Theory vs. Practice: The ARM parts.

[Themohawkninja]

Introduction: So, as many of you have heard, the new parts have a much higher ISP, TWR ratio, and other such stats as compared to the 2.5 meter parts. While this has caused many people to jump to the conclusion that the parts are OP, I have decided to conduct experiments in-game with the parts to see just how these numbers compare to the actual results of a flight. This is due in part to the fact that I only view "overpoweredness" by the results of an object's application.

This experiment tests the balancing of the parts by making what is essentially the same craft in a 2.5 meter and 3.75 meter scale. This should become better explained when the parts list for each craft is stated.

Method: Each craft was flown with SAS on and on full throttle so long as the engines wouldn't overheat. If the engines would appear to overheat, the engines would be throttled down as little as possible.

All craft were flown eastward as follows:

• 10 degrees at 2 km
  
• 45 degrees at 6 km
  
• 90 degrees when apoapsis reached 40 km
  
• Stop burning when apoapsis is at 80 km except for small burns to maintain 80 km apoapsis due to atmospheric effects
  
• Burn at apoapsis until in a roughly 80 km circular orbit. Circularization burns are omitted to keep the fuel usage as consistent as possible.
  
• Record fuel remaining
  

Results: It should be noted here that the ISP used to calculate dV is always the ground ISP.

Ship A1: Mk7 Nosecone, RC-L01 Guidance Unit, 2x Z-100 Rechargable Battery Pack, Rockomax Jumbo-64, Rockomax Skipper.

dV (300 ISP)=4488.31

Result:

• 2.8% liquid fuel remaining
  
• Remaining dV (300 ISP)=282.301
  

Ship A2: Mk7 Nosecone, RC-L01 Guidance Unit, 2x Z-100 Rechargable Battery Pack, Kerbodyne S3-14400, Kerbodyne KR-2L.

dV (280 ISP)=4495.55

Result:

• 6.2% liquid fuel remaining
  
• Remaining dV (280 ISP)=627.290
  

Ship B1: Mk7 Nosecone, RC-L01 Guidance Unit, 2x Z-100 Rechargable Battery Pack, 2x Rockomax Jumbo-64, Rockomax Mainsail.

dV (280 ISP)=4578.42

Result:

• 3.7% liquid fuel remaining
  
• Remaining dV (280 ISP)=405.136
  

Ship B2: Nosecone, RC-L01 Guidance Unit, 2x Z-100 Rechargable Battery Pack, 2x Kerbodyne S3-14400, Kerbodyne S3 KS-25x4 Engine Cluster.

dV (320 ISP)=5462.68

Result:

• 9.4% liquid fuel remaining
  
• Remaining dV (320 ISP)=626.331
  

Conclusion: While there was only one trial per craft, the results show that while the 2.5 meter parts gave the craft a remaining dV budget of approximately 280 and 400 m/s, the 3.75 meter parts gave the craft nearly 630 m/s dV after it entered orbit. It is now up to the reader's own interpretation of the data to determine whether he or she believes that these new parts require balancing.

Edited April 7, 2014 by Themohawkninja

[r4pt0r]

nice research. new parts work fine for me, they got me into orbit for my first ever successful manned duna return mission yesterday night.

[zarakon]

The results of this would vary greatly by just including different amounts of fuel.

For example, your KR-2L example has much less fuel than it is capable of lifting. This means you're shorting yourself 2000+ possible dV that you could have, and your TWR is excessively high which would lead to a huge amount of wasted fuel early in the flight

You should design the test rockets so that they all start at the same thrust/weight ratio. Maybe 1.6.

You might want to avoid using the new fuel tank parts if you want to isolate the engines as the only variable. The new tanks actually have a worse fuel/weight ratio than the old 1.5m and 2.5m tanks. Just use the 2.5m tanks with the new engines

Your numbers for remaining dV should also be based on the vacuum ISP, since you would no longer be in the atmosphere. This would also make a big difference for the KR-2L which has a very high vacuum ISP.

Edited April 7, 2014 by zarakon

[Caelib]

Good research. People are complaining because they don't understand how these new parts fit into the overall vision for KSP career mode -- as soon as the currency system is introduced, they will quickly understand why they don't want to use that 3.75m rocket to launch a satellite.

