Evolution of SSTOs (Also: Hello, pleased to meet you!)

[Athos]

Hey everyone! If you couldn't already tell, I'm new and this is my first post outside the welcome forum.

After spending a lot of time ignoring them since I started playing during the Steam Summer Sale, I've been busy working on SSTOs lately, reading the forums for help and inspiration here and there. Noob that I am, I figured I'd share the results of my work. Unfortunately, I've not thoroughly tested exactly what these craft are capable of, so I can't tell you precisely how much fuel or tonnage can be delivered to orbit with a good pilot, or what other destinations in the system they can reach.

I can tell you that they do achieve orbit, and are capable of docking with a space station in a 150km orbit. Everything spacegoing in this post has a standard-size docking port, so any of them can refuel in space to attempt other destinations. The larger ones can be used to deliver fuel, monoprop and Kerbals to a station, or attempt to transfer a payload from one orbit to another.

Before I get into specifics, my design philosophy was to keep things as simple as possible. After doing some research on the forums, I know that better results could be achieved with precision building, flying, engine management, and more patience during ascent, but I really enjoy playing this game with dead reckoning, and building an SSTO that can make an easy successful ascent is highly satisfying to me.

After unlocking most of the tree with rocket missions to Minmus and the Mun, I started building air-breathing aircraft with a very simple 'fighter': A single-engine jet with cockpit, two fuel tanks and four radial intakes, delta wings, tail, canards. I later upgraded it to a turbojet and added science equipment. Stinger is a joy to fly, even lifts off the runway with no control input.

I moved on to SSTOs using the Aeris 4A as initial inspiration. I built one with the wiki's recommended mods to see what made it tick, but had trouble achieving orbit. After consulting the forum a bit to learn about how to fly an SSTO, and discovering the wonders of proper flight paths, ram intakes, aerospikes, throttle, engine and intake management, airhogging, strut spam, and multiple canard sets, I put the Aeris-based design aside and tried a medium spaceplane. (This thread helped me understand what was necessary to build one with acceptable performance. In particular, SalmonellaDingDong's post and RoboRay's screenshot on page 3 inspired my preferred intake mounting method.)

Fuel Delivery Vehcile 1 (FDV-1) was a difficult beast to fly; she tended to tumble on the runway if you tried to steer at all, had to roll off the end of the runway before she could pitch up, unbalanced flameouts would send her spinning at the drop of a space helmet, and she required a fair amount of fuel management to keep the center of balance where it should be. But, Jeb managed to get her docked to a space station at 150km, with enough fuel that I could've dropped some off. She and Jeb stayed up there for a spell while I went back to work. I went back to the forums and noted the advice about center of mass and center of lift, and realized that FDV-1 couldn't take off because she might not have enough lift overall, and her CoM and CoL were not close enough together.

I decided to try smaller craft again, but after having trouble with engine management on FDV-1, I started looking for ways to simplify things. Enter RAPIER engines! I took Stinger, removed the turbojet, and bolted on two RAPIER engines with some extra fuel and extra lift. I replaced the single intakes with bi-couplers and dual intakes (didn't really have room for more). After some test launches and transitions to closed cycle with Hornet, I realized that I could probably make things even simpler with a single-engine craft and more airhogging. I would later come back, borrow the airhogged wings from my next project, and put the turbojet back on the centerline:

I went back to Stinger again, swapped the turbojet for a RAPIER engine, and replaced the forward fuselage with an FL-T200 and 400 and a monoprop tank. I mounted 8 intakes on cubic octagonal struts to the leading edge of the wings, added a bit of lift and an inline docking port. I still couldn't seem to get ascent right, though; I couldn't get Wasp fast enough before I rose too far and the engine switched modes, sucking up all the fuel. I could make orbit, but barely. (Wasp is pictured behind FDV-1 above.)

I decided to come back to it later, and using all that I'd learned thus far, I built a new heavy SSTO, FDV-2. I started with a Jumbo-64, ASAS module for additional torque, and large monoprop tank, adapter, and mk2 cockpit. Knowing I'd need a place to mount canards and a way to keep the center of gravity forward as fuel was used during ascent, I mounted an FL-T800 with fuel and ox flow disabled, and forward-facing shielded docking port. Mindful of FDV-1's failings, I mounted 4 turbojets at the rear of the Rockomax tank using a quad-adapter to keep them as close to the centerline as possible.

