SRB-powered SSTO

[Sharpy]

Okay, so I tend to have silly ideas that don't turn out so bad. Someone took my Poodle on MK2 body idea seriously and there are decent SSTOs that use Poodle as the "intermediate stage".

So today I got the idea... what if I used SRB as the intermediate stage? Godly thrust, moderate weight, size - less than most of my "intermediate" solutions.

So here's BAAAC. Another useless proof-of-concept that can be adapted into something useful - as it made it to the 80km orbit with no payload (other than Jebediah) but with 1500m/s left.

BAAAC is a run off the mill spaceplane, utilizing my "dual propulsion" engines (essentially, NERVs clipped into LF fuselages with Whips at the end) and it would be perfectly capable of orbit without the SRB, if I put, say, structural fuselages or LF fuselages instead of it. Except this family of spaceplanes takes more than a little time to reach the orbit. Way more. They are meant for the "Arduous climb" profile and make it really arduous, like fifteen minutes or so. So I made it into a "fish jump" plane.

It flies moderately well. I'd need to add canards to lift it before the end of the airstrip, but it climbs reasonably fast and handles rather okay.

1100 in horizontal flight, and we begin the climb.

Firing the nukes at 26km, when the jets die

The nukes managed to pull me up to 50km but not much more. No lift, no horizontal speed to speak of, and TWR way below 1.

This all changes the moment the BACC is fired.

It takes only a moment and the situation afterwards doesn't look all that different at a glance. Still the same nukes, still good 400m/s short of orbit, still not even above 70km. Oh, at least the SRB is full so the TWR is a little better...

But the truth is the 800 or so m/s of delta-V from when the SRB burned made a world of difference. The gravitational drag dropped so much that currently the nukes can keep up with it and keep pushing the apoapsis ahead of them. 80km apoapsis good 4 minutes ahead. Periapsis easily reachable. Oh, and hears deployed, whoops, forgot to fold them after launch.

At this point, 7 minutes after launch, orbit is just a formality.

Well, these are KER stats after I made it to the orbit.

Afterwards, it's a run-off-the-mill SSTO landing.

(with obligatory three crashes until I finally didn't botch my approach.)

Anyway, the point is:

SRBs make decent intermediate engines.

Edited July 21, 2015 by Sharpy

[EdusacconBR]

we can use them as an engine that decouples, but then it woundnt be a ssto anymore

[Sharpy]

Oh, but why?

Not to mention they are awesome structurally. A strong central beam that doesn't bend or break?

[ajburges]

Interesting concept. The weakness of a LF only design is the low TWR of Nukes. SRBs give you a good kick when you need it most and are pretty light once expended. (a BACC is 1.5 t a FL-T800 is .5 plus the extra engine mass) the bad ISP is offset by the efficiency of jets. I would light the SRB just as the jets flame out though.

Such a plane is less flexible than a liquid engine craft, but could be a relatively good way to get a nuclear space plane to orbit. Kickbacks would be the way to go because of their better Isp and dry mass ratio.

I will need to experiment with this.

Edited July 21, 2015 by ajburges

[Col_Jessep]

we can use them as an engine that decouples, but then it woundnt be a ssto anymore

Technically: yes. The goal of many SSTOs is to reduce cost for repeated launches and I could see that working out since SRBs are cheap. Refueling might be tricky though. =P

I wonder if it wouldn't be better to fire the SRBs earlier. Two reasons: You are shedding weight earlier which makes it easier for your NERVAs to finish the job and you can use the high TWR of the SRB to ascend steeper until you have left the thicker parts of the atmo behind you. At 35km you could point the nose back to the horizon and push your AP.

[cantab]

Nice!

Now I wonder if one using a replaceable booster could work. Perhaps make a fuselage of cargo bays with the rear open and dock the booster inside.

[spudman2]

a bloody SRB on a bloody SSTO

You... I...

Keep doin' what you're doin'. +rep.

