Amateur rocket to orbit

[DerekL1963]

3 hours ago, wumpus said:

The cat pack would presumably be *before* the combustion chamber (unless all your catalysts can survive combustion temperature.  Silver certainly won't).

It *should* be before the engine, with the decomposed products ducted to the combustion chamber.  But the current design of this engine shows it embedded in the combustion chamber.

[wumpus]

On 7/31/2017 at 1:03 PM, DerekL1963 said:

It *should* be before the engine, with the decomposed products ducted to the combustion chamber.  But the current design of this engine shows it embedded in the combustion chamber.

The last design I saw had nearly all napalm outside the combustion chamber.  I'm not sure why it was drawn and posted.

 

On 7/17/2017 at 10:44 AM, sevenperforce said:

You can't have first-stage use of SRBs, simply because burnout times will vary. If one of the side boosters burns out before the opposite one (and it will), the whole stack starts to cartwheel and then rips itself apart, Challenger-style. The only possible use of a first-stage SRB would be as the core, but that wouldn't help much.

I was googling side boosters to learn a few things about Falcon Heavy, when I ran into this: http://www.aircommandrockets.com/howitworks_1.htm
It describes how to build a water powered rocket (toy) with side boosters.  I'm wondering if anyone has any experience with model rocketry to be sure that side boosters are beyond amature rocketry.  I'd want to start with a TWR of ~3 and enough delta-v to get to just passed the transonic region (yes, this means nearly max thrust at maxQ.  This might not be ideal).

http://www.rrs.org/a-hydrogen-peroxide-rocket/  this seems to have failed entirely due to such rockets.  A backup means of removal might be needed (or simply fire them off in a korelev cross before they burn out).

 

Edited August 2, 2017 by wumpus
avoid doubleposting

[shynung]

@sevenperforce I'd say that having a catalyst bed directly exposed to the combustion chamber is generally a bad idea; silver won't stay solid at the temperatures that cat bed is going to be exposed in.

Much better to have a separate preburner containing the catalyst bed, into which a small amount of HTP would be decomposed, that exhausts into the combustion chamber joined with the rest of the HTP. Once combustion is achieved, the catalyst/preburner chamber can either be left on or closed off; undecomposed HTP can sustain combustion after it has been started.

 

[wumpus]

1 hour ago, shynung said:

@sevenperforce I'd say that having a catalyst bed directly exposed to the combustion chamber is generally a bad idea; silver won't stay solid at the temperatures that cat bed is going to be exposed in.

Much better to have a separate preburner containing the catalyst bed, into which a small amount of HTP would be decomposed, that exhausts into the combustion chamber joined with the rest of the HTP. Once combustion is achieved, the catalyst/preburner chamber can either be left on or closed off; undecomposed HTP can sustain combustion after it has been started.

I'd be fairly shocked if the whole thing didn't have to resemble a traditional hybrid (which in turn resembles a traditional SRB).  The oxidizer and all parts needed to supply the oxidizer are on top of the stage, and the fuel/combustion chamber is simply packed below as a tube of fuel with a hole in the center that acts as a combustion chamber.  Makes the whole "combustion chamber" problem moot until you get to the nozzle (which almost certainly won't have oxidizer cooled trickery, but that could be on the table just in case).

This assumes a sufficiently solid form of napalm that won't melt due to heating or otherwise shift position during acceleration and all the vibration of a rocket (do pressure fed rockets vibrate, or is that mainly thanks to turbopumps?).  Most of the drawings I've seen appear to be based on such a shift, but somehow halt the flow into a properly formed combustion chamber.  I have doubts about how you form a semi-liquid form into a combustion chamber (presumably some sort of screen, but that will require great testing of napalm being extruded under various weights of fuel and acceleration and all the backpressure from the combustion chamber).  Sounds like it involves many dangerous experiments.

[shynung]

@wumpus The original idea was having the oxidizer tank inside the fuel tank, so the design can use only one ullage tank, among other things.

And yeah, it'd need a great amount of tests to get it right. Using jellied propellant without any bladder system means there's a risk of the pressurant (air) simply blowing a tunnel through the jelly, not touching most of the propellant. There's also the issue of properly mixing the fuel and oxidizer, given that the fuel only has a very limited surface area exposed to oxidizer flow, unlike a common hybrid rocket.

