Amateur rocket to orbit

[mikegarrison]

54 minutes ago, Steel said:

Your sailboat analogy doesn't quite work, because equally the baseball bats used by amateur players are made by professionals, but they are still amateur players. In rocketry, the rocket is "the player" in a sense, rather than the tool used by the player (like a sailboat or a baseball bat).

Um, no. That was my whole point, which you seem to have missed.

Another way to put it is that I am a professional aeronautical engineer. If I were to build a plane in my garage, would it be an "amateur" plane or a "professional" plane? Planes are my profession, and have been for almost 30 years, but building one in the garage is classically "amateur".

The baseball or the sailboat racing is considered "amateur" even though the equipment is made by professionals. So does it matter that professional engineers build a Blue Origin rocket? What if Blue Origin built a rocket that sent Jeff Bezos to the moon, and that was the only thing they ever did? Would that have been an amateur trip to the moon?

If those university students that built that rocket got some grants to do it (or maybe even just to be in school), were they amateurs?

In sports, amateur has always been a rather fuzzy surrogate for "upper class making sure the lower classes can't play in the same event". That's really where "amateurism" came from -- if you needed to get paid to play the sport rather than doing it as a hobby because you were rich enough to support it, the upper classes didn't want you messing up their fun and possibly beating them at their own game.

[qzgy]

Can we stop arguing over the definition of "amateur"? It doesn't really matter IMO.

Yes, we should consider not to cheap out, but we should try to make it cost effective. End of story.

[sevenperforce]

And now that this semantic argument has been beaten to death...

Just now, qzgy said:

Can we stop arguing over the definition of "amateur"? It doesn't really matter IMO.

Yes, we should consider not to cheap out, but we should try to make it cost effective. End of story.

You can really just replace "amateur" with "minimum cost" since professional-grade turbopumps, SRBs, and the like are going to eliminate amateurs from a cost basis before anything else.

Once we have a good first-order approximation of stage mass fraction, I can start some basic optimization...if nothing else, to see how many OTRAG clusters you'd need if we took a purely serial approach. 

After I have the calculations down for a serial approach, we can start moving from serial to parallel to see how small we can make the overall vehicle.

Then start optimizing.

 

[mikegarrison]

1 minute ago, sevenperforce said:

And now that this semantic argument has been beaten to death...

You can really just replace "amateur" with "minimum cost" since professional-grade turbopumps, SRBs, and the like are going to eliminate amateurs from a cost basis before anything else.

Once we have a good first-order approximation of stage mass fraction, I can start some basic optimization...if nothing else, to see how many OTRAG clusters you'd need if we took a purely serial approach. 

After I have the calculations down for a serial approach, we can start moving from serial to parallel to see how small we can make the overall vehicle.

Then start optimizing.

 

Sounds good to me.

[sevenperforce]

Mass fraction will depend on tank and intertank wall thickness, which in turn will depend on combustion pressure. So if anyone can dig up something instructive regarding the range of possible combustion pressures for decomposed HTP burning with jellied petrol in a hybrid rocket engine, I'd appreciate it. Just a general range is enough to get things going.

The higher the pressure, the stronger (and heavier) the tank and intertank will have to be in order to contain it. I know it's easy enough to find tank pressures for pressure-fed peroxide-kerosene rockets, but I don't know how hybrid rocket combustion pressures differ.

[Steel]

I agree, it's a long argument!

Spoiler

 

(below is my reply to @mikegarrison in case he wants to read my reply. I've hidden it so as not to clutter the tread.)

Spoiler

20 minutes ago, mikegarrison said:

Um, no. That was my whole point, which you seem to have missed.

Another way to put it is that I am a professional aeronautical engineer. If I were to build a plane in my garage, would it be an "amateur" plane or a "professional" plane? Planes are my profession, and have been for almost 30 years, but building one in the garage is classically "amateur".

