Jump to content

Rocket engine ISP as thrust.

sal_vager
 Share

Recommended Posts

sal_vager

sal_vager

Posted
Posted (edited)

Currently, Kerbal rocket engines automatically vary their fuel flow to keep their thrust constant as atmospheric pressure changes, making the most use of their exhaust nozzles, and they are all most fuel efficient in a vacuum to a greater (LV909) or lesser (Mainsail) degree, but Human rocket engines don't do this.

Our engines keep a constant fuel flow (depending on throttle of course) and, depending on atmospheric pressure and engine nozzle design, can be tuned to be most efficient at sea level or vacuum, or even somewhere in-between.

So, what if Kerbal engines were more like ours?

For a start, they wouldn't all be best in vacuum, why tune a Mainsail to be most effective in orbit when it's a first stage engine?

And what'd be the midrange, second/third stage engines?

Here's a very rough idea of how it could be and I didn't do them all, feel free to pick holes in it, make suggestions, let me know how mod engines might also fit in.

[table=width: 500]

[tr]

[td]Engine[/td]

[td]Usage[/td]

[td]Sea level[/td]

[td]Altitude[/td]

[td]Vacuum[/td]

[/tr]

[tr]

[td]LV-T30[/td]

[td]Mid stage[/td]

[td]185[/td]

[td]215[/td]

[td]185[/td]

[/tr]

[tr]

[td]LV-T45[/td]

[td]Mid stage[/td]

[td]172[/td]

[td]200[/td]

[td]172[/td]

[/tr]

[tr]

[td]LV-909[/td]

[td]Vacuum[/td]

[td]38[/td]

[td]-[/td]

[td]50[/td]

[/tr]

[tr]

[td]Poodle[/td]

[td]Vacuum[/td]

[td]152[/td]

[td]-[/td]

[td]220[/td]

[/tr]

[tr]

[td]Skipper[/td]

[td]Mid stage[/td]

[td]562[/td]

[td]650[/td]

[td]562[/td]

[/tr]

[tr]

[td]Mainsail[/td]

[td]First stage[/td]

[td]1500[/td]

[td]-[/td]

[td]1333[/td]

[/tr]

[tr]

[td]Aerospike[/td]

[td]Mid stage[/td]

[td]174[/td]

[td]175[/td]

[td]174[/td]

[/tr]

[tr]

[td]LV-N[/td]

[td]Vacuum[/td]

[td]16[/td]

[td]-[/td]

[td]60[/td]

[/tr]

[tr]

[td]24-77[/td]

[td]Vacuum[/td]

[td]16[/td]

[td]-[/td]

[td]20[/td]

[/tr]

[tr]

[td]48-7S[/td]

[td]Vacuum[/td]

[td]25[/td]

[td]-[/td]

[td]30[/td]

[/tr]

[tr]

[td]O-10[/td]

[td]Vacuum[/td]

[td]15[/td]

[td]-[/td]

[td]20[/td]

[/tr]

[tr]

[td]KR-2L[/td]

[td]First stage[/td]

[td]2500[/td]

[td]-[/td]

[td]1842[/td]

[/tr]

[tr]

[td]S3 KS-25x4[/td]

[td]First stage[/td]

[td]3200[/td]

[td]-[/td]

[td]2844[/td]

[/tr]

[tr]

[td]PB-ION[/td]

[td]Vacuum[/td]

[td]2[/td]

[td]-[/td]

[td]2[/td]

[/tr]

[tr]

[td]Mk 55 Radial[/td]

[td]Vacuum[/td]

[td]108[/td]

[td][/td]

[td]120[/td]

[/tr]

[tr]

[td]RT-10 SRB[/td]

[td]First stage[/td]

[td]250[/td]

[td]-[/td]

[td]234[/td]

[/tr]

[tr]

[td]BACC SRB[/td]

[td]First stage[/td]

[td]315[/td]

[td]-[/td]

[td]289[/td]

[/tr]

[/table]

I took the max thrust, divided by max ISP, times min ISP :P

Update:

NathanKell has revised numbers that actually model ISP properly, both for FAR and for Stock, check out his posts to see how Kerbal engines would look if they used a more accurate depiction of ISP :)

Edited by sal_vager
Link to comment
Share on other sites
UmbralRaptor

UmbralRaptor

Posted
Posted

I like the idea.