[sevenperforce]

On 7/31/2017 at 2:12 PM, wumpus said:

I was googling side boosters to learn a few things about Falcon Heavy, when I ran into this: http://www.aircommandrockets.com/howitworks_1.htm
It describes how to build a water powered rocket (toy) with side boosters.  I'm wondering if anyone has any experience with model rocketry to be sure that side boosters are beyond amature rocketry.  I'd want to start with a TWR of ~3 and enough delta-v to get to just passed the transonic region (yes, this means nearly max thrust at maxQ.  This might not be ideal).

http://www.rrs.org/a-hydrogen-peroxide-rocket/  this seems to have failed entirely due to such rockets.  A backup means of removal might be needed (or simply fire them off in a korelev cross before they burn out).

Yeah, I've considered using a drop-away parallel booster design like that for some of my sugar rocket stacks.

The trouble is that no amateur motors will have simultaneous ignition, identical thrust curves, and simultaneous burnout.

The linked peroxide rocket had a very low TWR, so low that they added a single small SRB to give it added thrust climbing up the launch guide so it could be aerodynamically stabilized. The SRB was supposed to separate at the top of the launch guide, but instead it did not and produced off-axis thrust, which torpedoed the whole thing.

Wouldn't be an issue for us, because a hybrid has all the thrust we could possibly want.

6 minutes ago, shynung said:

@wumpus The original idea was having the oxidizer tank inside the fuel tank, so the design can use only one ullage tank, among other things.

And yeah, it'd need a great amount of tests to get it right. Using jellied propellant without any bladder system means there's a risk of the pressurant (air) simply blowing a tunnel through the jelly, not touching most of the propellant. There's also the issue of properly mixing the fuel and oxidizer, given that the fuel only has a very limited surface area exposed to oxidizer flow, unlike a common hybrid rocket.

The plan would be that gravity and acceleration are the primary forces promoting fuel feed; ullage pressure at the top is only high enough to prevent chamber pressures from pushing up on the propellant. A lightweight annular plastic disc may be used at the top of the fuel column to prevent tunneling.

The "chamber" would most likely need to be longer and narrower than shown above, as with most smaller rocket engines.

25 minutes ago, wumpus said:

This assumes a sufficiently solid form of napalm that won't melt due to heating or otherwise shift position during acceleration and all the vibration of a rocket (do pressure fed rockets vibrate, or is that mainly thanks to turbopumps?).  Most of the drawings I've seen appear to be based on such a shift, but somehow halt the flow into a properly formed combustion chamber.  I have doubts about how you form a semi-liquid form into a combustion chamber (presumably some sort of screen, but that will require great testing of napalm being extruded under various weights of fuel and acceleration and all the backpressure from the combustion chamber).  Sounds like it involves many dangerous experiments.

It'll be more solid than gel. We may be able to utilize non-Newtonian properties.

1 hour ago, shynung said:

@sevenperforce I'd say that having a catalyst bed directly exposed to the combustion chamber is generally a bad idea; silver won't stay solid at the temperatures that cat bed is going to be exposed in.

Much better to have a separate preburner containing the catalyst bed, into which a small amount of HTP would be decomposed, that exhausts into the combustion chamber joined with the rest of the HTP. Once combustion is achieved, the catalyst/preburner chamber can either be left on or closed off; undecomposed HTP can sustain combustion after it has been started.

Pressure in the tank is going to be higher than in the chamber, so flow will always be in the direction of the chamber. Thus, the flow of undecomposed HTP should be enough to cool the catpack.

[qzgy]

1 minute ago, sevenperforce said:

We may be able to utilize non-Newtonian properties.

Something like oobleck, or more like ketchup?

An oobleck-esque gel would be nice, reduces the chance of the gas blowing by, but then you run into the problem of having it not spill out everywhere.

[shynung]

@sevenperforce Throwing another idea here. If restartability is not a concern, we can do away with the catalyst bed if we inject a solution of calcium permanganate into the initial charge of HTP entering the combustion chamber. It acts as a decomposition catalyst, igniting the first load of HTP. The rest of the HTP can sustain the combustion without needing decomposition themselves.

[sevenperforce]

1 minute ago, qzgy said:

Something like oobleck, or more like ketchup?

An oobleck-esque gel would be nice, reduces the chance of the gas blowing by, but then you run into the problem of having it not spill out everywhere.