The baseball or the sailboat racing is considered "amateur" even though the equipment is made by professionals. So does it matter that professional engineers build a Blue Origin rocket? What if Blue Origin built a rocket that sent Jeff Bezos to the moon, and that was the only thing they ever did? Would that have been an amateur trip to the moon?

If those university students that built that rocket got some grants to do it (or maybe even just to be in school), were they amateurs?

In sports, amateur has always been a rather fuzzy surrogate for "upper class making sure the lower classes can't play in the same event". That's really where "amateurism" came from -- if you needed to get paid to play the sport rather than doing it as a hobby because you were rich enough to support it, the upper classes didn't want you messing up their fun and possibly beating them at their own game.

I may have phrased it badly, but the point I was trying to  make was that it could be considered amateur if the person building it was not employed to build it. For instance, you can be (and in fact are) a professional Aerospace Engineer, but since you're building the plane unpaid in your garage, it is an amateur plane. On the other hand if you were being paid to build the plane, it professional. That's where I draw the line.

However, the pilot can still be an amateur pilot whether the plane is amateur or not.

TL;DR: Basically, without complicating this with sports metaphors, this is what I think: Specifically in the case of rocketry, a rocket is amateur as long as none of the people working on it are employed to do so. (So university students are technically amateur still, since they're not employed)

Anyway I feel like we're getting bogged sown in semantics and the thread has moved on!

 

Back on topic: I've been reading through and it seems that you guys are looking at parallel staging and clustering. Surely for a low-cost rocket this is the opposite of what you want to do as you're introducing multiple failure points and extra monitoring requirements. It would only take one engine to underperform by 20% and your whole stack cartwheels off into the distance.

I think this is why most small launchers use a three stage solid motor design. Light and let it go, no monitoring, no complicated guidance and control systems, no complex stage separations. That's not to say solid motors are the way to go (for reasons discussed before) but I think having potentially 8 cores (including strap on boosters) for a first stage is an absolute nightmare for failure mitigation

Edited July 10, 2017 by Steel

[sevenperforce]

4 minutes ago, Steel said:

Back on topic: I've been reading through and it seems that you guys are looking at parallel staging and clustering. Surely for a low-cost rocket this is the opposite of what you want to do as you're introducing multiple failure points and extra monitoring requirements. It would only take one engine to underperform by 20% and your whole stack cartwheels off into the distance.

I think this is why most small launchers use a three stage solid motor design. Light and let it go, no monitoring, no complicated guidance and control systems, no complex stage separations. That's not to say solid motors are the way to go (for reasons discussed before)

The advantage of clustering over a single-stick approach is that amateur (read: low-cost) solid and hybrid rocket manufacturing does not scale well. With differential throttling, some underperformance can be compensated for. Besides, these would be designed for reuse, so they would all be static-fired ahead of time.

[Steel]

6 minutes ago, sevenperforce said:

The advantage of clustering over a single-stick approach is that amateur (read: low-cost) solid and hybrid rocket manufacturing does not scale well. With differential throttling, some underperformance can be compensated for. Besides, these would be designed for reuse, so they would all be static-fired ahead of time.

Ok, but with the lightweight and low cost flight computer that is required for these things, along with the limited amount of sensors that will be available due to mass restrictions, can you actually program enough redundancy into the computer so that it can make those throttle movements in the required time? Basically what I'm saying is that surely with clustering comes a decision to either install more sensors and computers to be able to compensate for harware failures in-flight, or you just have to accept a higher chance of mission failure due to a hardware failure?

EDIT: Also please understand I'm not here to discourage, just to play Devil's advocate.

Edited July 10, 2017 by Steel

[DerekL1963]

8 hours ago, TheEpicSquared said:

About the valves, wouldn't having 7 valves+piping significantly increase mass, and cost as well? Legitimate question, does a finely-moving valve really cost that much? 

The big thing it adds is a nightmarishly stupid level of complexity.  Not in the wiring harness and control, but in installing and checkout.  And a variable number of injection points just screams "combustion instability" to me.