I'd lean towards just an upper/lower stage break, given how fast you get through Kerbin's atmosphere. Also, dropping thrust at reduced pressure seems like a bad idea. (It's unrealistic unless we want to start introducing air-augmented engines)

That said, the big hole would be the KR-2L. The current Isp values make it an upper stage engine (and it's apparently a J-2 expy, despite the very high TWR).

Link to comment
Share on other sites
sal_vager

sal_vager

Posted
  • Author
Posted

The KR-2L is just a bit silly really, something like that shouldn't be so great in vacuum as it is.

As for dropping thrust, the idea is that the nozzles are designed for a specific atmospheric pressure, in vacuum some nozzle types are under expanded, so you don't get full thrust.

Just like in real life :)

Link to comment
Share on other sites
ferram4

ferram4

Posted
Posted

All rocket engines (that do not use any reaction mass from the atmosphere) have their peak Isp and thrust in vacuum, period. Even the ones that are optimized for non-vacuum, but still not SL pressures; they all have better Isp and thrust once there is no atmospheric back-pressure to fight against. There is no situation where a standard rocket engine produces more thrust at SL than in vacuum, regardless of what pressure they are designed for.

Honestly, the "less thrust in vacuum" idea is less realistic and worse than the constant thrust issue. At least with that issue, the engines sort-of follow realistic principles. With this, anything not designated as vacuum-optimized will be completely wrong.

Link to comment
Share on other sites
ferram4

ferram4

Posted
Posted

Yes, an engine in vacuum will have higher thrust, that's what I said. Your chart says otherwise, and shows engines that produce less thrust in vacuum than at SL. The expansion of the plume after the nozzle doesn't matter, it's the pressure difference at the nozzle exit that accounts for the thrust and Isp offset from the design point.

Link to comment
Share on other sites
sal_vager

sal_vager

Posted
  • Author
Posted

So engines would have the same power in vacuum that they have at sea level, unless the nozzles were over expanded robbing them of oomph below their optimum operating air pressure (vacuum) ?

So it'd look like this?

[table=width: 500]

[tr]

[td]Engine[/td]

[td]Usage[/td]

[td]Sea level[/td]

[td]Altitude[/td]

[td]Vacuum[/td]

[/tr]

[tr]

[td]LV-T30[/td]

[td]Mid stage[/td]

[td]185[/td]

[td]215[/td]

[td]215[/td]

[/tr]

[tr]

[td]LV-T45[/td]

[td]Mid stage[/td]

[td]172[/td]

[td]200[/td]

[td]200[/td]

[/tr]

[tr]

[td]LV-909[/td]

[td]Vacuum[/td]

[td]38[/td]

[td]-[/td]

[td]50[/td]

[/tr]

[tr]

[td]Poodle[/td]

[td]Vacuum[/td]

[td]152[/td]

[td]-[/td]

[td]220[/td]

[/tr]

[tr]

[td]Skipper[/td]

[td]Mid stage[/td]

[td]562[/td]

[td]650[/td]

[td]650[/td]

[/tr]

[tr]

[td]Mainsail[/td]

[td]First stage[/td]

[td]1500[/td]

[td]-[/td]

[td]1500[/td]

[/tr]

[tr]

[td]Aerospike[/td]

[td]Mid stage[/td]

[td]174[/td]

[td]175[/td]

[td]175[/td]

[/tr]

[tr]

[td]LV-N[/td]

[td]Vacuum[/td]

[td]16[/td]

[td]-[/td]

[td]60[/td]

[/tr]

[tr]

[td]24-77[/td]

[td]Vacuum[/td]

[td]16[/td]

[td]-[/td]

[td]20[/td]

[/tr]

[tr]

[td]48-7S[/td]

[td]Vacuum[/td]

[td]25[/td]

[td]-[/td]

[td]30[/td]

[/tr]

[tr]

[td]O-10[/td]

[td]Vacuum[/td]

[td]15[/td]

[td]-[/td]

[td]20[/td]

[/tr]

[tr]

[td]KR-2L[/td]

[td]First stage[/td]

[td]2500[/td]

[td]-[/td]

[td]2500[/td]

[/tr]

[tr]

[td]S3 KS-25x4[/td]

[td]First stage[/td]

[td]3200[/td]

[td]-[/td]

[td]3200[/td]

[/tr]

[tr]

[td]PB-ION[/td]

[td]Vacuum[/td]

[td]2[/td]

[td]-[/td]

[td]2[/td]