I kept the basic wing structure from FDV-1, and attached them in two rows, again supporting them with struts. I very carefully placed a total of 4 Mk1 structural fuselages at the rear wingroots on the Rockomax tank, and capped each with an intake and a RAPIER engine, rather than aerospikes, so I wouldn't be hauling idle engines during ascent. I abandoned monoprop RCS in favor of Vernor thrusters, two facing in each direction. I replaced the static solar panels from earlier SSTOs with 2x SP-Ls arranged radially at 45 degree upward angles.

Here I started airhogging and canard spamming in earnest. I mounted 3 pairs of advanced canards to the nose and a set of delta-deluxe winglets to the Rockomax adapter behind the cockpit, and used cubic octagonal struts and quad-couplers with 4 ram intakes to mount groups of them to the leading edges of the wings. I ended up with a total of 20 intakes, including the ones at the rear, and 8 engines.

Not a great ratio as airhogging goes, but this got the FDV-2 prototype into orbit with way more fuel to spare than FDV-1, she could actually lift off the runway, and with the engines all mounted close to the centerline, she was fantastically stable all the way up, with SAS able to prevent flameout spins on its own. Even better, with so few tanks, she requires very little fuel management through the mission cycle.

For FDV-2B I added a Hitchhiker so I could deliver redshirts--err, additional staff--along with the fuel and monoprop, replaced the delta-deluxe winglets with two pairs of standard canards, and later added two more intake groups for a total of 28. FDV-2B is definitely the most practical SSTO I've built, making orbital deliveries on manual control safe enough to be routine, even if it's not a huge amount of fuel per trip.

After a few successful test flights and orbits, I went back to the drawing board and decided to go bigger. For FDV-3, I wanted to make a super-heavy SSTO. For the prototype, I decided to go with a Mk3 cockpit, two fuselages, Mk3 to Mk2 adapter, Mk2 to Mk1 adapter, and a basic jet along the centerline for its tiny kick at takeoff. (I figured a well-meaning KSC engineer looked at the forlorn pile of basic jets abandoned after turbojets were invented, and decided to give one a home.) For payload and main engines, I went with a copy of FDV-2's payload and engine section mounted on either side of the fuselage, and for canards and reserve fuel, a copy of her nose tank in front. I also placed fuel lines from the frontmost fuselage to the Rockomax tanks; since engines pull fuel from the furthest tanks available to them, this ensured that all the engines mounted to the Rockomax tanks pull jet fuel from the rearmost sections of the main fuselage first, keeping the center of gravity forward.

I spammed everything: 4 SP-L solar panels, 8 sets of canards, 12 Vernor thrusters, a total of 17 engines, 3 rows of wings, a total of 72 intakes, and ye gods, the struts. I can't tell you how much time I spent launching and reverting and adding struts, and there's still some wing flex. But you know what? Despite having 300+ parts and causing time dilation on my PC, she flies pretty well, and can haul a lot more fuel, too. She's less stable at flameout, but with RCS, SAS, and optionally shutting down the outer 4 turbojets, still controllable.

For FDV-3B, I replaced the 4x SP-L solar panels with 2x Gigantors, added a centerline tail, a third Mk3 fuselage right behind the cockpit for additional fuel, a structural wing pair right there, and reworked the aerodynamics at the nose: I reduced the number of canards to 5 pairs, and added a set of delta wings to the main fuselage, which extend just barely over the top of the Rockomax tanks on either side.

She actually has the same number of parts as, is ever so slightly 'cheaper' than, and performs slightly better than the prototype. One item of note: I discovered that since the main fuselage has more ground clearance than the payload sections, the best way to mount the nose gear is with a large structural pylon. I have to be careful not to punch it off by inadvertently staging a second time. I did it on purpose to see what would happen: She can't take off without it. Doubt she could land either.

With how easy FDV-2 is to fly, I doubt FDV-3 will ever be used for anything other than special occasions, but she is an amazing beast as far as I'm concerned!

And after all that, I finally brought Jeb home from the space station and parked FDV-1 outside the SPH, deciding to leave it there as a permanent static display (as pictured earlier). He then flew Wasp up to my space station and back with a much better ascent than before, and parked it right beside FDV-1 afterwards (also as pictured earlier, behind FDV-1). Last on my checklist is a successful mission with Hornet. She really hauls arse into orbit, it's hard to keep her from breaking atmo too quickly.

And now, onto the ships themselves! I've included ascent profile and descent profile with most of these, but I'm pretty sure they can be improved upon. If you'd care to suggest improvements without too much complexity, feel free.

Stinger: This single-engine jet is light, fast, stable, has plenty of range, and mounts science equipment.

Download

Wasp: Simple light SSTO prototype. Her airhogged RAPIER provides excellent performance with the right flight plan, again leaving plenty of fuel to spare after ascent.