Edited July 11, 2017 by DerekL1963

[TheEpicSquared]

@sevenperforce If RWs don't work, would magnetorquers be too small? And another  thing, I just realized that we most likely will not be able to recover the center core, considering that it is being accelerated to nearly orbital velocities. 

Also, it wouldn't be that difficult (relatively speaking, of course) to just add another strap-on, if 3 aren't enough. In fact, I think this would be better since now you have direct control over each direction, left, right, forward and backward. This is better than a 3 strap-on variant, which would require multiple cores to throttle down to get the same four-directional control. 

Assuming this configuration, I have a rather complicated idea for throttling and staging.

If A, B, C, D and E are the cores, then looking at the rocket from the bottom would result in this view:

      A

B   C   D

     E

If thrust allows, I say that shortly after liftoff, cores B and D stay at full thrust while cores A and E throttle down to 75%. Core C, the sustainer, throttles down to 50% thrust. 

Because B and D are running at a higher thrust, they separate first. At this point, A and E throttle back up to 100% until they burn out, with C remaining at 50% thrust. When A and E burn out and separate, C would throttle back up to 100% until burnout and separation, at which point the kick motor would fire at apoapsis and complete orbit. 

Edited July 11, 2017 by TheEpicSquared

[wumpus]

15 hours ago, sevenperforce said:

The advantage of clustering over a single-stick approach is that amateur (read: low-cost) solid and hybrid rocket manufacturing does not scale well. With differential throttling, some underperformance can be compensated for. Besides, these would be designed for reuse, so they would all be static-fired ahead of time.

Re-use?  As in after launching?  We've done the parachute idea to death in the various spacex threads and it appears too heavy (possibly not for side boosters).  A pressure fed hypergolic engine wouldn't have the restart and throttling issues that powered landing traditionally has, but you just cranked your control logic requirements up exponentially.  You also need some sort of SONAR/RADAR/LIDAR system to measure final altitude.  I don't think this system is up to adding spacex-style paddles.

Parachute reuse on the side boosters (or possibly something like spaceship one's wings/vanes) might be a plan.  It also might require a few more boosters.  It also requires land-land flight, and landing legs need to support arbitrary landing sites (presumably any old desert landscape, although you might have some control over the final position if you get in line of site quickly enough).

[sevenperforce]

14 hours ago, Steel said:

Ok, but with the lightweight and low cost flight computer that is required for these things, along with the limited amount of sensors that will be available due to mass restrictions, can you actually program enough redundancy into the computer so that it can make those throttle movements in the required time? Basically what I'm saying is that surely with clustering comes a decision to either install more sensors and computers to be able to compensate for harware failures in-flight, or you just have to accept a higher chance of mission failure due to a hardware failure?

EDIT: Also please understand I'm not here to discourage, just to play Devil's advocate.

Absolutely. You won't hurt my feelings, haha.

From my numbers, I just don't think clustering is avoidable. Without six-or-seven-figure budgets, hobbyists/amateurs/whatever simply can't manufacture solid-fueled or hybrid-fueled stages large enough to launch an orbital rocket serially. Copenhagen Suborbitals has built and tested some of the largest hybrid rockets ever, and they encounter serious combustion instability once they get up to 100 kN or so. Of course they also struggle with using nitrous oxide, since its low gaseous density gives the stages a pitiful propellant fraction of 40%.

Amateurs can, however, construct peroxide-based hybrid rockets with much higher propellant fractions as long as they are kept reasonably small. And that's where clustering comes in. I don't like the computational requirements either, but I don't think there's another way.

9 hours ago, DerekL1963 said:

The big thing it adds is a nightmarishly stupid level of complexity.  Not in the wiring harness and control, but in installing and checkout.  And a variable number of injection points just screams "combustion instability" to me.

Yes, checkout is difficult. However, combustion instability won't be a problem; the injection is behind the catalyst bed, so as soon as the HTP hits the catalyst bed it will decompose anyway.

