[1.3.1] Ferram Aerospace Research: v0.15.9.1 "Liepmann" 4/2/18

[jrandom]

If stability is not an issue for you, you can pretty much throttle up as much as you want. If you use stock engines (or stock-alike ones) in terms of ISP, that usually is the most DeltaV-saving ascent.

If your rocket tends to flip/lose control at a certain altitude, this is usually caused by you going too fast at too low an altitude. In that case it makes sense to limit your throttle. A good and simple (if not entirely exhaustive) indicator for the stability issues is the MaxQ you can see in the FlightData window inflight.

The rule of thumb is: Throttle up as much as possible without losing control . A TWR of 1.3 is just a good starting point as it usually means a nice balance between stability and deltaV loss.

With Real Solar System and the Realism Overhaul, I'm finding I'm hitting 20km a little too fast, sometimes getting reentry effects shortly thereafter (this is lifting off at TWR 1.5 and leaving the throttle in place), and I'm having trouble flattening out by 50km. I'm still fine-tuning my ascent profile, as I'm very not used to this earth-sized Kerbin.

Right now my launch profile is to hit 45-degrees east at 20km and horizontal by 50-52km. This gets me to 250km orbit not much above spending 9500m/s delta-v so it's not that shabby, all things considered.

[NathanKell]

jrandom: you want liftoff TWR between 1.15 (or so ) and 1.5 (or so). Low liftoff TWR means cheaper, but more limited launch options (asmi is the source for this; the main concern is wind shear--low TWR on liftoff requires picture-perfect weather). Higher liftoff TWR means you can launch when/wherever. SpaceX has ridiculously low liftoff TWR and is therefore incredibly limited in their launch conditions.

Regarding TWR after launch, terminal velocity really isn't a concern in FAR (400m/s at sea level, goes up fast from there). You do want to be careful about dynamic pressure (and thus heating), but if you do a gradual turn (as you should--look up what a _real_ gravity turn entails) you'll be fine, even with high acceleration.

The main issues people run into with FAR are about flying their rockets as if it were stock: not pitching over until 10km up (rather than starting turn when velocity <= 100m/s, as you should), and flying with high AoA/sideslip (i.e. having nose pointed well away from surface-prograde indicator). If you keep AoA/sideslip -5 < x < 5, you should be absolutely fine.

[NathanKell]

And ninja'd, so: Yikes! You want to be turning nearly as soon as you clear the tower, though you needn't be horizontal before a good 100km or so (depends on upper stage TWR really). You shouldn't be using that much dV though (is it MJ's dV used, or "starting Vac dV - ending Vac dV"?)

You can get Gemini-Titan up with <9100m/s vac dV btw.

[jrandom]

My delta-v map says 9500m/s to 250km orbit (Edit: scratch that, it says 9400m/s), but I'm guessing that's a sort of average estimate. I do indeed start gradually turning right after liftoff and slooowly keep pitching over. I've been flying with FAR since my first week of stock KSP and haven't done the fly-up-to-10km-then-turn technique since that time.

I did get spoiled by the stock-sized Kerbin -- my ascent profiles were so tuned that I could circularize into an 80km orbit by burning at periapsis for only seven seconds or so. Beautifully flat trajectories I had! Can't quite pull off that exact finesse yet with bigkerbin.

[Frederf]

The point of TWR 1.5 is that TWR is expected to grow as the atmosphere recedes. You'd love to have TWR infinite up to terminal velocity then 2.0 to stick to TV all the way up. Most RL rockets aren't throttleable and even if they were max TWR adds weight so spending time at partial throttle is wasted weight penalty. With thrust stuck on max combined with thrust improving as you approach vacuum, your initial and final TWR will bracket 2.0. If you start at 2.0 then the average will be >2.0 and your total dV losses will be high due to aero. You'd want to balance it so you spend time slow which has a gravity penalty and time fast which has an aero penalty and minimize their total.

[NathanKell]

jrandom: I apologize for besmirching your FAR-honor. :]

If you fly with something like Centaur upper stage-level TWR (or even S-IVB) then you'll need a quite gradual curve that doesn't flatten until quite late and you'll still circularize after apogee. If you fly with something with reasonable TWR in the upper stage, you can get away with something that flat. If you pull it off right, you can circularize for about 50-100m/s

The 9400m/s estimate is presumably not including dV losses due to burning in atmosphere (i.e. it's for if you're just looking at vacuum dV required) rather than actual dV required? Or it's based on assuming a wimpy American upper stage? Dunno. But anyway, I've always heard 9.3 for a 300x300 orbit, and that's with a realistic trajectory where they accept gravity losses from going straight up longer, in exchange for not having to have stronger PLFs for the higher aerodynamic stresses / heat of a flatter trajectory.

Frederf: (not you specifically, but) what is it with the terminal velocity fixation? It's ~400m/s at sea level and you need one _heck_ of a TWR to even approach it after sea level. Seriously, you _will not_ be overspeeding in atmosphere with FAR. The real issue is burning so far from apogee that all you're doing is raising your apogee, not perigee.

Edited December 1, 2013 by NathanKell

[jrandom]

Circularize after apogee? What sorcery is this??

[NathanKell]

It's what you're forced to do when you have a single anemic RL-10 at <100kN pushing like 40 tons of Centaur and payload.

[jrandom]

It's what you're forced to do when you have a single anemic RL-10 at <100kN pushing like 40 tons of Centaur and payload.

But how do you do it? Aim 45-degrees skywards and fire away? I am befuzzled!