[AbacusWizard]

Just thought of something... an SSTO can essentially be defined as a vessel that is capable of reaching orbit without jettisoning anything except reaction mass (if it didn't jettison reaction mass, it wouldn't be a rocket in the first place).

So let's say you use a solid rocket booster to get a bunch of ∆v... and then jettison the SRB, but first position yourself in such a way that you're jettisoning it in the retrograde directon, and therefore the detachment pushes you in the prograde direction. That means you're using the empty SRB as reaction mass, so technically it counts as fuel, so you're technically an SSTO! Right? Eh? Eh?

[Sharpy]

You were right - launching the BACC right after jet flameout saved me some 200 dV to orbit - and a plenty of time too.

Right after the jet flameout

After the BACC burned out

And on the orbit:

[Edax]

Since there is no way to refuel solid fuel in orbit, that booster becomes totally worthless once it's been used right? Which means your better off with a chem/lf fuel tank and engine assuming you have some kind of refueling station around Kerbin.

[Col_Jessep]

You were right - launching the BACC right after jet flameout saved me some 200 dV to orbit - and a plenty of time too.

Nice! =3

[Sharpy]

Edax: Unless you're going to Laythe, the jets are a dead weight too. Plus empty SRBs are quite light.

Yes, LF tanks and LF engines wouldn't be bad, except it's hard to find ones of reasonable thrust in reasonable size. You won't get all that much kick out of a Reliant or Aerospike, and putting a Rhino on an SSTO is... well, a questionable practice.

...currently working on a Kickback-powered MK3 plane.

Besides, I'm not saying it's the best solution. It's just a viable solution. Especially for SSTO that carry payload to the orbit instead of refueling there and traveling through half the universe.

Edited July 21, 2015 by Sharpy

[Brofessional]

Engines inside engines is too much clipping for me.

I've used SRB's in SSTO's before, by placing the SRB inside Mk2 cargo bays and clipping an adapter on the end. The lack of throttle makes them quite impractical though.

[Vanamonde]

This report of a mission has been moved to Mission Reports.

[Sharpy]

Why not a showcase in spaceship exchange?

[spudman2]

you're technically an SSTO! Right? Eh? Eh?

Well.... no. The point of an SSTO is to keep the hardware (fuel is not hardware) for reuse. SRB's can't be reused (mostly), so from an economic standpoint they are a poor choice of engine.

[Sharpy]

Well, by that account, my decoupler propulsion would classify as a single-stage (even though it's all stages!) since it's all just reaction mass being expelled. But while yes, the line is somewhat blurred, I don't think getting a single 2.5KNs impulse while ejecting a mainsail with two orange tanks backwards allows you to classify that mainsail+tanks as reaction mass

On a separate note - while my MK3 attempts failed miserably yesterday (I just built the thing too big, too heavy and too unwieldy) for a while it was "not pure SSTO" with a very interesting approach to staging... as the payload is removed and CoM moves way back past CoL, to move the CoL back I placed a pair of canards on structural pylons. "Changing wing geometry" by discarding a pair of winglets.

Edited July 22, 2015 by Sharpy

[cantab]

Well.... no. The point of an SSTO is to keep the hardware (fuel is not hardware) for reuse. SRB's can't be reused (mostly), so from an economic standpoint they are a poor choice of engine.

In fairness, in real life SRBs can be refurbished and re-used, and in KSP if you can land back at the runway you can recover and then launch the craft anew for only the cost of the fuel.

[spudman2]

I placed a pair of canards on structural pylons.

Would you mind showing us (me) a picture of this craft?

for only the cost of fuel

Hence why I said mostly. Can't refuel on-orbit.

[Xeldrak]

I built something similar a while ago: the CRoBAR

However, the CRoBAR drop it's airbreathing engines so it's not a real SSTO more a Single-and-a-half-Stage-To-Orbit.

Edited July 24, 2015 by Xeldrak