Better than the Skipper, but not as powerful as the Mainsail

[CoriW]

So recently I was doing some Heavy Lifter development and I've discovered something that I thought I should share, although I'm sure some of you have probably figured it out by now as well.

When you use a TVR-400L Stack Quad-Adapter with four LV-T45 Liquid Engines mounted on it, you end up with better Thrust and ISP than you would get by using a Rockomax "Skipper" Liquid Engine. (Skipper: 650KN Thrust, 300/350 ISP, 4x LV-T45's: 800KN Thrust, 320/370 ISP) Now of course if you want a stage below the engine(s) it's probably a better idea to go with the Skipper, as that would be more structurally sound, but for ascent stages the Quad-Adapter 4x LV-T45's are pretty good.

[Sof]

why use 4 when you could use 7?

[CoriW]

why use 4 when you could use 7?

Actually my current heavy lifter uses 20 LV-T45 Liquid Fuel Engines, and 4 LV-T30 Liquid Fuel Engines. It can lift 36 Tons into LKO with fuel to spare.

[Munbeast]

You actually end up with less delta-V though, because the LV-T45 arrangement you describe is 2.2 tons heavier than a skipper

[-RanZ-]

You actually end up with less delta-V though, because the LV-T45 arrangement you describe is 2.2 tons heavier than a skipper

Those adapters have weight also

[CoriW]

You actually end up with less delta-V though, because the LV-T45 arrangement you describe is 2.2 tons heavier than a skipper

Wait, really? Well that's weird... Whenever I swap out my LV-T45's on my heavy lifter for Skippers, Engineer Redux says my DeltaV goes down. Hmm, this requires further investigation.

EDIT: So my Heavy Lifter had 5 clusters of LV-T45's, and when I would just switch out the center one it seemed that I would loose some DeltaV, but I just went ahead and replaced all clusters with Skippers... And I gained a total of 6m/s DeltaV.

Edited July 31, 2013 by CoriW

[Munbeast]

Wait, really? Well that's weird... Whenever I swap out my LV-T45's on my heavy lifter for Skippers, Engineer Redux says my DeltaV goes down. Hmm, this requires further investigation.

I've tried a few combinations now and it seems the 4 x LV-T45 arrangement is better for some designs but not others. It's starting to hurt my brain working it out. It does seem like it is something worth experimenting with though.

[CoriW]

I've tried a few combinations now and it seems the 4 x LV-T45 arrangement is better for some designs but not others. It's starting to hurt my brain working it out. It does seem like it is something worth experimenting with though.

I think I've figured it out! The reason it varies by design is because of the TWR, since the 4x LV-T45's have a higher thrust, it can lift a lighter ship faster and thus increase the ISP more quickly, where as if the ship is heavier, they won't be able to lift the ship as quickly and the ISP will remain lower for a longer period of time.

Or at least... That's the only solution I can come up with as to why it seems to vary per design.

EDIT: Although on a side note, if you ever have a ship with a TWR just below 1.00 (Say like 0.98 or something), you can always swap it out for the 4x LV-T45's and get the extra thrust you need to push it up above 1.00.

[dlmarti]

This is from engineer redux:

1 mainsail : 3942m/s and 3.64 TWR

1 skipper: 4737m/s and 1.66 TWR

Quad T45's: 4558m/s and 1.93 TWR

7 aero spikes: 4393m/s and 2.67 TWR

In my experience these numbers don't seem right, but I could be wrong.

[dlmarti]

My testing:

1 mainsail: 518,308m ( a lot of atmo effects, it lost a lot of delta-v )

1 skipper: 4,040,095m (flew nice, avoided more of the atmo effects )

Quad T45's: 1,729,854m ( I did not see any atmo effects, but it was certainly a little fast through the atmo )

5-aerospikes: 1,357,544 (invalid, I had to add a lot of reaction wheels to keep it stable)

All tests were at full power, with 2-X200's for fuel. Measured what the highest apogee could be attained straight up.

tbh, this is a completely bogus test, but what engineer redux was telling me just didn't sound right. This test, at least partially, backs up the engineer redux claims.

[AmpsterMan]

Those numbers feel correct as far as I am concerned. I find I use a cluster when I need more TWR than a skipper can provide.

Furtheremore, because the ratio of Full to Empty weight is the variable of a logarithmic function in the Rocket Equation, the greater Isp of the LV-Cluster will only do better than a Skipper when the full mass of that stage is comparatively large.

In other words, LV-Quads always do better TWR wise than Skippers, but only do better Delta-v wise when the full mass of the stage is "comparatively large"

[TheHengeProphet]

There were some very in depth discussions about this back in .18 and .19, but I can't seem to find them, so I wonder if they may have been lost in the crash.

Anyways, the results amounted to clusters being more efficient when the engine's T:W was less important compared to the rest of the craft (under a heavier payload), where something like the mainsail was more efficient where engine T:W was concerned for gaining efficiency.

