Jump to content

KSP engine's "Thrust Limiter" analog in real life


Recommended Posts

Hi there! It's me again with another of those uneducated questions of mine!

My knowledge of rocket engines is nearly zero (I know they exist, and that's pretty much it), hence the question.

I see KSP (at least under Realism Overhaul) allows you to limit the thrust on any engine - so let's say you are looking at a Russian RD-120 that produces 1500 kN by default; using this functionality you can arbitrarily reduce the engine's thrust to whatever you want. What would be the real life analog to this? Is this a concession the game makes oriented to better gameplay or would this actually be possible? I mean - I know thrust is a function of chamber pressure and propellants used (among other things but I'm guessing these 2 are the main items?) so my common sense tells me this "Thrust Limit" in-game functionality would have no place in real life without actually modifying the engine - is this the case?

Many thanks in advance, as always I gloat in your knowledge of science and space related stuff :)

Edited by hypervelocity
bad englando again
Link to comment
Share on other sites

Your right, real engines cannot arbitrarily reduce their thrust.  Most engines can throttle a little bit, like +-5% I think, but it takes special engine designs to get higher than that.  The Apollo landing engine was unique in its ability to throttle to 11% I think, and Be4 and Merlin can throttle down to 40% and 70% if I remember correctly.

Link to comment
Share on other sites

Thanks @ment18, that's what I thought - I noticed the throttling ability in the engines you mentioned (I have used the Apollo descent engine and a couple of Merlins). The game distinguishes between throttling and "thrust limiting" by whether it enables you to do it while in game or in the VAB only. Noticing how almost every engine is unthrottleable made me look wary at this thrust limiter option.

Thanks for your answer.

Link to comment
Share on other sites

The RS-25 (Shuttle main engine) was capable of 67%-109% throttle levels.  The Merlin 1D (Falcon 9) can throttle between 40% and 100%.

1 hour ago, ment18 said:

Your right, real engines cannot arbitrarily reduce their thrust.  Most engines can throttle a little bit, like +-5% I think, but it takes special engine designs to get higher than that.  The Apollo landing engine was unique in its ability to throttle to 11% I think, and Be4 and Merlin can throttle down to 40% and 70% if I remember correctly.

 

Link to comment
Share on other sites

1 hour ago, hypervelocity said:

The game distinguishes between throttling and "thrust limiting" by whether it enables you to do it while in game or in the VAB only. Noticing how almost every engine is unthrottleable made me look wary at this thrust limiter option.

I use VAB thrust limiter almost exclusively for solid boosters.

On those rare occasions where I use them for solids, it's to either balance an asparagus design, or to limit upper stage to something reasonable. I consider this to be fairly realistic, since I imagine one can design engines to suit specific launch needs.

In-game thrust setting is obviously cheaty, but it's a gameplay compromise. KSP is first and foremost a game and should be fun. It's not a simulator and if it needs a deep throttle setting so as not to drive players nuts, so be it. Hardcore players can still choose to go either 100% or 0% to stay (more) realistic, but for casual players, fine control is helpful.

One thing to consider:

Falcon 9 can throttle its Merlin engines down to 70%, but it has 9 of them, so by using only one engine it can effectively reduce the total thrust all the way down to 70/9~8% of total thrust.

Link to comment
Share on other sites

48 minutes ago, DAL59 said:

Why not define 109% as 100%?

That's 109% of it's rated power.  

More so, the initial engine was certified at 100% the rated power output.  But as the engine evolved, it was eventually certified to 109% of rated output.

Quote

Specifying power levels over 100% may seem nonsensical, but there was a logic behind it. The 100% level does not mean the maximum physical power level attainable, rather it was a specification decided on during engine development—the expected rated power level. When later studies indicated the engine could operate safely at levels above 100%, these higher levels became standard. Maintaining the original relationship of power level to physical thrust helps reduce confusion, as it created an unvarying fixed relationship so that test data (or operational data from past or future missions) can be easily compared. If the power level was increased, and that new value was said to be 100%, then all previous data and documentation would either require changing, or cross-checking against what physical thrust corresponded to 100% power level on that date.[12] Engine power level affects engine reliability, with studies indicating the probability of an engine failure increasing rapidly with power levels over 104.5%, which was why power levels above 104.5% were retained for contingency use only.[31]

 

Edited by Gargamel
Link to comment
Share on other sites

19 hours ago, Gargamel said:

The RS-25 (Shuttle main engine) was capable of 67%-109% throttle levels.  The Merlin 1D (Falcon 9) can throttle between 40% and 100%.