[Uncle Mort]

I prefer to use the term "better powered" rather than "overpowered". New equipment, new abilities. Personally I'm happy to build launchers that look like "proper" rockets rather than a mass of tanks, fuel pipes and struts to get a reasonable tonnage in orbit.

[regex]

as soon as the currency system is introduced, they will quickly understand why they don't want to use that 3.75m rocket to launch a satellite.

Making such claims is suspect at best because no one has any idea how the currency system, or any other future part of career mode, will actually work.

[Themohawkninja]

The results of this would vary greatly by just including different amounts of fuel.

For example, your KR-2L example has much less fuel than it is capable of lifting. This means you're shorting yourself 2000+ possible dV that you could have, and your TWR is excessively high which would lead to a huge amount of wasted fuel early in the flight

You should design the test rockets so that they all start at the same thrust/weight ratio. Maybe 1.6.

Your numbers for remaining dV should also be based on the vacuum ISP, since you would no longer be in the atmosphere. This would also make a big difference for the KR-2L which has a very high vacuum ISP.

I wasn't too sure about which ISP to use, because while yes the vacuum ISP would be more logical for remaining dV, it would also seem more logical to both (A) use the same ISP for both dV calculations, and ( use the ISP value that will be "used" the most (longest burn time), hence I decided to go with the ground ISP.

[Caelib]

Making such claims is suspect at best because no one has any idea how the currency system, or any other future part of career mode, will actually work.

Except that's the point of a currency system.

[regex]

Except that's the point of a currency system.

Sorry, what is the point of the currency system? Can you give me a link to prove your statement? Honestly, you have no idea how it will work or how it will limit launches so saying that putting a satellite into orbit with a grossly over-powered craft is somehow a bad idea remains to be seen.

[r4pt0r]

Sorry, what is the point of the currency system? Can you give me a link to prove your statement?

the smaller rocket will be cheaper, thats a given mate. so once currency is added you wont(read:shouldnt) be spending your kerbal monies on huge lifters for your smaller satalites.

[regex]

the smaller rocket will be cheaper, thats a given mate. so once currency is added you wont(read:shouldnt) be spending your kerbal monies on huge lifters for your smaller satalites.

We've seen nothing about how the currency system will work or how it will affect player behavior. Given SQUAD's propensity for making things easier for newer players I could argue that the game will have to be modded in order to actually provide a challenge to veteran players and that currency won't matter in the slightest. Given the stated goal that the three currencies be interchangeable we may find that enterprising players simply won't care because they'll be rolling in whatever currency they need, they just have a single standard launcher that puts 500 tons into orbit and call it good. Alternatively, it could end up being incredibly grindy and we'll find out that currency will get the rug pulled out from under it much like resources. The point being that blanket statements of fact are counterproductive and set people up for disappointment.

E: Yeah, I get it, smaller rockets will be cheaper, but that doesn't mean that people will somehow be forced to use them or will realize they're "bad ideas".

Edited April 7, 2014 by regex

[zarakon]

the smaller rocket will be cheaper, thats a given mate. so once currency is added you wont(read:shouldnt) be spending your kerbal monies on huge lifters for your smaller satalites.

Nothing about the potential currency systems is a given. Until it's actually added, we can't even be sure that it will ever exist. The whole idea could end up scrapped just like resource collection was.

[ScottyDoesKnow]

I don't entirely agree with the way you tested, so I did a quick one by the following method:

1. Enough fuel to have the same TWR (aimed for 1.6)

2. Same payload percentage

3. MechJeb ascent to 75km

I also used the largest tank for that engine as much as possible since realistically that's how they'll be used (disadvantage to the 3.75m engines since their tanks are less efficient).

Engine		TWR	Payload	Total	%	dV Remaining

Skipper		1.57	0.45	42.14	1.0679	896
Mainsail	1.57	1.1	97.1	1.1328	1064
KS-25x4		1.59	2.38	205.63	1.1574	1680
KR-1x2		1.58	1.5	129	1.1628	1899
KR-2L		1.58	1.9	160.9	1.1808	1969

dV remaining is vacuum.

