Terminal Velocity vs. Burning Low ISP Fuel First?

[arkie87]

Let's say you have a craft like this with FAR aerodynamics (aerodynamic disassebly is disabled), such that if you begin thrusting with only the two boosters, you will be below terminal velocity, do you want to ignite the liquid engine? Liquid engine ISP is higher than SRB's, but I've heard that it's better to burn lower ISP fuel first.

Which effect is worse on deltaV? Being below terminal velocity or burning low ISP and high ISP fuel together?

EDIT: Answer:

I believe the intent of that rule of thumb isn't "burn all your low-Isp fuels before you burn any higher-Isp ones," it's "burn off your lower-Isp fuels at the first opportunity."

FURTHER EDIT: Actually, the above answer is only correct if one is not allowed to drop the SRB's; if one is, then it should've been obvious that dropping the SRB's before turning on LFE is more efficient since LFE will be pushing drastically less dry mass, as pointed out by Starman:

if you don't start your LFE until after staging the SRBs, you get more dV, because you aren't spending any of your liquid fuel hauling the SRBs or their fuel. There's a good chance that will outweigh any savings from reducing gravity drag by going full-thrust from the start.

Edited December 18, 2014 by arkie87

[WanderingKid]

I can't speak for FAR, but I can speak in generalities. My understanding is that you are typically better off burning to near terminal velocity than you are saving gas for later unless the Isp is significantly different. The general idea is that the faster you are out of the soup, the better off everything is. The caveat is that you have to judge it for yourself. 5% under Terminal isn't worth firing off an engine with 20% Isp if you wait, but 10%-10% is certainly worth it, simply for savings later.

However, the flipside is "Really, for 50m/s?". How tight is your build to your mission?

[arkie87]

I can't speak for FAR, but I can speak in generalities. My understanding is that you are typically better off burning to near terminal velocity than you are saving gas for later unless the Isp is significantly different. The general idea is that the faster you are out of the soup, the better off everything is. The caveat is that you have to judge it for yourself. 5% under Terminal isn't worth firing off an engine with 20% Isp if you wait, but 10%-10% is certainly worth it, simply for savings later.

However, the flipside is "Really, for 50m/s?". How tight is your build to your mission?

Interesting. I would think in stock, since terminal velocity is so painfully low, you really really want to be going at it. In FAR, since terminal velocity is so high anyway, the gain in ISP might be much more significant, so you might want to keep it for later....

[WanderingKid]

You lose so much to Terminal Velocity that you typically will throttle down to stay in range in stock. As I mentioned, though... FAR is outside my experience. You'll have to launch the same mission a few times and judge for yourself.

[Starman4308]

The practical answer is "2.0 is already way too much TWR to have for more than ~30 seconds: throttle down those SRBs some".

The semi-practical answer is "You lose so little to aero drag in FAR that you probably want to leave the LFEs alone".

The theoretical answer is "Run simulations to get your answer, because it is one of those questions with the most evil answer ever: 'it depends​'".

[The_Rocketeer]

It's better to burn lower ISP fuel first because it's lower value - it contains less thrust per mass. While you're carrying it you're flying a heavier ship with less efficient fuel. If you burn your LFE at the same time, you're wasting high value fuel to lug your SRBs and their fuel up to a higher velocity for no reason.

Save the LF until you're flying a lighter ship and only accelerating the mass of what you actually want to go fast.

[magnemoe]

Using mechjeb launch autopilot, it typically burn in the beginning to get up to terminal speed, after this it reduces trust until off, it then stay off until the SRB drops as you TWR grows faster then the terminal speed, For the rocket above this should be the case too, don't use far.

Anyway the dry mass of the two SRB is so high it would be pointless to use the small core to push it and you should get 4g or more as it start to run dry.

[arkie87]

The practical answer is "2.0 is already way too much TWR to have for more than ~30 seconds: throttle down those SRBs some".

The semi-practical answer is "You lose so little to aero drag in FAR that you probably want to leave the LFEs alone".

The theoretical answer is "Run simulations to get your answer, because it is one of those questions with the most evil answer ever: 'it depends​'".

You think its more fuel efficient to throttle SRB's down? If I had a mainsail, would you tell me I should throttle it down to a skipper even though i already have the extra mass?

- - - Updated - - -

Using mechjeb launch autopilot, it typically burn in the beginning to get up to terminal speed, after this it reduces trust until off, it then stay off until the SRB drops as you TWR grows faster then the terminal speed, For the rocket above this should be the case too, don't use far.

Anyway the dry mass of the two SRB is so high it would be pointless to use the small core to push it and you should get 4g or more as it start to run dry.

I'm using FAR though...?