That is if I remember correctly.

[arq]

4 T45s are heavier than a Skipper (actually they weigh as much as a mainsail), and that's before the weight of the quad-adapter. The thing that determines dV is the ISP and the wet and dry mass of the vessel, the engine adds to both the wet and dry mass. A heavy engine requires higher dV to compensate for the mass that it adds to the vessel. Low engine mass is why the Ant engine is so efficient (dV-wise) with a single Oscar-B tank, despite the abysmal ISP. With any very small vessel (less than a few tons), the NRV is a terrible choice because it is so darned heavy. Almost every engine has some situation in which it provides the highest dV.

[numerobis]

2 T30s and a T45 is the same thrust, lower mass, slightly better Isp than a Skipper. So that wins in every category outside stackability and part count.

In general, if you have an engine that has higher mass and better Isp, then you need that engine to burn for a long time to make it a net improvement.

[Rjhere]

4 T45's are not necesserily better than one Skipper! 4 T45's weighs 6t, plus the adapter...

The Delta V with the adapter, quad T45's and a X200-32 are:

DV= ISP*g0*In(M0/M1) = 370 * 9.81 * In (24.2t / 8.2t) = 3928.1 DV

For only a Skipper and the same fuel tank:

DV= ISP*g0*In(M0/M1) = 350 * 9.81 * In (22t / 6t) = 4461 DV

Here the Skipper is clearly the winner. The reason being that not always is ISP the main thing you want, weight is a very important part of it, though that quad engine cluster does look much more badass!

Edited July 31, 2013 by Rjhere

[CoriW]

My testing:

1 mainsail: 518,308m ( a lot of atmo effects, it lost a lot of delta-v )

1 skipper: 4,040,095m (flew nice, avoided more of the atmo effects )

Quad T45's: 1,729,854m ( I did not see any atmo effects, but it was certainly a little fast through the atmo )

5-aerospikes: 1,357,544 (invalid, I had to add a lot of reaction wheels to keep it stable)

All tests were at full power, with 2-X200's for fuel. Measured what the highest apogee could be attained straight up.

tbh, this is a completely bogus test, but what engineer redux was telling me just didn't sound right. This test, at least partially, backs up the engineer redux claims.

4 T45's are not necesserily better than one Skipper! 4 T45's weighs 6t, plus the adapter...

The Delta V with the adapter, quad T45's and a X200-32 are:

DV= ISP*g0*In(M0/M1) = 370 * 9.81 * In (24.2t / 8.2t) = 3928.1 DV

For only a Skipper and the same fuel tank:

DV= ISP*g0*In(M0/M1) = 350 * 9.81 * In (22t / 6t) = 4461 DV

Here the Skipper is clearly the winner. The reason being that not always is ISP the main thing you want, weight is a very important part of it, though that quad engine cluster does look much more badass!

Although I tested my heavy lifter using MechJeb (To make sure it was roughly the same flight both times), and whether I used the Skippers (5 Skippers and 4 LV-T30's) or the LV-T45's (20 LV-T45's and 4 LV-T30's), it seemed to get into orbit with pretty much the same amount of fuel left... This is really interesting, because it seems the outcome is much different depending on the ship you use. I tried another ship earlier that was just a RC-L01 Remote Guidance Unit, a Rockomax X200-32 Fuel Tank, and then tried both variations of just a Skipper as well as just the 4x LV-T45's, and in that test the Skipper seemed to get further.

Oh and I probably should of mentioned this earlier, but I have FAR installed... Not sure how much that's affecting what I'm seeing, could be quite a bit.

EDIT: I guess pretty much to sum up what I'm seeing is the math says the Skipper is better, but in the case of my Heavy Lifter it seems to be pretty much exactly the same.

(Although I did end up switching to Skippers since the math says it should be better.)

EDIT2: I just realized when I posted this I didn't even notice there was a page 2 in this thread... Woops.

Edited July 31, 2013 by CoriW

[TheHengeProphet]

Many of these tests are using light craft. If you push the load to the edge of what the Skipper can lift, which is less than what the LVt-40 cluster can lift, you might begin to notice a difference in efficiency. A cluster is much better for a centre stage, for example, if it gives you a noticable difference in I^SP, due to a better utilisation of its burn time.

This is not to say that for general purpose isn't better, as it is a considerable reduction in parts count (and likely stability as a result).

[CoriW]

Many of these tests are using light craft. If you push the load to the edge of what the Skipper can lift, which is less than what the LVt-40 cluster can lift, you might begin to notice a difference in efficiency. A cluster is much better for a centre stage, for example, if it gives you a noticable difference in I^SP, due to a better utilisation of its burn time.

This is not to say that for general purpose isn't better, as it is a considerable reduction in parts count (and likely stability as a result).

This makes a lot of sense.

[Whirligig Girl]

I'd much rather have something as good as skipper, almost, but size of LV-T series.