Simple, low-level SSTO rockets

[Sharpy]

I remember someone advertizing their family of SSTO rockets for different weight payloads... No jet engines/Rapiers used - just classic rockets that don't drop anything all the way until the orbit (where they drop the designated payload and perform reentry for recovery, dropping on parachutes). I can't seem to find these anymore, or any non-spaceplane SSTOs that don't use air-breathing engines.

Can you point me towards some cost-efficient solutions for getting consistent payloads (specifically, 5 tons) into LKO or a bit beyond?

Edited July 6, 2015 by Sharpy

[Streetwind]

Skipper + orange tank + probe core + parachutes should be a decent SSTO rocket.

Problem is that the 2.5m probe core comes very late in the tech tree. Also, keeping the parachutes from burning up during reentry may be a serious challenge, which is probably why you don't see as many rocket SSTOs anymore.

[Sharpy]

Airbrakes aren't very heavy and they do wonders to the speed of reentry. I'll try what you suggest. Plus it *just* might be possible to do it without the probe core.

In VAB:

- Set the parachutes to deploy at 0.75 atmospheric pressure. (that converts to 1800m altitude).

In LKO:

- deploy the airbrakes

- activate (stage) the chutes

- burn for 48km periapsis

- separate the payload (with the core/capsule), switch to it

- burn for 70km periapsis with the payload

- switch to the (probeless) SSTO (visible as "debris").

- accelerate time as optimal

- pray for no mountains.

[ajburges]

I tend towards 2 stage lifters (core + SRBs) if I'm forced to use rockets (I focus on planes), but similar concepts apply.

You do reduced stage lifters to get that recovery money. Deorbit control is essential. Get a large service bay (or fairing, adapters, and struts) and put a probe core and some batteries in it (I like the HECS core for hold prograde/retrograde). Since you already have a service bay, stuff it with chutes. My 2.5 orange tank + mainsail core needed 9 chutes for safe landing and another 3 for a soft tip on its side. A quartet of tail fins will give you aerodynamic stability and some drag on reentry.

[Warzouz]

That should be me... But those rockets are high-tech (probe core, batteries...)

http://forum.kerbalspaceprogram.com/threads/123195

Rocket SSTO makes a lot of sens on carreer mode, especially middle game. They are VERY cheap. Maybe not as cheap as plane SSTO, but they are easily scalable (plane SSTO can't and have to be redesigned). I extended my heavy 300T launcher to a 600T in 10 min and succeeded on the first launch with nearly the same ascent profile. Put the needed dV, TWR and make your rocket streamlined : you're good. The only limitation are your CPU and the structural integrity of the payload fixation on the SSTO stage.

Design hints for a lower tech version :

- You'll need to put some smaller parts (probe, batteries...) somewhere, probably in a service bay. Those service bays are quite fragile and I had them crumbling under the payload weight. That's why I used to put them at the bottom of the rocket, protected by a heatshield (partially filled). I only used higher tech parts because I could get rid of the service bay.

- Chutes can become heavy if you want to land with them. I chose powered landing to avoid using hundreds of them. An alternative is to use plane landing gears (medium or heavy). They have a much higher crashing tolerance, but they look... strange on a rocket.

- Airbrakes are usefull to tweak landing zone. I think they are necessary in 1.0.4. Drogues may be as well, depending of the weight of your stage. To be tested with your design.

- Vernors can be an alternative to reaction wheels on heavy SSTO. You can save on batteries. I didn't used them : I've placement issues.

- For a successful reentry, try to local the deorbit location. In 1.0.4, I found that the western ridge of the crater 90° of KSC was the best considering a 60m/s deorbit burn. But again, that depends on the drag of your stage (airbrakes or not...)

[Sharpy]

Probe cores aren't the problem, got HECS already. The Mainsail is. I really avoided researching that node because I got a Boar through a silly contract (test in suborbital Minmus flight) and kept using it for everything except testing But if I'm ever to get NERV, I have to research Mainsail, so... okay.

- With airbrakes drogues are definitely not necessary. Tweaking the landing zone is one thing. Getting the upper thousand m/s off the orbital speed (when burning up is a real concern and drogues are of no use) is another, and once that's taken care of, the rest is peanuts: with four airbrakes, the twin boar + orange tank + several extras gets down to 250m/s before 10,000m altitude.

- I'll definitely need to try the powered landing. It seems tricky to pick the right moment and the right thrust. So far, before airbrakes I was used powered braking - getting the speed down by 500m/s over the 40km-20km altitude range makes a world of difference during reentry and the launch stage freed from payload and with nearly empty fuel tanks gets some impressive dV on the last few drops of fuel. (OTOH if I missed the point to shut off the engines and perform the separation in suborbital flight, I had to refuel the launch stage from the transfer stage supply... ugly.)