Yeah, I'm not sure whether it should be shear-thickening or shear-thinning.

We would likely use a paraffin or similar liner to form the initial combustion chamber shape, so flow on the pad isn't a problem. If it's something like oobleck, then the pressure from the combustion chamber should keep it fairly firm while it flows from above, though I'm not entirely sure that would work. 

2 minutes ago, shynung said:

@sevenperforce Throwing another idea here. If restartability is not a concern, we can do away with the catalyst bed if we inject a solution of calcium permanganate into the initial charge of HTP entering the combustion chamber. It acts as a decomposition catalyst, igniting the first load of HTP. The rest of the HTP can sustain the combustion without needing decomposition themselves.

We could simply have the initial combustion "wall" be painted/sprayed with a suitable catalyst. The reason I like a catalyst bed is that it would probably allow lower throttling.

[DerekL1963]

1 hour ago, sevenperforce said:

The plan would be that gravity and acceleration are the primary forces promoting fuel feed; ullage pressure at the top is only high enough to prevent chamber pressures from pushing up on the propellant

Well then, this engine is DOA.  (Or, more accurately, yet another way this engine is DOA.)  No way gravity provides sufficient flow given the small size of your combustion chamber, not with a fuel thick enough that it doesn't simply flow out the nozzle in the first place.  Gravity (and acceleration) requires distance and time to accelerate the fuel, and there simply isn't enough vertical height in any practical launch vehicle.
 

1 hour ago, sevenperforce said:

The "chamber" would most likely need to be longer and narrower than shown above, as with most smaller rocket engines.

Given that this is a hybrid, your chamber is going to need to be ten to twenty times as long as shown in the diagram to provide sufficient burning area.  The problem is you have a burning surface of a solid/hybrid rather than the burning volume that a liquid engine has.  (Basically, since you don't have an injector for the fuel, you're not going to have efficient mixing of the propellants.)

Seriously, I don't grasp why you're stuck on a radical (and extraordinarily unlikely to work) engine design, rather than a conventional (and well understood and characterized) design.

[sevenperforce]

5 minutes ago, DerekL1963 said:

Well then, this engine is DOA.  (Or, more accurately, yet another way this engine is DOA.)  No way gravity provides sufficient flow given the small size of your combustion chamber, not with a fuel thick enough that it doesn't simply flow out the nozzle in the first place.  Gravity (and acceleration) requires distance and time to accelerate the fuel, and there simply isn't enough vertical height in any practical launch vehicle.

Not sure what geometry you're imagining, but neither gravity nor acceleration require time or distance to "accelerate" the fuel. The downward force on the fuel column will be several gees from launch to burnout.

5 minutes ago, DerekL1963 said:

Given that this is a hybrid, your chamber is going to need to be ten to twenty times as long as shown in the diagram to provide sufficient burning area.  The problem is you have a burning surface of a solid/hybrid rather than the burning volume that a liquid engine has.  (Basically, since you don't have an injector for the fuel, you're not going to have efficient mixing of the propellants.)

Seriously, I don't grasp why you're stuck on a radical (and extraordinarily unlikely to work) engine design, rather than a conventional (and well understood and characterized) design.

Yes, the chamber will need to be longer and thinner. With a gel, however, you aren't dealing solely with a burn surface; there will be significant droplet entrainment and mixing.

The only reason I'm still proposing this design is that IF it turns out to work in preliminary tests, and works well, it offers significant advantages. Of course it might not.

[DerekL1963]

5 hours ago, sevenperforce said:

Not sure what geometry you're imagining, but neither gravity nor acceleration require time or distance to "accelerate" the fuel. The downward force on the fuel column will be several gees from launch to burnout.

o.0  I'm not imagining anything, I'm using real world physics.  For your fuel to flow, it has to accelerate from zero speed - and that takes time and distance. And the force is only one gee at ignition, which just adds further difficulty - the force flowing your fuel varies radically from ignition to burnout.
 

5 hours ago, sevenperforce said:

Yes, the chamber will need to be longer and thinner. With a gel, however, you aren't dealing solely with a burn surface; there will be significant droplet entrainment and mixing.

o.0  A few messages back, it wasn't a gel...  and now it's a gel again.  Except when you're mistakenly relying on the properties of oobleck to make it once again, not a gel.  Seriously, your fuel magically changes properties to magically address whatever objection is being made.