Edited December 17, 2014 by arkie87

[Kerbart]

(...)but I've heard that it's better to burn lower ISP fuel first.

I would think it's better to burn any fuel first; the less mass you have to carry upward the better. As long as you're not exceeding terminal velocity burn anything you can.

Now, if you are exceeding terminal velocity you still want to shed mass as quickly as possible and those empty booster do weigh something. So in order to get rid of the boosters ASAP run those first and then light the LFE when terminal velocity < 100% (or when you drop the boosters, obviously). That's what I would do.

(...)However, the flipside is "Really, for 50m/s?". How tight is your build to your mission?

When Robert Rodriguez made El Mariachi for $7000 he went through great pains to deal with the fact that two key props (guitar cases) had different colors but were supposed to look the same. “Of course the whole problem could have been solved painting the brown case black. But then I would have had to buy black paint. And that's how these Hollywood productions go way over budgetâ€Â

Being a real tight-ass on your DV requirements can make for massive reductions at launch time. Willing to cut your reserves down to nearly nothing goes a long way. I'm way too leisurely for that, and too lazy to plan my missions that tight, so I usually end up with these Whackjobian monstrosities on the launch pad. But the few times I did run these white-knuckle missions I was amazed at how much lighter and smaller things can be. Who needs 2.5m tanks, anyway?.

[Kerbart]

(...)but I've heard that it's better to burn lower ISP fuel first.

I would think it's better to burn any fuel first; the less mass you have to carry upward the better. As long as you're not exceeding terminal velocity burn anything you can.

Now, if you are exceeding terminal velocity you still want to shed mass as quickly as possible and those empty booster do weigh something. So in order to get rid of the boosters ASAP run those first and then light the LFE when terminal velocity < 100% (or when you drop the boosters, obviously). That's what I would do.

(...)However, the flipside is "Really, for 50m/s?". How tight is your build to your mission?

When Robert Rodriguez made El Mariachi for $7000 he went through great pains to deal with the fact that two key props (guitar cases) had different colors but were supposed to look the same. “Of course the whole problem could have been solved painting the brown case black. But then I would have had to buy black paint. And that's how these Hollywood productions go way over budgetâ€Â

Being a real tight-ass on your DV requirements can make for massive reductions at launch time. Willing to cut your reserves down to nearly nothing goes a long way. I'm way too leisurely for that, and too lazy to plan my missions that tight, so I usually end up with these Whackjobian monstrosities on the launch pad. But the few times I did run these white-knuckle missions I was amazed at how much lighter and smaller things can be. Who needs 2.5m tanks, anyway?.

[thereaverofdarkness]

Looks like you've got a TWR of 2.45 with just the two boosters. That's a bit high. I try not to launch above 2.2. I don't have much experience with FAR and how that pans out for desired TWR--higher might be fully acceptable (unless you have deadly reentry), but lower is supposed to be fine, too. I've heard tell that 1.5 TWR is all you need to launch a streamlined rocket with FAR.

[Godot]

You think its more fuel efficient to throttle SRB's down? If I had a mainsail, would you tell me I should throttle it down to a skipper even though i already have the extra mass?

...

The answer to both questions is:

It depends.

If your TWR in the thick parts of the atmosphere (< 10km or even < 5 km) results in your spaceship accelerating way over the Terminal Velocity, then yes, it may be more efficient to theottle down your Mainsail to a skipper (at least until you are out of the thick parts of the atmosphere).

Similarly with SRBs, which also, if you throttle them down, will generate theor thrust over a longer time.

On the other hand:

If your Mainsail will be kept throttlesd down to a Skippers TWR for most of the time (that it is connected to your ship) then is may be better to use a Skipper instead.

Similarly, if your SRBs produce so much thrust, that you have to reduce their thrust to a very low value in order to not speed over Terminal Velocity, it may be better to use less SRBs (and/or rather invest a few more tons into fuel for your LFEs)

[arkie87]

The answer to both questions is:

It depends.

If your TWR in the thick parts of the atmosphere (< 10km or even < 5 km) results in your spaceship accelerating way over the Terminal Velocity, then yes, it may be more efficient to theottle down your Mainsail to a skipper (at least until you are out of the thick parts of the atmosphere).

Similarly with SRBs, which also, if you throttle them down, will generate theor thrust over a longer time.

100% agree. But for this case, i have explicitly stated that I am below terminal velocity (this was specified in the OP--emphasis added):

Let's say you have a craft like this with FAR aerodynamics (aerodynamic disassebly is disabled), such that if you begin thrusting with only the two boosters, you will be below terminal velocity, do you want to ignite the liquid engine? Liquid engine ISP is higher than SRB's, but I've heard that it's better to burn lower ISP fuel first.