Anyway, this is a smaller factor.

5 hours ago, TheEpicSquared said:

@sevenperforce I just realized that we most likely will not be able to recover the center core, considering that it is being accelerated to nearly orbital velocities.

Yeah, most likely not. Of course, given that decomposed HTP is hypergolic with petrol, we can potentially use a second hybrid stage in serial, in which case we would be able to recover the core but not the second stage:

Although not shown, you could have anywhere from 2 to 6 parallel boosters, and they could separate simultaneously or pairwise.

5 hours ago, TheEpicSquared said:

Also, it wouldn't be that difficult (relatively speaking, of course) to just add another strap-on, if 3 aren't enough. In fact, I think this would be better since now you have direct control over each direction, left, right, forward and backward. This is better than a 3 strap-on variant, which would require multiple cores to throttle down to get the same four-directional control. 

Assuming this configuration, I have a rather complicated idea for throttling and staging.

If A, B, C, D and E are the cores, then looking at the rocket from the bottom would result in this view:

      A

B   C   D

     E

If thrust allows, I say that shortly after liftoff, cores B and D stay at full thrust while cores A and E throttle down to 75%. Core C, the sustainer, throttles down to 50% thrust. 

Because B and D are running at a higher thrust, they separate first. At this point, A and E throttle back up to 100% until they burn out, with C remaining at 50% thrust. When A and E burn out and separate, C would throttle back up to 100% until burnout and separation, at which point the kick motor would fire at apoapsis and complete orbit. 

That's not too terribly complicated, really. A very kerbal solution to be sure. There's definitely a limit to asparagus staging; unless you have crossfeed, there's only so much you can do with throttle control. If you go much beyond 1+2+2, you'll end up with viciously diminishing returns.

As a very basic initial approach, what if we use the height, diameter, and mass of the successful HEROS-3 sounding rocket, and simply swap in our propellants? I'll try to do the math on that and see what kind of performance we would have.

1 minute ago, wumpus said:

Re-use?  As in after launching?  We've done the parachute idea to death in the various spacex threads and it appears too heavy (possibly not for side boosters).  A pressure fed hypergolic engine wouldn't have the restart and throttling issues that powered landing traditionally has, but you just cranked your control logic requirements up exponentially.  You also need some sort of SONAR/RADAR/LIDAR system to measure final altitude.  I don't think this system is up to adding spacex-style paddles.

Parachute reuse on the side boosters (or possibly something like spaceship one's wings/vanes) might be a plan.  It also might require a few more boosters.  It also requires land-land flight, and landing legs need to support arbitrary landing sites (presumably any old desert landscape, although you might have some control over the final position if you get in line of site quickly enough).

A hybrid pressure-fed rocket is going to be MUCH sturdier than the wispy flying tin can that is the Falcon 9 first stage. Side boosters (and possibly the first-stage core, if serial staging was used) would chute down and land on their sides.

[qzgy]

7 hours ago, TheEpicSquared said:

@sevenperforce If RWs don't work, would magnetorquers be too small? And another  thing, I just realized that we most likely will not be able to recover the center core, considering that it is being accelerated to nearly orbital velocities. 

Also, it wouldn't be that difficult (relatively speaking, of course) to just add another strap-on, if 3 aren't enough. In fact, I think this would be better since now you have direct control over each direction, left, right, forward and backward. This is better than a 3 strap-on variant, which would require multiple cores to throttle down to get the same four-directional control. 

Assuming this configuration, I have a rather complicated idea for throttling and staging.

If A, B, C, D and E are the cores, then looking at the rocket from the bottom would result in this view:

      A

B   C   D

     E

If thrust allows, I say that shortly after liftoff, cores B and D stay at full thrust while cores A and E throttle down to 75%. Core C, the sustainer, throttles down to 50% thrust. 

Because B and D are running at a higher thrust, they separate first. At this point, A and E throttle back up to 100% until they burn out, with C remaining at 50% thrust. When A and E burn out and separate, C would throttle back up to 100% until burnout and separation, at which point the kick motor would fire at apoapsis and complete orbit. 