[/tr]

[tr]

[td]Mk 55 Radial[/td]

[td]Vacuum[/td]

[td]108[/td]

[td][/td]

[td]120[/td]

[/tr]

[tr]

[td]RT-10 SRB[/td]

[td]First stage[/td]

[td]250[/td]

[td]-[/td]

[td]250[/td]

[/tr]

[tr]

[td]BACC SRB[/td]

[td]First stage[/td]

[td]315[/td]

[td]-[/td]

[td]315[/td]

[/tr]

[/table]

Link to comment
Share on other sites
ferram4

ferram4

Posted
Posted

ALL engines will make more thrust in vacuum than at SL, regradless of whether they are underexpanded at SL or overexpanded at altitude. Even the SL optimized ones. Even the ones optimized for 20 atms pressure. The only difference is that SL optimized engines will not be as efficient in vacuum as the vacuum-optimized ones (obviously), but the vacuum-optimized ones will become amazingly inefficient at SL. As an example, the Poodle is making about 3 times the thrust it should be at SL if it were actually vacuum optimized; that's a little closer to a mid-to-upper range engine, I think.

Link to comment
Share on other sites
NathanKell

NathanKell

Posted
Posted

Yeah, this is also the fallacy of the aerospike. A nozzle being optimized for a given pressure (0atm all the way to supidly-high-dozens-of-atmospheres) does not mean it will produce the *most* thrust and be the most efficient at that pressure compared to all other pressures; it means that no *other* nozzle could be *more* efficient/produce more thrust at that pressure.

Think about it like this: efficiency (aka thrust per fuel unit burned) is a function of pressure, modified by an efficiency function which itself is a function of pressure. You can never have better than 100% efficiency, however, and how much less-than-100% efficiency you get is itself a function of pressure (i.e. you can't be all that inefficient in vacuum no matter how under-expanded you are, but you can get near-0% efficiency at high enough ambient pressure because of flow separation and instability).

Link to comment
Share on other sites
Kegereneku

Kegereneku

Posted
Posted

The core of the problem is that as of now the game follow an incorrect equation of the relation between thrust/ISP/pressure

The fuel consumption do not actually vary with altitude, the same amount of fuel/propellant is used (proportionally to the throttle I mean).

What is normally supposed to change is the thrust. Some rocket engine give more thrust at low altitude (higher pressure) while other do give more thrust in a vacuum.

The interest of the Aerospike is that is should be able to keep the same thrust regardless (nearly) of atmospheric pressure.

Link to comment
Share on other sites
NathanKell

NathanKell

Posted
Posted (edited) 

Engines			Thrust	Thr(SL)	Isp(SL)	Isp(V)	Gimbal
S3 KS-25x4		3200	2900	290	320	3
Kerb. KR-2L		2500	1838	250	340	4
Mainsail		1500	1422	275	290	3
Skipper			650	563	260	300	2
Poodle			220	73	120	360	3
LV-T30			215	180	260	310	0
LV-T45			200	156	250	320	3
LV-909			50	14	100	350	2
Rock 48-7S		30	24	250	310	2
LV-1			4	1	100	300	0
LV-1R			4	4	260	280	8
Rock 24-77		20	18	265	290	8
Mark 55Rad		120	108	270	300	8
Aerospike		175	156	280	315	0
LV-N			60	17	220	800	1
KR-1x2			2000	1770	270	305	3

Solid Boosters		Thrust	Thr(SL)	ISP(SL)	ISP(V)	
SRB KD25K		650	613	245	260	
BACC			315	290	230	250	
RT-10 SRB		250	234	225	240	
Sepratron 1		18	16	100	115

Note that these Isps are for use with FAR; if you're playing with the souposphere, you probably should bump them back up by about 10% (and then claim they're some kind of exotics, rather than standard nitrogen/hydrazine-based storables, for which these Isps make sense).

That should make upper stage engines of limited use on the ground, but very helpful up high, give boosters a solid role (that was denied in .24/25 with the Mainsail buff), and let you use the radial engines as verniers (note high gimbal) so you can have gimbal roll control now that Harv made the gimbal work with it in .24+

Obviously how each particular engine is given a role will depend on taste; but you can see some examples:

1. Early engines have low Isp

2. Vacuum optimized engines have *crap* for sea level Isp; this is more than pure math would suggest, since they endure flow separation at sea level.

