SKRV - SSTO, airbreathing, fully reusable, precising landing capable 25 tons to LKO booster rocket

[Temstar]

When the word "SSTO" is mentioned, the imagine that immediately pop into people's head is something like Skylon - airbreathing, reusable, spaceplane.
When the word "reusable" is mentioned, the first thing that people think is "spaceplane", "SSTO" and "airbreathing".

But really, "reusable" and "SSTO" are not synonymous with spaceplane. When NASA put out orders for their Space Transport System concept most companies did come back with a spaceplane design, EXCEPT Chrysler:


The SERV was a very unique design that is basically tubby rocket shaped in the form of a overgrown Apollo capsule. It takes off under rocket power and reaches orbit in a single stage. Release its payload, deorbit itself and ploughs through the atmosphere using that well known shape and heatshield. Then once it nears the surface turbojets fire up to allow it to fly back and land at a precise landing site.

The design was too radical for NASA so was not chosen. But it is sound...

Presenting:
Single-stage Kerbin-orbit Reusable Vehicle


Craft file: https://www./?o74dqcpb10a1s2m

 

SKRV features:

• rocket single stage to orbit - no restriction because of CoM issues or cargo bay dimensions unlike a spaceplane
• 25 tons payload to 75km x 75km orbit
• fully reusable
• flyback booster with precision landing capability

Action groups:

1. toggle jet engines between wet and dry mode (by default dry)
2. toggle on/off jet engines
3. toggle airbrakes

Let's have a typical flight profile:


Launch! You may notice that in the staging sequence the jet engines are activated first - seen as we have them we might as well get some use of them at launch to get the launch vehicle up to speed. Stage the jet engines, throttle up to max, hit action group 1 to activate the afterburners to make the engines spool up faster then hit stage again to light the main rocket engine. Watch the resource gauge and once intake air drop to zero hit action group 2 to turn off the jets. We won't need them again till landing time. Don't worry too much about delayed switching off jets causing flameout - the Mammoth engine has so much steering authority that it can compensate for any number of jet engine flameouts during ascent.


The SKRV has a take off TWR of 1.60 and being single stage this TWR steadily increases during flight, so an very aggressive ascent profile is needed - if you simply follow the prograde the booster will hit the 75km target AP extremely quickly. So active turn towards the horizon over and above the gravity turn.


And here we have it, 75km x 75km orbit with 343m/s of delta-V left. I've been playing with it for the last few days so I can get a fairly optimal ascent profile now. Conceivably I could put more than 25 tons into orbit but until you have gained lots of experience flying the SKRV I wouldn't recommend this.


Here is the 25 ton test payload in orbit.


Now we get to the interesting bit. After releasing the payload and having made its way around Kerbin, it's now time to deorbit the SKRV and return to KSC. Having reached orbit with 343m/s of delta-V left with the payload attached I had more than 400m/s of delta-V after releasing the payload, resulting in this rather steep deorbit burn. Of course the steeper your reentry trajectory the more accurate your landing will be and less trouble you will have with reentry heating.


Depending on your trajectory reentry could be pretty... exciting. When I was designing the SKRV I had lots of problem with the Mammoth overheating. I was able to overcome this issue by adding the airbrakes. The airbrakes serve to slow you down just enough in the upper atmosphere that peak heating no longer destroy your engines - in effect the airbrakes reduce the heat load from the Mammoth by spreading out the absorption of kinetic energy. Deploy the airbrakes once the deorbit burn is completed and leave them on as long as possible.

Note for very shallow reentry trajectory you may see the airbrakes heat up so much that they would reach max heat, before that happen retract the airbrakes so they can cool down a bit then deploy them again. The Mammoth engine has enough heat capacity that it can take the reentry heat for some time without the help of airbrakes. In this particular case since I have a rather steep reentry profile the airbrakes heated up to a max of just under 84% before cooling down. So I elected to leave the brakes out for the entire reentry.


At some point around 14km the turbojets will start to work. Now is the time to switch them on (action group 2) if you need to do landing site adjustments. In this case it was clear that I was going to overshoot KSC so I fired them at full retrograde. If you find yourself undershooting you want to set SAS to radial and fire the jet engines.


Here we see the SKRV flying back to the KSC after overshooting the landing site. It can be a bit tricky to fly but basically it's not all that different from hovering across the surface of the Mun in a Munlander. From the smoke trail you can see I've already built up forward velocity and I have the engines thrusting radial (skywards) to hover horizontally across the sky.


Coming into land at the KSC. Although it's quite possible to do pure powered landing the SKRV is equipped with four parachutes to help make landing easier. Once you are over a good place to land stage the parachutes, set SAS to radial, afterburner off and throttle down for a soft landing.