Her ascent profile is simple; activate SAS, close most of the intakes with group 1, full throttle, stage for takeoff, gear up and 50 degree AOA, watch intake air and reopen intakes with group 1. Zero AOA at 20km and continue accelerating as vertical speed bleeds off, pitch up slightly as rate of climb approaches zero to maintain slow climb, then pitch back up to 45 degree AOA once you've hit orbital velocity (or when the engine switches modes, if you rose a little too fast). Group 2 toggles all intakes. Raise apoapsis and circularize. Group 0 toggles docking port.

Her descent profile is just as simple; deorbit to a 40km periapsis, open your intakes with group 2, switch the engine back to air-breathing with group 3, and fly home.

Download

Hornet: High-performance light SSTO. 12 intakes for 2 RAPIER engines and centerline turbojet.

Use Wasp's ascent profile, but go to zero AOA earlier, say about 19km, to prevent her from overshooting the atmosphere. More care must be taken to prevent spins with throttle management. Also, group 2 shuts down the turbojet in addition to toggling all intakes. Use a similar descent profile; group 2 toggles the intakes back open, group 3 shuts down the RAPIER engines and activates the turbojet, group 4 reactivates the RAPIER engines and toggles them back to air-breathing if you want more thrust.

Download

FDV-1: Medium SSTO. 4 turbojets with centerline aerospike. She's a museum piece, not worth regular use on manual control, but good for a challenge, if you like that sort of thing. I'll let you figure out your own ascent profile if you so choose.

Download

FDV-2B: Heavy SSTO. It's the most practical I've built; she carries a fair amount of fuel and monoprop, and with her engines all mounted close to the centerline, SAS alone is enough to keep her stable during ascent until they've all flamed out (or at least, it was during my test flights). I'd consider her the best of the bunch, and I'm proud to share. Achieves 100km orbit with 4 kerbals, 750 monoprop, 1000+ fuel, 2000+ oxidizer, and reserve FL-T800. Better pilots should be able to achieve better results. Update: I think I'm getting the hang of SSTO ascent. I tried again and achieved 100km orbit with ~1550 fuel and ~2450 oxidizer, plus the reserve. Ascent profile updated accordingly. Also, I've made some minor strut changes for fuselage rigidity and updated the craft download.

Before takeoff, activate SAS and resource display, throttle up to max, close two intake groups with action groups 1 and 2.

Spacebar to get the party started. Rotate just after passing the VAB. Careful--it's easy to pitch up too fast and start tumbling. After liftoff, gear up and 50 degrees AOA. Open intake group 1 after passing out of the lowest atmosphere layer, and group 2 once your intake air drops to 1.

Start pitching forward at about 19km. Zero your AOA at 20km. SAS on its own should keep you stable through the rest of your ascent. The ship will continue to rise while you accelerate. As your rate of climb approaches zero, pitch up to 15 degrees to maintain slow climb. Try to reach at least 1700 m/s before passing through 30km, maintaining max throttle. Faster is better. You will notice some wobble as your engines start to flame out, but SAS will compensate.

Eventually, the RAPIER engines will switch modes, and temporarily, some or all of your turbojets will spring back to life. Pitch carefully up to 45 degrees to get your apoapsis rising and break atmo ASAP. SAS should still keep you stable. If you're shooting for a 100km orbit, your apoapsis will get there before your jets completely flame out. Group 3 will kill them all and close all intakes.

If you're going higher, might as well leave the jets on until they flame out completely, then hit group 3. Once your apoapsis hits the desired point, kill your engines. You may need to goose them a bit to keep your apoapsis from dropping below where you want it as you continue rising through the stratosphere. Eventually, circularize.

Optional:

The docking port and solar panels can be toggled with group 0.

The fuel in the FL-T800 should be enough to get you home, and the oxidizer in it should help keep your center of gravity forward for stability during descent. (Don't forget to enable the fuel feed from that tank.) If you want to do a little flying around (or, like me, you have trouble with precision deorbiting), put fuel back in the main tank as necessary.

Use your deorbit burn to lower your periapsis to about 40km. Once you've done that, hit 3 again to reopen all intakes, then 4 to shut down the RAPIER engines and reactivate the turbojets. That should be plenty of thrust for flying and landing, but if you want more, 5 will reactivate the RAPIER engines and toggle them back to air breathing mode. Don't forget to close the docking port and solar panels above 30km. Let your speed bleed off to the point you're comfortable flying, apply thrust, and bring it home. Use groups 1 and 2 to toggle your intakes, using them as airbrakes as necessary.

