[far] SSTO climb recovery woes

[endl]

so im trying to orbit this heavy plane:

the numbers seem good i do my first attempt to gain speed and the bottom falls out. it takes forever to climb back up and i net very little speed. seems to max at around 1.5kms at about 25km. what can i do to gain more speed/ more climb recovery

Edited November 19, 2014 by endl

[Rakaydos]

Apparently FAR nerfs turbojets so they cap out at 1600m/s or so.

[endl]

im not using turbojets, im using sabre M those other engines are nukes

[zxczxczbfg]

I suppose the answer would be "moar boosters". Though actually you might be able to get by with less boosters: Chop off two of those nuke engines and the weight reduction might give you better performance, unless you need them for something specific that I'm not considering. I don't have much experience with FAR, but I think this would still apply.

[LABHOUSE]

More lift is needed.

[funk]

Put more wings on it, so you can climb higher, where the air is thinner to gain more speed. I'm using FAR and DRE, in my experience SABRE combined with precooler gains thrust at high altitudes, so the velocity is about 1500m/s at 30km. SABRE flame out about 1730m/s or 35-37km height, in best case both is met at the same time for switching mode...

[Voculus]

It actually looks like you have plenty of lift. I know for certain two SABRE Ms will carry at least 115 tons into orbit. (Ship and payload). Without going into the ins and out of ascent profiles, the one single thing you can do differently is to simply bring the nose down, and trade altitude for speed. Don't sweat the fuel consumption. It's still a pittance next to rocket motors firing.

As the air thins high up, the engines will starve for air when going too slow. Descending into slightly thicker atmosphere, and increasing the airflow into the SABREs solves your woes. As you speed up and then begin to climb again, you'll find that the engines are kicking out more thrust at the same altitude than before. The SABREs will get you up to about Mach 5.3, and up to 35k or better in altitude, before you switch them to closed cycle.

[endl]

It actually looks like you have plenty of lift. I know for certain two SABRE Ms will carry at least 115 tons into orbit. (Ship and payload). Without going into the ins and out of ascent profiles, the one single thing you can do differently is to simply bring the nose down, and trade altitude for speed. Don't sweat the fuel consumption. It's still a pittance next to rocket motors firing.

As the air thins high up, the engines will starve for air when going too slow. Descending into slightly thicker atmosphere, and increasing the airflow into the SABREs solves your woes. As you speed up and then begin to climb again, you'll find that the engines are kicking out more thrust at the same altitude than before. The SABREs will get you up to about Mach 5.3, and up to 35k or better in altitude, before you switch them to closed cycle.

this is the case when it works with my lighter craft my problem is that when i gain speed i cant climb out fast enough

[Wanderfound]

SABREs or not, you won't be getting too much above 1,500m/s on jets in FAR (about 1,750m/s is the max for a lightweight low-drag speedster, which this ain't). As soon as you max out your jet speed, light the rockets.

[endl]

ok so i shouldnt expect better speeds i can probably space it then without having to go into such a hard dive where recovery is so difficult.

[Hodo]

SABREs top out at just over Mach 5 before they start to lose power so that isn't the main issue. The thing I can see you are having is a loss of pitch control at those speeds. You are dealing with a serious amount of loss of lift at those speeds. Additional wings isn't going to solve this. I am sure if you look at your Statistic tab in FAR you will see your CL line drop drastically the faster you go. I also imagine your CoL is shifting back further away from the CoM of the craft at hypersonic speeds.

But there are two ways of fixing this issue.

1- Redesign and scrap the current design and add more control surfaces and shift the CoL closer to the CoM.

2- Just add canards to the nose of the craft set to only pitch and a very limited control angle.

I have a bit of experience with SSTOs in FAR and large SSTOs expecially.

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[Voculus]

Hodo, every time you post pics of that beast, I smile! One of these days I'm going to tackle a Goliath spaceplane like that.

[Hodo]

Hodo, every time you post pics of that beast, I smile! One of these days I'm going to tackle a Goliath spaceplane like that.

The funny thing is that is one of I think 4 in that line of monsters.

The one above is the SP-409.

This one is the SP-406 the original heavy hauler of mine.

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It actually built a space station in one challenge.

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Then there is the SP-407 which was the VTOL version of the SP-406.

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And the one that started it all, SP-400A.

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[Vigelius]

Hodo, I'm curious about your beasts' TWRs - nothing I build gets anywhere close on what looks like similar engine setups. Are they from before the Great Nerfing, or have you found a way around that?

[Hodo]

Hodo, I'm curious about your beasts' TWRs - nothing I build gets anywhere close on what looks like similar engine setups. Are they from before the Great Nerfing, or have you found a way around that?

The SP-400A and the SP-406 were before the jet nerf. The SP-406 still worked after the nerf, but it had a drastic change in flight profile. The SP-400A TWR fully loaded was .96:1, the SP-406 was around .6:1 after the nerf.

The SP-409 came after the engine nerf and had a TWR of .75:1 fully loaded on jet engines.

The trick was the flight profile was real gentle. I would climb at about 20-25deg till 12km then bring the nose down to 10deg and accelerate till I hit mach 5 or 25km, then switch over to closed cycle and then pitch up to 24deg and climb till 100km AP. The whole flight would take between 20-30min to achieve orbit.

The other thing was to cut down on accessive drag as much as possible.