[Doctor Axel]

While I don't much mind the 3.75m parts, since they're the only stock entries in their class, what I DO mind is the all-in-one 2.5m fueltank+engine combo that is straight up, objectively better than a mainsail+orange tank. It has the exact same total weight, fuel capacity, and thrust as a mainsail+tank, with the added bonus of being a single part AND having a better ISP. I call shenanigans.

[Superluminaut]

I did the math but my thread got moved because mods suck.

http://forum.kerbalspaceprogram.com/threads/75088-S3-Engine-Clusters

[Rastaman]

I don't entirely agree with the way you tested, so I did a quick one by the following method:

1. Enough fuel to have the same TWR (aimed for 1.6)

2. Same payload percentage

3. MechJeb ascent to 75km

I also used the largest tank for that engine as much as possible since realistically that's how they'll be used (disadvantage to the 3.75m engines since their tanks are less efficient).

Engine		TWR	Payload	Total	%	dV Remaining

Skipper		1.57	0.45	42.14	1.0679	896
Mainsail	1.57	1.1	97.1	1.1328	1064
KS-25x4		1.59	2.38	205.63	1.1574	1680
KR-1x2		1.58	1.5	129	1.1628	1899
KR-2L		1.58	1.9	160.9	1.1808	1969

dV remaining is vacuum.

I did a similar thing with FAR and procedural parts. Launch TWR was 1.1 for each. Payload was 4.2 tons each. I used the same ascent profile as the OP, a pitch of 80 at 2 km, pitch of 45 at 6 km, upon an apoapsis of 40 km a pitch of zero until an apoapsis of about 120 km, then circularization at apoapsis. Pitch and circularization was done by mechjeb. Always single state to orbit. Expended dv was always about 3700 m/s.

Engine	dV Remaining

Skipper		1120
Mainsail	        1800
KS-25x4		2850
KR-1x2		2670
KR-2L		3050

It looks like if the new engines are overpowered, the Mainsail always was overpowered compared to the Skipper, yet nobody noticed.

Edited April 7, 2014 by Rastaman

[Liowen]

While I don't much mind the 3.75m parts, since they're the only stock entries in their class, what I DO mind is the all-in-one 2.5m fueltank+engine combo that is straight up, objectively better than a mainsail+orange tank. It has the exact same total weight, fuel capacity, and thrust as a mainsail+tank, with the added bonus of being a single part AND having a better ISP. I call shenanigans.

A small tweak to this engine that could, and maybe should be, is get rid of vectoring, electric generation and the ability to stack other tanks on top of it. Make it more like a solid rocket booster and less like fuel tank + engine combo. A different way that things could work is make boosters only attachable radially (solid or liquid) to a main engine.

One thing with these test I would suggest use a payload they were designed to do, 40t can be done by a small lifter make it a payload of 100t + with fuel amounts and part counts of each. The reason for parts count is not to show that one would make a player lazier than another, but rather if a player has a system that might become a slideshow if they use a higher part count setup there is an alternative. I have seen a lot other users on here saying that if one player uses the new parts to move a 100t+ payload they are being lazy or unskilled, or what way they feel the need to belittle others, when it might be that the person simply might not have a system able to run high part crafts. Should this limit them from having fun as they see fit in their own game? It is not a matter of being too easy for players to do something, it is about there not being pretty little "snowflakes" thinking they are special....its a game not a competition.

If anything these forums are ruining the fun of the game.....(I am sure I will get a warning for that but it is how I feel currently just looking at the posts here as of late)

[ScottyDoesKnow]

It looks like if the new engines are overpowered, the Mainsail always was overpowered compared to the Skipper, yet nobody noticed.

You're using the same payload weight instead of the same payload fraction. So yes, more fuel and a more powerful engine will push the same payload farther.

[Jouni]

A small tweak to this engine that could, and maybe should be, is get rid of vectoring, electric generation and the ability to stack other tanks on top of it. Make it more like a solid rocket booster and less like fuel tank + engine combo. A different way that things could work is make boosters only attachable radially (solid or liquid) to a main engine.