- with Airbrakes, I can afford making the reentry far more aggressively - entering the atmosphere just as the deserts end, path ending about 1/3 the ocean width past KSC, makes a landing between the mountains and KSC. (since my orbits are usually quite ugly - poor ascent trajectory and never bothering to circularize - it's easier to describe the landing system that way. OTOH if my launch stage doesn't go orbital (it rarely does), the landing is somewhere before the Badlands, and recovery yields some 70% only. That's a primary reason why I'm aiming for a true SSTO. As for probe cores and batteries, I'm really gonna try the coreless approach. Might *just* work, and that's, what, 60m/s dV wasted from the transfer stage?

And the aforementioned 5 tons is my new approach to orbital recovery of stranded Kerbonauts (contracts) - with their vessels. The 5 ton vehicle is a single medium-length 1.25m fuel tank with a probe core, 12 Vernor engines, Klaw, 4 airbrakes, 8 parachutes, a battery and some solar panels. I don't know the exact number but it has lots of dV, something of order of 3000 I think, through the Vernor engines alone - plenty enough to rendezvous any stranded Kerbal, grab their vehicle with the Klaw, and deorbit it safely. No problems with crew cabin getting occupied by Jeb, no EVA, no problem with extracting kerbals stuck inside a probe core, the only actual problem is lack of informational display - delta-V, predicted burn time etc for Vernor engines. Oh, and it fits in an MK2 cargo bay. Although with the vertical SSTO I believe the small fairing would be better.

ps. Rocket SSTO because landing is by far less hassle than an airplane... unless I drop the airplane on a parachute.

Edited July 6, 2015 by Sharpy

[Warzouz]

Well. It seems I wasn't clear enough

As for power landing, I meant using chutes but not too many of them. I'm not trying to land from 250m/s but from 20m/s. I'm not using engines to slow down (airebrakes and drogue do that). I use engine to land (litteraly).

On my heaviest rocket SSTO (230T, capable of lifting 300T to LKO and return), I have 24 chutes and make a final burn of 17m/s IIRC. If I wanted to land it with chutes only, I think I should put more than 100 chutes. On this big baby, I need 2 drogue chutes or I can't open the 24 chutes before crashing. With airbrakes, drogue and chutes I usually cross the 250m/s below 3000m. Lighter SSTO opens higher and higher so they don't need drogue (but I kept them to have a standard procedure).

Since 1.0.4, I'm not able to land in the moutains anymore (I can't open chute soon enough). But that's not a real loss because I never landed on the mountains without destroying a most of the rocket. Now I overshoot KSC on purpose.

Edit : And I agree, rocket SSTO are easier to land than plane SSTO. You only have to manager the vertical speed right on landing point. When you know the terrain around KSC this is easy. OTOH, plane SSTO are very pleasant to pilot and land, but you need more training.

Edited July 6, 2015 by Warzouz

[Zephram Kerman]

You can use the small nose cone to cluster reliants around a swivel. Use whatever combination of engines you need for just the right TWR. Put some of them on an action group to take advantage of the swivels' better ISP in vaccum.

Control surfaces have a "deploy" feature, which makes it fully deflect without any ASAS. So I've been putting my fins in pairs. During early reentry, deploy one pair so that the whole rocket flies a little bit sideways and slows down greatly in the upper atmo.

If your rocket is light enough to land with chutes only, consider putting them at the empty CoM. It will land sideways on a chute and fin, but cannot tip over.

[Sharpy]

Eh. Reentry without the probe core is impossible. The chutes don't deploy.

[Laie]

I've been recovering a lot of dumb stages. You need a command module to trigger the chutes, but once active, they will deploy at the set pressure/altitude. Works for me.

But for SSTOs, I'd really recommend to bring along a probe core. If service bays work for you, they're lightweight and offer plenty of room for a core and a few batteries (you don't need many, and no solar at all). A few well-placed rudders will act as wings on the way down, giving you some crossrange capability. No serious flight, but enough to avoid the mountains or at least dive for a valley.

Hint: LFBs have insane impact tolerance and are quite easy to recover.

[ajburges]

Dead stick reentry requires you to stage parachutes before you lose control with settings that will have them deploy correctly.

You can find tune it later, but a good starting point is to have all chutes require .6-.7 atm of pressure. At that point you should be subsonic or incinerated. Set a minimal amount of chutes (1-3) to fully deploy at 800-1000 meters. This will bring speed to less than 100 m/s. Have the rest fully deploy at 300 m. The earlier chutes will give them enough time to open that low. Your goal is to be 6 m/s or slower at touchdown.