3. Even "sea level optimized" boosters do better in vacuum, though as the tech increases, the divergence between sea level and vacuum Isp decreases (since sea level Isp is highly dependent on chamber pressure but increasing chamber pressure doesn't help much in vacuum).

4. The aerospike is "just another engine" -- it has decent performance everywhere, but in any *given* area is worse than an optimized nozzle, and also worse than the super-high-tech x4 engine, which I have taken to be the SSME.

Edited by NathanKell
Link to comment
Share on other sites
Laie

Laie

Posted
Posted
The KR-2L is just a bit silly really, something like that shouldn't be so great in vacuum as it is.

That's solely a game balance issue. There's no technical reason why good ISP has to come with low thrust. And the OP-ness is mostly due to TWR, really (66% more powerful than a Mainsail at nearly the same weight). It's ISP, though better than a Skipper or LV-T30, is still short of the Poodle and far short of Nukes. And frankly, I would use the KR-2L much less if I could get 5000 Newtons from Nukes without driving the part count to insane levels.

1. Early engines have low Isp

Cute idea, but just look at the Jool-5 thread for why the LVT30 is indispensable even in late-game missions. If the LVT30 is nerfed beyond recognition, we will need an LVT33 somewhere higher in the tech tree.

Generally, I don't favor ISP trutherism. Designing an Eve lifter is difficult enough even if you don't have to worry about variable thrust.

Link to comment
Share on other sites
Duxwing

Duxwing

Posted
Posted
...5000 Newtons from Nukes without driving the part count to insane levels.

Also consider the dry mass: 2.25 tons each, LV-Ns greatly reduce dV when excessively-many.

-Duxwing

PS I think you mean 50,000 Newtons or 5,000 Kilo-Newtons, which are KSP's thrust unit.

Link to comment
Share on other sites
NathanKell

NathanKell

Posted
Posted (edited)

Added info. Here's all-stock stats, just thrust corrected.


Engine			Mass	Thrust	SL Thr.		IspSL/V
LV-1/LV-1R		0.03	4	3		220	290
24-77			0.09	20	17		250	300
48-7S			0.10	30	26		300	350
LV-909			0.50	50	38		300	390
LV-N			2.25	60	17		220	800
Mark 55 Radial		0.90	120	109		290	320
Aerospike		1.50	175	174		388	390
RAPIER (Rocket)		1.20	175	156		320	360
LV-T45			1.50	200	173		320	370
LV-T30			1.25	215	186		320	370
Poodle			2.00	220	152		270	390
Skipper			3.00	650	562		320	370
Mainsail		6.00	1500	1333		320	360
LFB KR-1x2		6.00	2000	1706		290	340
Kerbodyne KR-2L		6.50	2500	1842		280	380
S3 KS-25x4		9.75	3200	2844		320	360

And finally, assuming all engines are on an equal-tech-level playing field, here is my quick rebalance (keeping mass as a given):

Engine			Mass	Thr	ThrSL	IspSL/V		Gimbal
LV-1			0.03	4.0	1.1	100	350	0
LV-1R			0.03	3.0	1.6	200	370	8
24-77			0.09	10.0	9	310	345	8
48-7S			0.1	12.0	6.3	200	380	2
LV-909			0.5	60.0	23.1	150	390	3
Mark 55 Radial		0.9	140.0	133.8	325	340	8
Aerospike		1.5	175.0	150	330	385	0
RAPIER (rocket)		1.2	175.0	160	320	350	2
LV-T45			1.5	200.0	177.8	320	360	4
LV-T30			1.25	215.0	193.8	320	355	0
Poodle			2	210.0	51.2	100	410	4
Skipper			3	440.0	289.5	250	380	2
Mainsail		6	1000.0	929.6	330	355	2
LFB KR-1x2		6	950.0	868.6	320	350	1.5
Kerbodyne KR-2L		6.5	850.0	425	200	400	2
S3 KS-25x4		9.75	1800.0	1747.8	335	350	1

Edited by NathanKell
Link to comment
Share on other sites
11of10

11of10

Posted
Posted

A bit of a side question, perhaps, but:

I think I picked up somewhere that rocket engines optimised for atmospheric work (IE: F-1) tend to malfunction/explode in vacuum?

Doesn't make much sense considering that first stage of Saturn went high up and kept working, but can someone say something about this?

Link to comment
Share on other sites
This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
 Share
Go to topic listing
×
×
  • Create New...