Terminal Velocity vs. Burning Low ISP Fuel First?

[Starman4308]

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...

In terms of pure delta-V available, you still should burn the SRBs first. Otherwise, you burn efficient liquid fuel hauling around inefficient solid fuel. In the presence of atmo and gravity drag, it returns to a question of "it depends", because the solution involves the standard batch of partial differential equations.

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

I think I might've been in a rush to catch a bus or something. I apologize for the vague wording.

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

I don't, at least not from a practical standpoint of "actual FAR launches". The #1 point is based on gameplay experience of trying to wrestle 1.6 TWR rockets down onto a proper gravity turn instead of shooting off vertically to space. A 2.0 TWR is simply too much for an efficient launch in FAR: you lose too much going straight up against gravity, and not enough going horizontal, and that outweighs any savings you might gain from the quicker acceleration.

The reason a lot of people say it is because it's true: high TWR rockets are nightmares to handle in FAR, and real rockets shy away from high launch TWR* because they would tear themselves apart from aerodynamic stress: they have to be designed for max-Q. That doesn't really happen in FAR, probably because Ferram gets too many complaints from inept designers already, so you need to really try to get aerodynamic failure in FAR.

*Other than a few which use SRBs for the first 30 seconds or so. Then they go back down to reasonable TWR.

EDIT: Unless what you meant was "ignore, ignore, ignore the practical issues", in which case it returns to the #1 point: does the dV increase from not lighting up the LFE until after SRB burnout counteract reduced gravity/atmo drag? I don't know, because I don't have any quick simulators handy.

Edited December 18, 2014 by Starman4308

[arkie87]

I don't, at least not from a practical standpoint of "actual FAR launches". The #1 point is based on gameplay experience of trying to wrestle 1.6 TWR rockets down onto a proper gravity turn instead of shooting off vertically to space. A 2.0 TWR is simply too much for an efficient launch in FAR: you lose too much going straight up against gravity, and not enough going horizontal, and that outweighs any savings you might gain from the quicker acceleration.

The reason a lot of people say it is because it's true: high TWR rockets are nightmares to handle in FAR, and real rockets shy away from high launch TWR* because they would tear themselves apart from aerodynamic stress: they have to be designed for max-Q. That doesn't really happen in FAR, probably because Ferram gets too many complaints from inept designers already, so you need to really try to get aerodynamic failure in FAR.

*Other than a few which use SRBs for the first 30 seconds or so. Then they go back down to reasonable TWR.

EDIT: Unless what you meant was "ignore, ignore, ignore the practical issues", in which case it returns to #2: does the dV increase from not lighting up the LFE until after SRB burnout counteract reduced gravity/atmo drag? I don't know, because I don't have any quick simulators handy.

I fully understand this, but i specified in the problem statement in the OP that aerodynamic disassembly feature of FAR is disabled (though perhaps i should have highlighted this fact a bit more). People, who use stock aerodynamics, have mocked my disabling of this feature because it's unrealistic, which for obvious reasons, is ironic. I like FAR with aerodynamic dis-assembly disabled because it gives spaceplanes more ability to actually control where they go, and the ability to bounce off the atmosphere during re-entry i.e. the control over where you re-enter and how steeply. However, since I'm using a keyboard, and not a joystick, it is wayyyyyyy too easy to increase G load wayyyyy too fast even with a quick tap. Even if i play with quicksaves, it still a huge pain.... thus FAR with aerodynamic dis-assembly disabled is more realistic than stock, but less than FAR. Alternatively, it's like playing with NEAR but with Mach effects...

Thus, i was wondering if aerodynamic dis-assemblies arent a problem, what optimum TWR for FAR was (which was discussed in another thread), and I arrived at this craft...

I can test this rocket with dis-assembly enabled and see if it would have been a problem. If it is, then I obviously,reducing thrust on SRB's even is beneficial. Furthermore, I and 5thHorseman have tested getting it into circular orbit around Kerbin and it seems performing a somewhat good gravity turn with apoapsis around 200 km is possible. I can try reducing thrust to see if that makes performing the gravity turn easier, and therefore, saves fuel, despite the wasted atmospheric efficiency (velocity/terminal-velocity). Props to Ferram on being able to calibrate his correlations so accurately that they force players to match real-world rocket TWR pretty well.

[Starman4308]

I fully understand this, but i specified in the problem statement in the OP that aerodynamic disassembly feature of FAR is disabled (though perhaps i should have highlighted this fact a bit more).

I've got to go soon, but two things. First, were you trying the rocket in FAR or stock aero? Second, FAR doesn't have to destroy your rocket to cause problems: if it causes your rocket to flip over, you're just as doomed.

[arkie87]

I've got to go soon, but two things. First, were you trying the rocket in FAR or stock aero? Second, FAR doesn't have to destroy your rocket to cause problems: if it causes your rocket to flip over, you're just as doomed.

I was using FAR with aerodynamic disassembly disabled (or equivalently, NEAR with Mach effects).

Right. But that can happen just the same with a "proper" (i.e. low) TWR.

[GoSlash27]

IRT the OP,

I've been thinking a lot along these lines over the course of the last few days, and it has led me to some counter-intuitive conclusions on this matter...

Perversely, burning low Isp fuels is beneficial to saving DV in the early stages, particularly in low-drag scenarios.

This is because burning low Isp fuel sheds mass faster, thus allowing your thrust to weight ratio to increase more rapidly. BUT

Saving DV is not necessarily beneficial to the efficiency of a launch.

This is because what you really need to be saving is fuel, not DV. Saving DV doesn't do you any favors if you needed a larger, more expensive, and costly booster to get the same job done. It doesn't matter how much DV a stage wastes so long as it has adequate DV to do it's work and is still lighter and cheaper.

The mass of the first stage of your rocket is irrelevant in the general scheme of things, since there's no stage that's got to lift it. It's just got to be cheap and do the job.

Subsequent stages need to be as light and cheap as possible in order to minimize the workload on previous stages and budget. Again, we don't care about how much DV it saves, just that it has enough DV to work.

Exception here is where saving DV *does* save you mass and overall cost. For instance, if you have a low enough thrust to weight ratio, the penalty from gravity losses during a launch may raise your DV budget far enough that the additional mass of fuel and tanks to meet it winds up eliminating the initial advantages you picked up with the lower t/w vehicle. This is a thing on airless bodies and I'm just now delving into that.

Once gravity losses are no longer a concern, it's a different story.

This is all thought process, not established fact. So caveat emptor, etc.

Best,

-Slashy

[Starman4308]

I was using FAR with aerodynamic disassembly disabled (or equivalently, NEAR with Mach effects).

Right. But that can happen just the same with a "proper" (i.e. low) TWR.

The primary point is that, for a typical ascent, a 1.4 TWR rocket is going to find it much easier to make a proper gravity turn without flipping out, whereas a 2.0 TWR rocket is going to go shooting off for some silly apoapsis (200km, say) and waste a lot of its dV in the process: this is because the overpowered rocket would flip out if you tried to make it follow the same gravity turn. This is in addition to the fact that, in rocket design, going with a lower TWR design is going to help your overall dV, because less TWR needs less engine mass, and less engine mass means more dV. It's inapplicable to this scenario, since we're given a rather overpowered, mostly SRB-driven rocket, but to sane designs, adding engine mass is going to hurt.