[endl]

whats the TWR of the sp-407, i originally wanted to add vtol to this plane but im not confident i can do it without a massive redesign

[Hodo]

whats the TWR of the sp-407, i originally wanted to add vtol to this plane but im not confident i can do it without a massive redesign

Actually about the same as the SP-406. It sacrificed cargo for VTOL and STOVL abilities.

The VTOL TWR sits at 1.4:1 without cargo, and just over 1.02:1 with a 72 ton cargo load on Kerbin.

But it uses Mainsail boosters and aerospikes as its VTOL engines.

EDIT-

One of these days I will get around to replacing the SP-400 line of cargo haulers. They were useful on so many levels. But lately I have been designing military craft so been thinking a bit smaller. Although I have the design process of Grumman, small is not in my vocabulary.

Edited November 19, 2014 by Hodo

[endl]

part of my reluctance to try is that i dont want a massively over engineered craft. bahamut has some nice LFO surface mountable engines that are for vtol use (you can tweak scale them) but since i want cargo this would mean a new wing and fuselage profile, although i would probably be able to carry more fuel for the eventual trip to duna. im pretty sure ill definitely need vtol cargo at some point though

[Hodo]

part of my reluctance to try is that i dont want a massively over engineered craft. bahamut has some nice LFO surface mountable engines that are for vtol use (you can tweak scale them) but since i want cargo this would mean a new wing and fuselage profile, although i would probably be able to carry more fuel for the eventual trip to duna. im pretty sure ill definitely need vtol cargo at some point though

I had that parts pack for a while, but found that non-rocket based VTOL engines were a bit underpowered and useless anywhere without an atmosphere. The one that is like a RAPIER that has a closed and open cycle mode, that was a wonderful engine, but it had problems when .25 came out and I haven't reinstalled that mod to see if it works again.

I also have to many conflicts with tweakscale so I don't use it unfortunately.

You can make a pretty decent VTOL cargo craft with the B9 parts and stock parts. Actually the Aerospike is a wonderful engine because it is so low profile.

[endl]

I had that parts pack for a while, but found that non-rocket based VTOL engines were a bit underpowered and useless anywhere without an atmosphere. The one that is like a RAPIER that has a closed and open cycle mode, that was a wonderful engine, but it had problems when .25 came out and I haven't reinstalled that mod to see if it works again.

I also have to many conflicts with tweakscale so I don't use it unfortunately.

You can make a pretty decent VTOL cargo craft with the B9 parts and stock parts. Actually the Aerospike is a wonderful engine because it is so low profile.

i want to be able to land this craft in a 0 atmo enviroment so i need a low profile LFO that can has enough thrust for VTOL use

[Hodo]

i want to be able to land this craft in a 0 atmo enviroment so i need a low profile LFO that can has enough thrust for VTOL use

B9 has the VTOL engines, there is a jet version and a rocket version. They both swivel 180 deg in 22.5 deg incriments. The rocket version puts out 100kn of thrust at a ISP of 370 I think. And the jet version has a 55kn thrust after the FAR nerf, and dies off around mach 1.5.

I prefer either the rocket B9 VTOL engine because of its low mass .5 tons, or the stock aerospike because of its low profile and high power output and great ISP in all situations.

The hard part is performing the transition from hover to forward flight and back.

[endl]

i find that the b9 vtols dont scale well for my designs either i need to spam alot of them which is messy and hard to work with or they get so big they just arent practical. i coded in tweak scale support for some bdynamics parts and was able to make a few minor changes to get the power up to spec, here is the final version of my plane with vtol functionality at 100k orbit

the conards really improve the pitch control dramatically i was able to hit 30k going at 1900m/s when the orbital burn was done i had 40% fuel left in my reserves which translates to about 1400 LF. this is while carrying my heavy fuel rover which weighs 10-15t cant remember how much exactly

[Hodo]

Not bad, but I see you are still having some nose pitch problems at speed. This could be fixed by adding a bit more wing forward or extending your wing root so it pushes forward a bit more. The reason is as you reach supersonic and hypersonic speeds, your CoL actually pushes back towards the rear of the wing.

[endl]

it was actually really easy to fly, the plane loses altitude (around 17km) when the run first starts but regains it on its own while never dropping below the horizon marker on the ball. at that point i can just hold S button to space. i added the conards for vtol stability mainly but it did make flying it easier. i do have some wiggle room to move/resize the wings a bit more but im not sure why you think i have to do it. is it because of the wing tip engines? i added those because my TWR fell below 1

take off has to be the most difficult part for me i have to maintain a precise angle or the plane starts to stall before clearing the ground, its not that hard but it isnt as smooth as my lighter craft

Edited November 20, 2014 by endl

[Hodo]

No that isn't the reason why I think you need to add a bit more length to your wing root. It is because as I said, the CoL actually shifts towards the rear of the wing at supersonic speeds. So if your CoL is just behind your CoM when in the SPH, it will stay roughly around there till you start going faster than the speed of sound, at Mach 1 it will shift back a small amount, and the closer you come to hyper sonic speeds, in excess of mach 3 your CoL actually can be as far off as a couple of meters, depending on the wing.

There are two ways to counter this affect, you can either extend your wing root so you have your CoL sitting closer to the CoM when in subsonic flight, which will make the craft very twitchy. Or you can shift your fuel load from front to rear as you go faster to counter the shift of the CoL, this is what real supersonic aircraft do. I also do this with my craft, it helps a great deal for a lot of reasons.