And that's setting aside that there's no mechanism creating droplets, let alone driving entrainment and mixing.
 

6 hours ago, sevenperforce said:

The only reason I'm still proposing this design is that IF it turns out to work in preliminary tests, and works well, it offers significant advantages.

Feel free to enumerate them.

[wumpus]

55 minutes ago, DerekL1963 said:

o.0  I'm not imagining anything, I'm using real world physics.  For your fuel to flow, it has to accelerate from zero speed - and that takes time and distance. And the force is only one gee at ignition, which just adds further difficulty - the force flowing your fuel varies radically from ignition to burnout.

I can only assume that it needs to be pressurized in a way similar to the oxidizer, and with nearly the same pressure (the forces pushing back from the combustion chamber and the forces pushing down by gravity+accelleration are the same).  No idea if a traditional hybrid tube will hold its shape.

[Steel]

From the comments I'm seeing (along with some of the questions I've raised previously) I think maybe ditching the radical hybrid engine design might be for the best. After all, if the aim of all of this is to produce a white paper detailing a cheap and simple way for amateur to attempt big-boy orbital-class rockets, surely it's hard to justify a novel and completely untested design over one that has 60 years worth of experience behind it?

[DerekL1963]

3 hours ago, Steel said:

After all, if the aim of all of this is to produce a white paper detailing a cheap and simple way for amateur to attempt big-boy orbital-class rockets, surely it's hard to justify a novel and completely untested design over one that has 60 years worth of experience behind it?

That's pretty much my point.  If you're trying to build on the cheap, and your first step is a complex chemical and mechanical engineering research program to study a completely unknown engine - you're already on the wrong path.

[qzgy]

I think, at this point, it seems enough points have been raised to make a novel hybrid engine a bit of a stretch, and also not really viable.

Edited August 4, 2017 by qzgy

[shynung]

Our goal is a paper design of an amateur-accessible orbital rocket, right? I agree with the others that we should start with a proven design, to lighten the load on engine R&D. We can scale the design as needed later on.

[wumpus]

While the current design has few virtues for the first 1[.5] stages, the ease of ignition at altitude and high Isp make it ideal for a second stage.  I'm still curious to see if there are any ways to salvage this design (if only for an upper stage).  One quick way to do it would be to find a sufficiently solid hydrocarbon (I'm guessing coal is too heavy, but perhaps a conglomerate of anthracite and napalm could be held back by a thin screen) and a traditional hybrid.  Solving this could well be easier than coming up with a new high Isp rocket + ignition.

I'd recomend going straight to a traditional hybrid for stages 1.5 (first sustainer plus side boosters).  While N₂O might have lousy Isp, that isn't terribly important when you have the dry mass of two more stages on top of the stage.  Furthermore, the early stages have to be *big* and will make up most of the mass of the rocket (slightly bigger with poor Isp, but they would be big even with LOX+RP1).  This type of rocket is relatively easy for the amature to create, and presumably scale up and no materials really draw huge oversight flags (the scale involves reduces sourcing the HTP for stage 2 to a much less significant problem than doing the first stages as well). [PS. After looking at the real delta-v requirements, I'm less sure.  I just really doubt amature construction of "real rocket fuel" at the size these boosters would need for a few km/s of delta-v.]

One possible idea for a high Isp hybrid would be to use alcohol as a fuel.  This is used (in liquid form) by Copenhagen Suborbitals and was infamously used in the V2s.  It has nearly (with a few percent) the Isp of RP1 but a temperature nearly 1000 degrees less (presumably C/Kelvin, but this information might be from Ignitition! and thus F).  The important thing for building a hybrid is that the "alcohol" is typically only 100 proof (50% alcohol, 50% water.  Ignition! mentions that such tanks had losses from thirst techs), and this isn't far from "jello shots" (a quick google showed me 25% alcohol), I have to wonder if non-potable thickeners (soap, parrafin, any napalm ingredients) might allow for a sufficiently solid alcohol at 50%.  This wouldn't have the hypergolic properties of HTP+napalm, but it looks like we are drifting away from that anyway (hopefully it won't need restarting).  Note that as this likely uses LOX (with all the cryogenic and explosive issues) it likely shouldn't be used in the first stage unless necessary.