Dead stick reentries also need to have passive control systems either land a rocket on its side, or have sufficient stability to stay upright.

Make sure you use a good reentry profile. My 2 stage to orbit launchers nether need nor have any additional bits to control reentry heat. I simply renter with a Pe of 45 km or so over my landing site (disclaimer: I use the trajectories mod for more accurate trajectory planning). The mass of the Mainsail makes it aerodynamically stable falling backwards. The thermal mass of the engine takes the worst of the heating and the wings and probe core get toasty, but never critically hot. I can deploy the radial tail fins for rotation and a (very) small amount of extra drag. The parachutes fire as I describe. If it has a probe core (course corrections give another 5% recovery FTW) I keep a reserve of three chutes to control any possible tip over after touchdown.

On engines:

Thrust is the king of SSTO design. By not staging, you are needing to haul a lot of fuel (and worse tank) mass. Favor atmospheric Isp after thrust. You want maximum thrust at liftoff and that means you need better ASL Isp.

Mainsail is the king of 2.5 m engines. Nothing that size delivers as much thrust and the vacuum Isp is bearable to boot!

Skipper has a better mass and vacuum Isp (over 50% of ascent burn is in vacuum like pressure) at the cost of a little less Isp ASL and half as much thrust. It gets points for being the first real SSTO engine in the tech tree.

Twin boar is serviceable, but the low Isp will eat away at your mass fraction.

In the 1.25 parts your only real option is the LVT-30. The TWR is not high enough for inspiring payload fractions. Best to stick to staging for this diameter. The parts are cheap enough.

For 3.75 m there is no better engine than the Mammoth!

When/if you transition to space plane SSTOs the advice remains similar, except you stop caring about Isp ASL ask together. You realize over 70% of the difference at just 10 km high and don't rely on rickets until at least 17 km!

Edited July 7, 2015 by ajburges

[Wolf Baginski]

On my heaviest rocket SSTO (230T, capable of lifting 300T to LKO and return), I have 24 chutes and make a final burn of 17m/s IIRC. If I wanted to land it with chutes only, I think I should put more than 100 chutes. On this big baby, I need 2 drogue chutes or I can't open the 24 chutes before crashing. With airbrakes, drogue and chutes I usually cross the 250m/s below 3000m. Lighter SSTO opens higher and higher so they don't need drogue (but I kept them to have a standard procedure)

The bear cleans his spectacles and peers at you over the top edge of his drawing board. "Try Action Groups," he says. "Attach the chutes using symmetry, and you don't have to click on each individual chute to set it up. 24 chutes is three symmetry groups of eight chutes. You can have a Group of drogue chutes to pop first, stabilise the payload and slow it down a bit. Then the main chutes, with that initially-reefed canopy if they're set up that way. Finally, you can use a retro rocket, like the Russians do with a Soyuz. They use the same method for tanks, and no way is this bear standing on that drop zone."

He checks a notebook. "There's a Mod called Smart Parts. They're a sort of single function probe core that can trigger staging or an action group, So you could release the chutes at specific altitude, or after a delay. The Altitude sensor could fire that retro-rocket automatically." He pauses. "Remember, when you remove the pin from the grenade, Mr Grenade is no longer your friend..."

[ajburges]

Another tip for powered landing. Find the throttle setting for .5-.8 TWR. As long as you don't exceed unity, your rockets will never zero out velocity (and disastrously remove chutes). Parachute effectiveness is inversely proportional to the net force on your ship. KER is a real help with this, but you can manually reckon mass to determine TWR ASL.

Edited July 7, 2015 by ajburges

[Hcube]

I personally use mainsail+2 orange jumbos. I put some of the longest (striped ones) 1.25m tanks in 4X symmetry around the bottom jumbo, for extra fuel with fuel lines, and i attach the gear to those for a wider base. On top of this a 2.5m bay with a HECS and reaction wheels/power inside when i don't have a 2.5m probe core yet. On top of this, a fairing base, and then whatever decoupler+payload i want.

I put 8 airbrakes upside down on top of the top jumbo tank to keep it stable during re entry. This can totally land without chutes provided you keep around 400LFO in the tanks and a bit of skill/practice. I managed to launch 8-9ton payloads and land this bad boy without chutes. But then adding a pair of chutes will make it easier and won't cost much deltaV.

./\

| | <-- payload in fairing

--- <-- fairing base and decoupler

__ <-- cargo bay with power, probe core and reaction wheels

| |

| | <-- jumbo orangex64 with a pair of chutes

| |

| |

---

| | <--other jumbo with radial tanks, fins and gear

| |

| |

| |

__

\ /

/\ <--mainsail

Could have posted a pic, but where's the fun in that ?

Edited July 8, 2015 by Hcube
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