Which effect is worse on deltaV? Being below terminal velocity or burning low ISP and high ISP fuel together?

http://i.imgur.com/mDKAFUW.jpg

On the other hand:

If your Mainsail will be kept throttlesd down to a Skippers TWR for most of the time (that it is connected to your ship) then is may be better to use a Skipper instead.

It almost certainly will, but that is not what was suggested. What was suggested was limiting thrust, even though the additional thrust/TWR comes at no additional mass/deltaV penalty since the heavier engine is already ON the ship (and since it is below terminal velocity).

Similarly, if your SRBs produce so much thrust, that you have to reduce their thrust to a very low value in order to not speed over Terminal Velocity, it may be better to use less SRBs (and/or rather invest a few more tons into fuel for your LFEs)

From your answer here, it seems like you agree that if you are below terminal velocity, you do not want to limit thrust...

Edited December 17, 2014 by arkie87

[numerobis]

The "match terminal velocity" heuristic is the answer to the Goddard problem: how to design a rocket that flies as high as possible when fired straight up on a body with atmosphere.

I haven't seen a good analytical treatment of how things change when you aren't pointing straight up: when should you pitch over, how fast should you go, etc.

[magnemoe]

You think its more fuel efficient to throttle SRB's down? If I had a mainsail, would you tell me I should throttle it down to a skipper even though i already have the extra mass?

- - - Updated - - -

I'm using FAR though...?

In stock an SRB TWR of 2 or more will send you into terminal speed very hard as it uses up its fuel, with far is a less problem as terminal speed is higher, you have also reduced the trust of them a lot so they will burn long. What is your trust level on them?

Bringing up the second issue, you have an good TWR, your SRB is most of the mass of the rocket it looks like the upper stages also have lots of TWR so I would waited.

Many real world designs including the space shuttle and ariane 5 has an TWR of less than 1 without the SRB, same with my workhorse in KSP 0.25, it used an skipper and four large SRB, two had trust on 70% and was dropped later, at this point the core TWR was around 1.5, it burned at launch then after dropping two SRB.

I'm a bit curious about the design. The boosters huge who is an good thing in it self, but why two small stages and a T30 or T45 on upper stage, had been smarter to just make the core longer as it would still have good enough TWR, optionally use an 909 for the upper stage.

[thereaverofdarkness]

From your answer here, it seems like you agree that if you are below terminal velocity, you do not want to limit thrust...

Terminal velocity is the optimum ascent velocity for fuel efficiency and it is especially important that you remain near it when below 10km altitude. It is better to limit your thrust than to go way over terminal velocity, but it is better still to use a less powerful yet lighter and/or more efficient engine if that one has enough thrust.

[pincushionman]

I believe the intent of that rule of thumb isn't "burn all your low-Isp fuels before you burn any higher-Isp ones," it's "burn off your lower-Isp fuels at the first opportunity."

[arkie87]

I believe the intent of that rule of thumb isn't "burn all your low-Isp fuels before you burn any higher-Isp ones," it's "burn off your lower-Isp fuels at the first opportunity."

Thank you for actually addressing and answering the question I asked.

So it seems like firing up the liquid engine during ascent will be beneficial... Thanks!

[arkie87]

The "match terminal velocity" heuristic is the answer to the Goddard problem: how to design a rocket that flies as high as possible when fired straight up on a body with atmosphere.

I haven't seen a good analytical treatment of how things change when you aren't pointing straight up: when should you pitch over, how fast should you go, etc.

Yes, exactly. i might try to think about this... but i suspect there is no analytical answer i.e. it requires numerical simulation (for which, i have already made one in Matlab-- i assume you snicker whenever you hear matlab, since you studied scientific computing hehehe)...

[arkie87]

Terminal velocity is the optimum ascent velocity for fuel efficiency and it is especially important that you remain near it when below 10km altitude. It is better to limit your thrust than to go way over terminal velocity, but it is better still to use a less powerful yet lighter and/or more efficient engine if that one has enough thrust.

Not sure what you are trying to add that wasnt already said by me or someone else, but i think it seems like you are agreeing with me, so i'll take that...

Anyway, the first two points made by Starman are flawed:

(1) Reducing thrust when below terminal velocity is not optimal

(2) If the drag is so low in FAR, then that is a reason to TURN ON the LFE, not leave it off...

The practical answer is "2.0 is already way too much TWR to have for more than ~30 seconds: throttle down those SRBs some".

The semi-practical answer is "You lose so little to aero drag in FAR that you probably want to leave the LFEs alone".

The theoretical answer is "Run simulations to get your answer, because it is one of those questions with the most evil answer ever: 'it depends​'".