Speaking of afterburner, the operation of afterburner is quite important to SKRV's hovering capability. Hovering with jet engine has always been a pain in the ass because jets with their turbines are very slow to respond to throttle input unlike rocket engines. When designing the SKRV's hovering system a stroke of genius flashed across my head - although jet throttle is slow to respond afterburner is instant. And so SKRV was specifically designed to use the afterburner capability to control the hover. The jets have been arranged so that at full throttle and with a nearly empty booster their TWR is just under 1. That means with full throttle thrusting radial the SKRV will fall slowly (but at an increasing rate). To hover stationary you simply pulse the afterburners on and off.


And here we have it, happy landing back at the KSC not far from where we took off.


Note to subassembly users:

For some reason subassembly mangles up the SKRV's fuel lines. If you are using SKRV as a subassembly do note you need to take off the nose cone of those 1.25m sections and reattach the fuel line so they run from the ring of 1.25m tanks inward to the center large tank. The 1.25m tanks are divided into two sets of six. Once you've fixed the fuel lines you can reattach the nose cones.

Edited December 14, 2015 by Temstar

[n.b.z.]

Very nice. An inspiring vehicle, and a cool historical reference to a concept design I've never come across.

Seems both practical and cool.

[Rithsom]

Nice! This reminds me of Scott Manley's Reusable Space Program.

Assuming it lands at the KSC for 100% recovery value, how much does this rocket cost per flight? In other words, what is the cost of the fuel it uses?

[Temstar]

[quote name='Rthsom']
Assuming it lands at the KSC for 100% recovery value, how much does this rocket cost per flight? In other words, what is the cost of the fuel it uses?[/QUOTE]

The fuel cost $16,549, so $661.96 per ton to orbit.

[Rune]

Awesome! You know, consider me inspired to keep going down the "shuttles that never were" route. Utterly fascinating, I tell you!

 

Rune. And I think I have some ideas about how to make it look like the real thing...

[Majorjim!]

Just now, Rune said:

Awesome! You know, consider me inspired to keep going down the "shuttles that never were" route. Utterly fascinating, I tell you!

 

Rune. And I think I have some ideas about how to make it look like the real thing...

Put a fairing on it!

 Didn't you make some alternative shuttles last year?

[Rune]

2 hours ago, Majorjim said:

Put a fairing on it!

 Didn't you make some alternative shuttles last year?

Yup, Lockheed's Phase A proposal, the LS-200, and the soviet MAKS spaceplane, if you refer to this thread. However, they had nowhere near a useful role, apart from being replicas, while this one can actually lift up 20mT. Vectors really open up possibilities!

 

Rune. Surprisingly enough, it was exactly one year ago. Must be Shuttle day...

[selfish_meme]

Except this was supposed to use Aerospikes, which would make it much more interesting to build. And a payload bay in the inside cylinder...wow now that would be interesting

Edited December 14, 2015 by selfish_meme

[Majorjim!]

2 hours ago, Rune said:

Yup, Lockheed's Phase A proposal, the LS-200, and the soviet MAKS spaceplane, if you refer to this thread. However, they had nowhere near a useful role, apart from being replicas, while this one can actually lift up 20mT. Vectors really open up possibilities!

 

Rune. Surprisingly enough, it was exactly one year ago. Must be Shuttle day...

I had a feeling. Trust your feelings Luke Rune.

   

  

[Temstar]

1 hour ago, selfish_meme said:

Except this was supposed to use Aerospikes, which would make it much more interesting to build. And a payload bay in the inside cylinder...wow now that would be interesting

I did actually try to make it aerospike with the same sort of thinking that if you have a good Isp all the way up it would greatly help a SSTO spacecraft. Turns out that KSP's aerospike TWR is so poor that coupled with the lack of control authority in KSP via differential throttling an aerospiked rocket SSTO is only able to lift itself and tiny payloads, making the whole thing rather moot.

Funnily enough that's also one of the main reason why NASA rejected it. NASA knew that Chrysler's numbers were pretty aggressive and if critical components like the fuel tanks or the aerospikes engine cannot hit those performance specification then this being a SSTO the SERV might not reach orbit at all. If that ended up being the case NASA could well have a two decade hole in its manned spaceflight capability.

That and NASA really wanted a spaceplane.

[selfish_meme]

Some quick testing showed almost 20% payload fraction for 2 turboramjets and an aerospike, so its doable

[Temstar]

That can't be right:
Aerospike has a vac thrust of 180kN. This is an SSTO, so assuming a low lift off TWR of 1.3, one aerospike can power a rocket that's a hair over 14 tons.

20% payload means 2.8 tons of payload
Aerospike is 1 ton, turboramjets are 1.8 ton each. So that's 6.4 ton dry weight not including tankage.

Assuming the other 7.6 ton is all bipropellent fuel tank, that should get you 6.76 ton of fuel and 0.84 ton for tankage.