Have we even looked at a delta-v budget?  Last I looked, the project was to throw a satellite massing a handfull of grams into orbit (I suspect that recent 4g "picosat" supplier could supply/sponser one).  I've found in KSP that starting with equal budgets per stage for delta-v is a good starting point (other methods involve geometrically reducing mass.  If using N₂O in one stage and LOX in another, I'd happily use vastly more N₂O than LOX).  Knowing that you need at least 2km/s in the first stage or 5km/s in your second coudl cull plenty of design options.
http://www.hobbylinc.com/aerotech-g40-4w-white-lightning-motor-g-composite-model-rocket-engine-74004 (one of these for a final stage.  I wouldn't be too shocked if we had a multi-stage kicker, especially with a sufficiently small payload).  Expect to want the whole system to have a few thousand m/s of deta-v (I didn't see enough information for detailed calculation).
The second stage: minumum 3,000 m/s delta v and could be requierd up to twice that (I'd go to multiple stage kickers before trying that much).
First stage sustainer + side boosters.  Too many requirements: Needs a TWR>1 for liftoff (preferably >1.5, possibly as high as 3 to negate gravity losses), needs to escape the atmosphere (you don't want the atmosphere interferring with the stage 2 nozzle design).  Should supply a few thousand delta-v, especially since it is likely eating a couple km/s of gravity and aero losses.  Expect it to be far larger than you really intended.  Note that Space Ship One, easily the most famous rocket with a hybrid engine, only needed a  1.4km/s delta-v budget.  I'm less sure that hybrids are up to the full task.

Looking at the big picture I'm wondering just how much we could gain from a sufficiently small sat [10g] and at least 3 solid (amonium percholorate or similar, commercially sourced) kicker stages.  Having a small dry mass works wonders for the rocket equation.  Also this means that much of the final control logic need not be on the last stage, but could sit a stage or two behind and "not count" as final dry mass.  Cutting down any delta-v requirements on the "nasty" second stage would be a huge benefit (although since it likely has the highest Isp, it still will remain the workhorse for delta-v).

[Hesp]

6 hours ago, wumpus said:

 I have to wonder if non-potable thickeners (soap, parrafin, any napalm ingredients) might allow for a sufficiently solid alcohol at 50%.

Try to investigate glycerol. It's been used as a fuel for internal combustion engines even if difficult to burn. The high % of oxygen in the molecule may reduce the oxidizer needed.

[DerekL1963]

7 hours ago, wumpus said:

While the current design has few virtues for the first 1[.5] stages, the ease of ignition at altitude

It's theoretical ease of ignition - we don't know for certain (or really at all) if it's even possible to ignite and attain a stable burn in the first place.  There's a raftload of known unknowns, and who knows how many unknown unknowns.

[wumpus]

19 hours ago, DerekL1963 said:

It's theoretical ease of ignition - we don't know for certain (or really at all) if it's even possible to ignite and attain a stable burn in the first place.  There's a raftload of known unknowns, and who knows how many unknown unknowns.

I've even pointed out that igniting the solid rocket motors might be harder than we think.  LOHAN had issues with that (I think they were solid), but that probably stablized to high atmospheric temperatures.  I think most of the unknowns can be determined with small amounts of HTP, but you would still need to source it in some quantities.  I suppose that any serious rocketery project will require at least one "dangerous chemicals chemist", and that such experiments with HTP really isn't out of the ordinary in such a project.

But I understand that somebody expecting such a whitepaper to be useful could fairly quickly do a few experiments and realize that the HTP portion won't work.  If the rest of the rocket simply relies on that level of Isp (it pretty much has to, any replacement would likely be LOX+vodka or better) you could pretty much have a pointless paper.

Basically "rocket science" is going to have a lot of unknowns.  Anyone trying to build a rocket is going to be spending a lot of time with google/a library, then even more time in the lab doing things that aren't *quite* in the textbooks (and even stuff that are, thanks to simplifications and misunderstandings).  Plenty of engineering has "fudge factors" that include a lot of areas that might have been poorly approximated, or perhaps the materials weren't quite followed exactly right.  Rocket science has some of the smallest "fudge factors" around, and that requires an exact understanding of what is going on that you won't get in an internet whitepaper.

We can tell a newbie how to build a rocket that will make orbit in KSP.  We might even be able to describe a minimal one (my favorite is a thumper+small fuel tank+terrier+mark 1 capsule) or one for RSS.  But writing a paper that tells you how to build a rocket without access to NASA parts is going to require a great deal of time in the lab.  The best we can hope is that the lessons learned won't invalidate the basic concept of the rest of the rocket (a real danger with a high Isp center stage that might not work).