Edited December 17, 2014 by arkie87

[Starman4308]

You think its more fuel efficient to throttle SRB's down? If I had a mainsail, would you tell me I should throttle it down to a skipper even though i already have the extra mass?

Yes, because if you try launching with an overly-high-thrust rocket in FAR, you'll have a lot of difficulty making your gravity turn. Even 1.6 is difficult to manage: 2.0 would be a nightmare. You'd go shooting off to a ridiculous apoapsis without having gotten much horizontal velocity.

EDIT: In short, FAR's changes to aerodynamics aren't just drag: it also brings in various other questions of aerodynamics, and those limit TWR as well.

Edited December 17, 2014 by Starman4308

[The_Rocketeer]

Arkie isn't making a gravity turn. He's ascending vertically to test whether it's ever better than a gravity turn.

If you try want to tell him it's not, bring proof. All encompassing proof.

[arkie87]

Yes, because if you try launching with an overly-high-thrust rocket in FAR, you'll have a lot of difficulty making your gravity turn. Even 1.6 is difficult to manage: 2.0 would be a nightmare. You'd go shooting off to a ridiculous apoapsis without having gotten much horizontal velocity.

EDIT: In short, FAR's changes to aerodynamics aren't just drag: it also brings in various other questions of aerodynamics, and those limit TWR as well.

That is a factor as well, but I am approaching from a theoretical standpoint, not necessarily a practical one.

I am wondering if maybe the science labs are a better home for my threads... since i am more interested in the theory than the practice

Speaking of which, i was wondering what about FAR's change in aerodynamics makes rockets unstable. I'd prefer as specific/detailed reason as possible, not just something vague like it changes the aerodynamics.

I imagine it has something to do with the changing lift and drag coefficients vs. AoA, but i cannot figure out how this would make the vehicle flip around, rather than get forced back to pro-grade direction...

Edited December 17, 2014 by arkie87

[Starman4308]

Speaking of which, i was wondering what about FAR's change in aerodynamics makes rockets unstable. I'd prefer as specific/detailed reason as possible, not just something vague like it changes the aerodynamics.

I suspect what is going on in FAR is that, once you start getting some AoA, you start to get body lift: the entire rocket starts acting as a wing, causing your CoP to go far forwards. Once CoP is in front of CoM, all sorts of fun things start happening. In stock, since all drag is proportional to mass, etc, drag remains centered on CoM, so you never get these problems.

EDIT: Also, 95% sure what I was thinking on part #2 of my initial post was that, if you don't start your LFE until after staging the SRBs, you get more dV, because you aren't spending any of your liquid fuel hauling the SRBs or their fuel. There's a good chance that will outweigh any savings from reducing gravity drag by going full-thrust from the start. I also tested the body lift thing: when you use FAR to tint lift, you start getting a lot of tint on your fuel tanks if you tilt your rocket at high velocity.

Edited December 17, 2014 by Starman4308

[arkie87]

I suspect what is going on in FAR is that, once you start getting some AoA, you start to get body lift: the entire rocket starts acting as a wing, causing your CoP to go far forwards. Once CoP is in front of CoM, all sorts of fun things start happening. In stock, since all drag is proportional to mass, etc, drag remains centered on CoM, so you never get these problems.

I actually PM'ed Ferram and he said something along those lines, though i still dont quite understand it... it has something to do with high AoA's being non-linear...

EDIT: Also, 95% sure what I was thinking on part #2 of my initial post was that, if you don't start your LFE until after staging the SRBs, you get more dV, because you aren't spending any of your liquid fuel hauling the SRBs or their fuel. There's a good chance that will outweigh any savings from reducing gravity drag by going full-thrust from the start. I also tested the body lift thing: when you use FAR to tint lift, you start getting a lot of tint on your fuel tanks if you tilt your rocket at high velocity.

I actually tested this just now, and i think you are right-- when i burned with LFE simultaneously, i definitely did not have more deltaV.

I suppose it was a stupid question since dropping the giant masses of the humongous SRB's before burning should be seemingly obvious, since it will be pushing less mass.

However, pretending I wasnt allowed to drop those, I wonder if the quoted answer in the OP is correct...

I suppose your answer #2 was a bit vague and i interpreted the logic/point incorrectly. sorry for that.

However, you do admit your #1 point was misinformed I hear lot's of people on the forums say that and I think its a common misconception

The practical answer is "2.0 is already way too much TWR to have for more than ~30 seconds: throttle down those SRBs some".

The semi-practical answer is "You lose so little to aero drag in FAR that you probably want to leave the LFEs alone".

The theoretical answer is "Run simulations to get your answer, because it is one of those questions with the most evil answer ever: 'it depends​'".

Edited December 18, 2014 by arkie87