Isn't Merlin 1D more than 200% of Merlin 1A?  I don't think it is 40% to 200%, but I have to wonder how much bigger they have to make the first stage merely to keep the hoverslam possible with the more powerful engines.

Don't forget that  changing the thrust of solid rockets is far more likely in real life than throttling liquid rockets.  You can't change them in flight, but you can manufacture the rocket to give higher/lower thrust as it burns through the fuel.
https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster

Link to comment
Share on other sites

Thanks for all the replies.

For a bit of context: I am creating a spaceplane that is inspired in Boeing's Phantom Express. The thing is I want to make it a SSTO, hence using a Raptor engine instead of a RS-25. The Raptor produces roughly 3000 kN of thrust, which would yield a TWR of over 75 when the craft is dry. What I thought was I could use 2 engines instead of one, limiting their throttle using the (now declared) unrealisitic KSP feature, and furthermore using the engine's throttling capabilities to adjust thrust on the go.

This plan is no longer pursued given what was discussed in this thread. I need to find a more suitable engine to avoid liquefying my payloads (I am fully aware some payloads can handle 100's or even 1000's of G-loads, but I ultimately want to rate the craft for crewed missions).

Thanks everyone for the involvement in the discussion.

Link to comment
Share on other sites

2 hours ago, hypervelocity said:

Thanks for all the replies.

For a bit of context: I am creating a spaceplane that is inspired in Boeing's Phantom Express. The thing is I want to make it a SSTO, hence using a Raptor engine instead of a RS-25. The Raptor produces roughly 3000 kN of thrust, which would yield a TWR of over 75 when the craft is dry. What I thought was I could use 2 engines instead of one, limiting their throttle using the (now declared) unrealisitic KSP feature, and furthermore using the engine's throttling capabilities to adjust thrust on the go.

This plan is no longer pursued given what was discussed in this thread. I need to find a more suitable engine to avoid liquefying my payloads (I am fully aware some payloads can handle 100's or even 1000's of G-loads, but I ultimately want to rate the craft for crewed missions).

Thanks everyone for the involvement in the discussion.

I'd recommend using a main booster engine and a pair of smaller engines more suitable for finer control, like the shuttle OMS engines or the vernier thrusters on an ICBM. If you use a suitably powerful pair of verniers, you could even shut down the"booster" engine as G-loading approaches your imposed limit and finish your push on the verniers.

Link to comment
Share on other sites

1 hour ago, MaverickSawyer said:

I'd recommend using a main booster engine and a pair of smaller engines more suitable for finer control, like the shuttle OMS engines or the vernier thrusters on an ICBM. If you use a suitably powerful pair of verniers, you could even shut down the"booster" engine as G-loading approaches your imposed limit and finish your push on the verniers.

OMS uses other fuel but agree, note you could also have only the small engines gimbal like on Soyuz, this would probably negate much of the weight of two engine types. 

And engines has an minimum trust, below this the engine might cut off or even blow up if fuel and oxidizer suddenly ignite again.
Its also technical issues like control of valves so you get the correct mix at all throttle settings, and for an first stage you don't really need it much, you might want to reduce trust to reduce max q or max g then stage start running dry but if you have lots of engines you can shut some down as done on saturn 5 who as I understand could not throttle first stage. 

Now throttle has some benefits, you use SRB or an heavy configuration (delta or falcon), you also has various payloads, some might be lightweight but fragile like an space telescope, other max out your capacity. 
With SRB or heavy you want all engines running before liftoff, however you want main stage to throttle down and let the boosters do the work as long as your twr is good enough. 
You also want to limit your trust on first stage burn out if you carry something light and fragile 

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.

×
×
  • Create New...