Scott Manley recently had a few things to say about the phrase "how hard can it be*?" (the "fly through the Mun" clip).  Let's just understand that doing the whitepaper will be the most trivial job of making a real rocket.  The final itteration is unlikely to have any bearing on the original design, but it just might help to have something to at least start your experiments on (another reason I suggested the first stage be a standard hybrid.  That presumably will scale up in a known way and keep the big parts roughly in a well predicted budget).  

* there was a youtube channel called 'how hard can it be' that tried to build a rocket (to orbit/space?) on its second show (the first show was a flying house).  Their big rocket was destoryed returning no telemetry.  Oddly enough, there was at least one more show.

 

AMMENDUM (and possible helpful breakthrough):

I was poking around for background on another thread when I came across a paper detailing a simple bipropellant mixture: both easily obtainable and non-cryogenic (hopefully ideal for pressure-fed engines), hypergolic with an Isp~300 (in vacuum).  Sorry, no spoilers.
https://tfaws.nasa.gov/TFAWS06/Proceedings/Aerothermal-Propulsion/Papers/TFAWS06-1026_Paper_Herdy.pdf

Edited August 5, 2017 by wumpus
1. "how hard can it be". 2. Ammendum.

[DerekL1963]

3 hours ago, wumpus said:

I think most of the unknowns can be determined with small amounts of HTP, but you would still need to source it in some quantities.

o.0  Why?  The issue (outside of the cat pack, a serious and significant development task in and of itself) isn't the HTP, it's the magical-pixie-dust fuel.
 

3 hours ago, wumpus said:

Basically "rocket science" is going to have a lot of unknowns.  Anyone trying to build a rocket is going to be spending a lot of time with google/a library, then even more time in the lab doing things that aren't *quite* in the textbooks (and even stuff that are, thanks to simplifications and misunderstandings).

o.0 Huh?  There's a huge difference between a conventional LOX/NOX+Hydrocarbon engine, where you *can* look stuff up - and this Rube Goldberg engine about which darn near nothing is known.   Not to mention you greatly exaggerate (or fail to completely grasp) the nature of the difference between the "lab time" (virtually all developmental) required for a conventional engine*, or that (complex research + development) required for this monstrosity.  They aren't even in the same league.  Apples and the thing least like apples you can imagine.

* People build them in their garage for heaven's sake.

[wumpus]

1 hour ago, DerekL1963 said:

* People build them in their garage for heaven's sake.

People build "normal" rockets in their garage.  9km/s is a completely different requierment.  This was the whole point of doing something more or less like that for the first 1.5 stages, because we know it works (and Isp isn't nearly the issue).  There would need to be a rather large second stage that has a nearly "pro" level Isp and delta-v, this can't be made by googling 'rocket engine'.

I also can't recall many multi-stage large garage-built rockets.  Multi-stage is such a basic design feature that I'm guessing it is either vastly more difficult than I would expect and/or (probably and) has a ton of regulations heaped upon it to keep the amatures from getting too high.

I'm really wondering what it would take for the N₂O+Propane pressure-fed system.  It looks like a pretty straigtforward "build in a garage" setup, I'm wondering if there is a great big catch (exhasut products are lethal? exhaust temperature melts the nozzle? something else?).  It appears to have the same requirements as any non-crygenic liquid fuel, less the crazy catalyst requirements from HTP.  Of course you have two to liquids to deal with instead of only one as in a hybrid, but everything else looks pretty good.

[Hesp]

5 minutes ago, wumpus said:

I also can't recall many multi-stage large garage-built rockets.  Multi-stage is such a basic design feature that I'm guessing it is either vastly more difficult than I would expect and/or (probably and) has a ton of regulations heaped upon it to keep the amatures from getting too high.

Basically because a (multistage) rocket with that delta-v and a guidance system is often recognized as a ballistic missile.

[DerekL1963]

4 hours ago, wumpus said:

This was the whole point of doing something more or less like that for the first 1.5 stages, because we know it works (and Isp isn't nearly the issue).

No - we do not know it works.  It's a completely new engine requiring a fuel with...  let's just say 'unusual' requirements.  Claiming that the difficulty is on par with a conventional engine is ludicrous at best.