[Frederf]

Frederf: (not you specifically, but) what is it with the terminal velocity fixation? It's ~400m/s at sea level and you need one _heck_ of a TWR to even approach it after sea level. Seriously, you _will not_ be overspeeding in atmosphere with FAR. The real issue is burning so far from apogee that all you're doing is raising your apogee, not perigee.

I've taken it onboard that TV is the speed at which dV losses during (at least vertical) ascent are minimized. Assuming the craft steers properly and stays in one piece it should be the minimal dV to orbit. The best ascent would be to 50/50 your dV losses from effective gravity (cosine attitude) and aerodynamic ones until your Ap was as desired then MECO and circularize at Ap. And I absolutely can exceed TV during ascent without too much difficulty. MRS's thrust fix means that a small TWR at launch can quickly surpass TV as thrust is increasing and weight is decreasing.

[asmi]

Atlas-V was optimized for direct injection into high-energy orbits. Here is my reference mission (4t payload to 185x35786 km GTO) for Atlas-V 502 (dual Centaur configuration):

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Basically as you can see on the screenshot #13 that it's 24 minutes direct injection burn. The idea is that you throw Centaur+payload stack with the booster high enough that you'll stay in space all that time. From the same screen, TWR at the beginning of the burn is 0.46, so you need to make sure you have sufficient time until your TWR becomes over 1 (that would allow you to maintain constant altitude while still accelerating).

[Camacha]

And I absolutely can exceed TV during ascent without too much difficulty. MRS's thrust fix means that a small TWR at launch can quickly surpass TV as thrust is increasing and weight is decreasing.

Are you sure? It has been stated a couple of times that it is nearly impossible to hit terminal velocity. One of the reasons is that the terminal velocity is also increasing along with your TWR.

[Guest]

Atlas-V was optimized for direct injection into high-energy orbits. Here is my reference mission (4t payload to 185x35786 km GTO) for Atlas-V 502 (dual Centaur configuration):

Yeah, now keep in mind that Atlas V was never actually flown like that. It always using a single RL-10 engine on it's Centaur. And with much larger payloads, since it can have SRBs added for increased launch TWR. It's actually one of the best LVs we have, too.

[asmi]

Yeah, now keep in mind that Atlas V was never actually flown like that. It always using a single RL-10 engine on it's Centaur. And with much larger payloads, since it can have SRBs added for increased launch TWR. It's actually one of the best LVs we have, too.

It's very good for high-energy orbits, but not so good for LEO missions (you really want to have better TWR for low orbits). For LEO mission I think Zenit is better efficiency-wise.

[Frederf]

Are you sure? It has been stated a couple of times that it is nearly impossible to hit terminal velocity. One of the reasons is that the terminal velocity is also increasing along with your TWR.

I don't know if I was getting there exactly but my speed readout in surface navball exceeded the FAR fight data readout. TV increases with altitude (air density) so it is a game of catchup but it stays pretty low for the first 5000m or so. After 20,000m, yeah, forget it.

[ferram4]

Question: How many engines are you using to do this, and what does this launch vehicle look like? I'm slightly terrified at what my imagination is coming up with for you to achieve that, but I wanna see.

[Frederf]

I'll post it later but it looked rather tame. I think a couple of BACC boosters with a skipper throttled lightly for steering control. TWR was no more than 4. Perhaps a high Cd in the 1-2 range is to blame; I'll have to check if it was that high.. As sensible as the craft looked you know how weird aero issues can crop up.

[Captain_Party]

Yeah, now keep in mind that Atlas V was never actually flown like that. It always using a single RL-10 engine on it's Centaur. And with much larger payloads, since it can have SRBs added for increased launch TWR. It's actually one of the best LVs we have, too.

Ah... You sir, are wrong, as the Atlas V XX2 has two RL-10's and the Atlas V CCB has one, albeit with two nozzles.

[Guest]

Nope, I checked. Atlas V XX2 configuration was never used to date. I'm not saying that it can't use a DEC, merely that it never did so.

[Captain_Party]

Crap. My mistake, sorry... *slowly walks backwards out of the room*

[DragonEG]

Ferram, i want to thank you for your mod! Actually, i even managed to remember my login and password for this...

Without your aerodynamic model this... thing... would never become airborne!

[KerbMav]

It seems my rockets fly more stable without using tailfins currently?

They quite regularly tilt more and more at 45 deg (north or south going easterly) and dont let themselves be pulled back up, randomly entering a summersaulting and regardless where I set the throttle.

Just as it looked like that I managed to launch every time nice and safely at TWR ~1.5 ...

[forsaken1111]

It seems my rockets fly more stable without using tailfins currently?

They quite regularly tilt more and more at 45 deg (north or south going easterly) and dont let themselves be pulled back up, randomly entering a summersaulting and regardless where I set the throttle.

Just as it looked like that I managed to launch every time nice and safely at TWR ~1.5 ...

I had the same problem, and eventually just uninstalled FAR. Rockets with tailfins for control would end up doing flips at random. I've seen it happen a few times in Scott Manley's videos as well, so I know its not just me having this problem. With or without tailfins I was unable to get rockets with significant payloads into orbit with FAR enabled because the rocket would just flip end over end as soon as I went off of total vertical, and this occured no matter how much aerodynamic control I attempted to add or remove, with or without fairings around the payload. The same rocket flies flawlessly without FAR.

Oh well.

[KerbMav]

Only that plucking the tailfins/winglets from my design made it quite stable again.

[forsaken1111]

Only that plucking the tailfins/winglets from my design made it quite stable again.

I wish that had worked for me, but its a bit nonsensical. The tailfins are supposed to provide stability and control, no?