The entire point of the LFB is the ability to stack more fuel tanks on top of it. The proposed SLS liquid booster is about 66 m high and 5.49 m in diameter. If we scale it to 2.5-meter scale, the entire booster should be about 4 orange tanks high, including the engines and the nosecone.

A more reasonable way to balance it would be to reduce its Isp. The Isp values of the real booster using F-1A engines are supposed to be 269.6 s at sea level and 303.1 s in vacuum, while the values for the SLS core are supposed to be 366 s / 452.1 s. Give it more thrust at a higher TWR and a lower Isp than the Mainsail with a similar amount of fuel, and it should get its job done.

The real booster should have TWR 1.63 at liftoff. If we use 2 additional orange tanks, we get similar results from a 6-tonne engine with 1800 or 1900 kN of thrust.

[merendel]

the smaller rocket will be cheaper, thats a given mate. so once currency is added you wont(read:shouldnt) be spending your kerbal monies on huge lifters for your smaller satalites.

While probably true we have no idea to what degree. For one thing we have no idea what our budget will look like. For another the price listed on the parts is all over the map and make no sense. The Skipper, Mainsail, and KR-2L all have the same price listed. The same price would only get you 3 LV-T30s. The thrust to weight and thrust to cost ratios as they stand are drasticly in favor of the KR-2L over a cluster of T30s.

If the prices are not placeholders then any time you need 4 or more LVT's in a cluster your better off with the KR-2L. If you want to argue that the prices are placeholders(and I'd agree) then we are left with absolutely no way to evaluate relitive costs of various rocket building strategies making this whole argument pointless till squad releases more info on the currency system. Logicly smaller rockets will be cheeper but were playing a game where little green men with 50% of their body mass contained in their heads gleefuly ride unsafe rockets to wherever, logic kinda got left at the door a long time ago.

[boxman]

Good research. People are complaining because they don't understand how these new parts fit into the overall vision for KSP career mode -- as soon as the currency system is introduced, they will quickly understand why they don't want to use that 3.75m rocket to launch a satellite.

Alot of people dont care about career mode at all so overpowered engines could affect the sandbox mode in a very negative way for those who wants at least some pseudorealism.

[Rastaman]

You're using the same payload weight instead of the same payload fraction. So yes, more fuel and a more powerful engine will push the same payload farther.

It means the new engines are not overpowered at all. There are right there with the others. With the same payload, you get at most 70% more dv than the mainsail out of it. The mainsail gets 60% more than the skipper.

Costs aside, there might well be objects in the solar system which are much further out than Eeloo or Jool. Objects you need more dv to get to and explore. For this you need a bigger rocket, more mass into orbit with linear rockets.

[cantab]

It looks like if the new engines are overpowered, the Mainsail always was overpowered compared to the Skipper, yet nobody noticed.

The Skipper was always lousy, I thought that was reasonably well known. Clusters of LV-T30's or T45's are better than a Skipper in all respects but structural convenience. The same arguably holds, though by a smaller margin, for the Poodle vs clustered LV-909's.

As for the main topic, well comparison methodology is always a bit tricky. "Real game" tests will capture things like the impact of the atmosphere well and are perhaps more convincing, though I would advise the use of an autopilot. However they can't practically explore the same "space" as calculations.

Ultimately I'd say the main performance criterion is payload fraction (payload mass/launch mass) for a given payload. Maximising that, though, requires exploring a massive design space, we can't be anywhere near comprehensive enough "by hand".

[maccollo]

When the discrepancy is this huge you don't need a super computer to demonstrate it.

Like sure, I get that the ARM engines could be statistical slightly more efficient but this is kind of silly.

Edited April 7, 2014 by maccollo

[ScottyDoesKnow]

It means the new engines are not overpowered at all. There are right there with the others. With the same payload, you get at most 70% more dv than the mainsail out of it. The mainsail gets 60% more than the skipper.

Costs aside, there might well be objects in the solar system which are much further out than Eeloo or Jool. Objects you need more dv to get to and explore. For this you need a bigger rocket, more mass into orbit with linear rockets.

No, what it means is that you're testing 2 variables at once and so you can draw no conclusions. You'd get the same result with a high thrust engine with terrible ISP and with a low thrust engine with huge ISP. You're completely ignoring how much fuel was expended.