Download (Some minor strut changes have been made since initial posting)

FDV-3B: Super-heavy SSTO. The biggest I'll be attempting with this old PC. 312 parts and one huge advancement over previous SSTOs: A ladder. Causes time to slow down about 25%, for me. With my imperfect flying, it can deliver 2 Kerbals, 1500 monoprop, ~4000 fuel, and ~5000 oxidizer, into ~100km orbit, with reserve FL-T800. Better pilots will achieve better results. Update: Ascent profile changed slightly. Also, reuploaded because I failed to re-lock steering on all the wheels and re-save after taking the photo. This caused the ship to depart the runway in an unexpected direction upon takeoff. Oops. Update 2: Reuploaded after I realized I forgot the forward pair of lateral Vernor thrusters. Double oops. A few other minor changes.

Before takeoff, activate SAS and resource display, throttle up to max, close two intake groups with action groups 1 and 2. This leaves a total of 16 intakes open at takeoff to feed the 17 engines.

Spacebar to get the party started. Only hit it once, or you'll punch off your nose gear! She isn't safe to take off or land without it. Use about 3/4 of the runway, then rotate. Careful--it takes a few moments to get the nose up, but once it is, it's easy to over-roll and start tumbling. After liftoff, gear up and 50 degrees AOA. Open intake group 1 after passing out of the lowest atmosphere layer, and group 2 at around 17km. Toggle the centerline basic jet off with group 9.

Start pitching forward at 19km. Zero your AOA at 20km. The ship will continue to rise while you accelerate. As your rate of climb approaches zero, pitch up 10-12 degrees to maintain at least a slow ascent. Activate RCS; the Vernor thrusters will help prevent spins. Try to reach at least 1700m/s before passing through 30km. More speed is better. SAS and RCS should keep you from spinning out of control as your engines flame out, but if your flameouts are too unbalanced, quickly kill the outer four turbojets with group 8; that should help stabilize you while still allowing you to eke out some speed from the inner four.

Eventually, the RAPIER engines will switch modes, and temporarily, some or all of your turbojets will spring back to life. Pitch carefully up to 45 degrees to get your apoapsis rising and break atmo ASAP. SAS and RCS should still keep you pretty stable, but if you didn't before, you might want to hit group 8 now to shut down half your turbojets. In any case, once your turbojets have all flamed out again, group 3 will kill them all and close all intakes.

Once your apoapsis hits the desired point, kill your engines. You may need to goose them a few times to keep your apoapsis where you want it as you continue rising through the stratosphere. Eventually, circularize.

Optional:

The docking port and Gigantor panels can be toggled with group 0.

If you offload most of your payload, deorbit, and return to atmospheric flight near KSC, the fuel in the FL-T800 should be enough to get you home, and the oxidizer in it should keep your center of gravity forward for stability during descent. (Don't forget to enable the fuel feed from that tank.) If you want to do a little flying around (or, like me, you have trouble with precision deorbiting), put fuel back in the main fuselage as necessary, filling from the front.

Use your deorbit burn to lower your periapsis to about 40km. Once you've done that, hit 3 again to reopen all intakes, then 4 to shut down the RAPIER engines and reactivate the turbojets. That should be plenty of thrust for flying and landing, but if you want more, 5 will reactivate the RAPIER engines and toggle them back to air breathing mode. Don't forget to close the docking port and solar panels above 30km. Let your speed bleed off to the point you're comfortable flying, apply thrust, and bring it home. Use groups 1 and 2 to toggle your intakes, using them as airbrakes as necessary.

The first time I attempted to land this thing's prototype, the tailfins over the Rockomax tanks detonated for no apparent reason not long before reaching the runway. I'm at a loss to explain this, and I have no idea if it will happen to anyone else flying it. Makes yawing a bit more difficult. Despite that, I managed a safe landing. Hopefully, if it happens again, the additional centerline tail on FDV-3B will at least help a bit.

Download (adjustments and fixes have been made since the photo was taken)

Whew! Well that was fun, especially with the forums being up and down today. I hope you SSTO fans enjoy some simple, straightforward craft!

Edited September 8, 2014 by Athos
Uploaded fixed FDV-3B

[Rainbowtrout]

These look quite good, and incredibly heavy. I will try them later, hopefully.

[Athos]

Let me know what you think! Now, off to bed!

[LvL]

All these SSTOs, WOW. rep for you you deserve it. PS, not many people get rep on their first post out of the welcome forums.

[Roflcopterkklol]

Neato, nice job man

[Athos]

Thanks for your kind words! I hope someone will fly them and tell me what they think. I look forward to sharing some of my rockets, too.