So wet weight 14 ton, dry weight 7.24 ton. We'll use vac Isp 340 for rocket equation. That only gets you 2200m/s of delta-V. You will save some delta-V by using just the jet engines and liquid fuel, but you increase dry weight again from all that air plumbing. I'm not so sure without wings two jets can support a vertical climb for very long.

Can you show us your test vehicle?

[selfish_meme]

27.43t on the Pad, payload 5.5t of ore, makes a 20% payload fraction, of course it's going to be less once you add in other things like decouplers and such.

If you had 6 such boosters, in a more compact form you could lob 20-30t into LKO

Edited December 15, 2015 by selfish_meme

[Temstar]

The decoupler won't add much, it's mostly the guidance (read flywheel) and the stuff that you need for reentry (heat shield, airbrake, hovering fuel, landing legs, parachutes etc etc) that will eat into payload fraction I find. But fine I'll give aerospike another try.

[selfish_meme]

I wasn't trying to convince you, I was not even really replying to you, I was just geeking out about building an actual replica, that looks and works the same, it seemed doable at first glance, I did a quick test and it looked good. It took me a while though to get that rocket into orbit with such a high payload fraction. In the replica I would be surprised to achieve higher than 15%.

[selfish_meme]

By the way I spooled up the jets and then staged the aerospike and run it all the way into orbit, just let the jets die naturally.

[selfish_meme]

The payload section in the middle is hard because apparently interstage fairings are as much drag as no fairing at all and heavier (someone worked out they were not occluded correctly on reddit) so it needs to be made out of something else like wing parts.

[Rune]

1 hour ago, selfish_meme said:

The payload section in the middle is hard because apparently interstage fairings are as much drag as no fairing at all and heavier (someone worked out they were not occluded correctly on reddit) so it needs to be made out of something else like wing parts.

You didn't see the pic I posted on the thread I did yesterday, did you? In a fit of inspiration I decided payload bays make the best payload bays, and replicating a design is the best way to replicate a design... Ok, maybe I cheated and didn't use spikes either, but it works with 20mT in the bay and looking like it does. The point being that the distinctive feature of the SERV was being a bunch of tanks surrounding a payload bay... and that is very doable in KSP:

 

Rune. Now, I only need a gazillion of hours devoted to it to solve all the little and not-so-little bugs.

Edited December 15, 2015 by Rune

[selfish_meme]

I did see it, I just didn't look closely, its very similar to what I had in mind but without aerospikes. I doubt I would make one now as it would be too close to yours.

[Rune]

18 minutes ago, selfish_meme said:

I did see it, I just didn't look closely, its very similar to what I had in mind but without aerospikes. I doubt I would make one now as it would be too close to yours.

If you went the spike route, I guarantee you they would look very dissimilar... The margin on this thing is slim enough for the extra engine weight to increase launch mass horribly. You are probably looking at something at least 50% bigger for the same payload, probably twice the GLOW with the knock-back effect on the airbreathing system... And besides, this thing is clamouring for a hinge like MJ's!

Also, there are a ton of other SSTO designs, like the R.O.M.B.U.S., DC-X...

 

Rune. Far be it from me to dissuade you form a cool build!

[selfish_meme]

16 minutes ago, Rune said:

If you went the spike route, I guarantee you they would look very dissimilar... The margin on this thing is slim enough for the extra engine weight to increase launch mass horribly. You are probably looking at something at least 50% bigger for the same payload, probably twice the GLOW with the knock-back effect on the airbreathing system... And besides, this thing is clamouring for a hinge like MJ's!

Also, there are a ton of other SSTO designs, like the R.O.M.B.U.S., DC-X...

 

Rune. Far be it from me to dissuade you form a cool build!

It really does look close but I used 6 way symmetry with three engines each instead of 8 way. And I hadn't actually built the detachable nose (isn't it supposed to be a little orbiter?) yet.

[selfish_meme]

24 minutes ago, Rune said:

If you went the spike route, Iguarantee you they would look very dissimilar... The margin on this thing is slim enough for the extra engine weight to increase launch mass horribly. You are probably looking at something at least 50% bigger for the same payload, probably twice the GLOW with the knock-back effect on the airbreathing system... And besides, this thing is clamouring for a hinge like MJ's!

Also, there are a ton of other SSTO designs, like the R.O.M.B.U.S., DC-X...

 

Rune. Far be it from me to dissuade you form a cool build!

It really does look close but I used 6 way symmetry with three engines each instead of 8 way. And I hadn't actually built the detachable nose (isn't it supposed to be a little orbiter?) yet.

Just looking at that diagram, 12 aerospikes and 28 turbojets in two rings, crazy, would be down to Juno's to make it fit hmmmm

[kmMango]

Awesome. I love the SERV concept, but could never build a working replica under the old aerodynamics. 

Rune, yours is pretty awesome too. I might need to build a RHOMBUS when I get back to my laptop.

Edit: Or an Ithacus...

Edited December 31, 2015 by kmMango