, most of the tanks are 8/9 fuel and 1/9 tank by mass. That means that even at the most extreme case where the mass of the engines and payload are negligible compared to the fuel tanks, the best you can do is ISP * 9.81 * LN(9) = 21.55 * ISP. For chemical engines, the Wolfhound has an ISP of 380 s => dV = 21.55 * 380 = 8188 m/s.

With the asparagus staging, you reduce the dry mass part of the way through. For example, let's say you have three tanks - a central core with the engines, and two side boosters. For the first stage (i.e., until the side tanks are empty), the rocket equation looks like this:

dV = g0 * ISP * LN(27 / 11) = 8.81 * ISP, or for a Wolfhound 3346 m/s. So you've added about 40% to your dV to get it up to 11,534, but you've also tripled you starting mass.

If you added another pair of tanks, it would only increase dV by another 1637 m/s, or about 14% over a single pair of drop tanks. A third pair of drop tanks would only give 1092 m/s extra, and so on. I don't know what the infinite sum ultimately converges to, but as a practical matter it's hard to do much better than doubling the single tank dV without an obscene number of radial tanks.

[magnemoe]

1 hour ago, StrandedonEarth said:

Nice, although I wasn't trying for a record setter. It was just an example of a compact, kinda light, high-dV manned craft. I could have gone for more or longer pairs if I thought I needed it. And that doesn't include the kick stage to get it out of LKO. And if it was just a probe...

Obviously, your ship was designed for an mission same as mine. Mine mission was just more extreme, not an very advanced ship design after all. 
 

[jimmymcgoochie]

I use radially attached drop tanks for a lot of interplanetary stuff, it's more efficient to drop the empty tanks and shed the extra weight than to use one huge tank and hauling it around the whole time. It really helps when you get into the realms of rescaled solar systems- I'm playing with JNSQ (2.7x rescale) and the delta-V requirements are considerably higher than the stock system so strapping extra tanks on makes a lot of sense.

However I don't bother with fuel ducts or extra engines on vacuum stages, just setting the decouplers to stage at different times and enabling crossfeed on them is enough (nuclear engines are expensive!). The only time I ever used true asparagus staging was when I built a rocket to launch a 1000 ton payload into space, and with an orbital velocity of 3800m/s that required over 100 meganewtons of thrust on the launchpad with a TWR of about 1.3!

Edited July 5, 2020 by jimmymcgoochie