As @sevenperforce said, it could work and could be a lot more complex. I'm just curious how we would measure stage burn out. Like, would we use some sort of fuel sensor able to detect the napalm or would we use an accelerometer to detect when thrust from the 2 side boosters cuts out?

1 hour ago, sevenperforce said:

Side boosters (and possibly the first-stage core, if serial staging was used) would chute down and land on their sides.

Assuming of course, that a successful deployment system is found.

[wumpus]

1 minute ago, qzgy said:

As @sevenperforce said, it could work and could be a lot more complex. I'm just curious how we would measure stage burn out. Like, would we use some sort of fuel sensor able to detect the napalm or would we use an accelerometer to detect when thrust from the 2 side boosters cuts out?

Assuming of course, that a successful deployment system is found.

I'm really wondering about "just parachute down".  Their velocity at staging is roughly the delta-v they (+sustainer) are providing.  A good place to start is to divide the stages evenly by delta-v, which means that a "half stage" should either provide 1/4 to 1/3 of the delta-v.  I'd have some doubts about it hitting the atmosphere at ~2000m/s, let alone the rest of the flight down.  Perhaps a back burn could help things.  Expect a few drogues or something like spaceship 1, as the parachute will be heavy enough if it only has to handle landing duty.

Landing on land sounds iffy (especially the "flying over land" license), although a fairly hefty aluminum tube on it's side should be able to land in the desert easily enough.  Landing on water sounds easier (just have enough empty space above the booster, preferably made of fiberglass) assuming you can get to it by boat, but this lowers the acceptable max-Q the rocket can sustain.

[sevenperforce]

As a random note, I just found out that GIRD-9, the first liquid rocket ever launched by the USSR, used LOX and jellied gasoline. Here's a photo of the unique combustion chamber (GIRD-9 at left; GIRD-10, a true liquid bipropellant rocket, at right): 

Additional thought: if we pressurize with N₂O rather than atmospheric air, we could squeeze a little extra dV out of everything, but I dunno if it's worth the extra trouble.

Anyway, back to the drawing board. I took a look at the HEROS-3 rocket and its propellant fraction is just so poor that it would be virtually impossible to get it into orbit no matter how many clusters we used. Thankfully, we have some improvements.

The optimal O/F ratio for nitrous+paraffin (from page 18 here) is around 9.5, so if we go back to the HEROS-3 rocket, we can estimate that the 88 kg of propellant was 8.4 kg of paraffin and 79.6 kg of N₂O. HTP is 2.2x as dense as N₂O and jellied gasoline about the same as paraffin (0.9 kg/L). So the HEROS-3 carried around 9.33 L of paraffin and 119 L of pressurized N₂O, for a total propellant volume of 128 L. HTP+kerosene has a mass O/F ratio of 7, corresponding to a volume O/F ratio of 3.23 (VERY nice), so I estimate we could pack 30.2 L of jellied petrol and 97.8 L of HTP into the same rocket, for a total propellant mass of 169 kg.

This increases our propellant fraction from a damning 54%  on HEROS-3 to a much more respectable 69.2%. Other improvements can be made, since we don't need avionics packages on each booster, but given the likelihood of including chutes, etc. we can ignore that. A 69% propellant fraction at an average specific impulse of 300 seconds gives us a whopping 3.47 km/s of dV on a single-stick. NOW we're cooking with gas (pun intended)!

1 hour ago, qzgy said:

As @sevenperforce said, it could work and could be a lot more complex. I'm just curious how we would measure stage burn out. Like, would we use some sort of fuel sensor able to detect the napalm or would we use an accelerometer to detect when thrust from the 2 side boosters cuts out?

Repeated static fires will give us a very good projection of stage burnout time, and a pressure sensor in the main chamber above the fuel load (near the head pressure valve in the earlier diagrams) will show an instant pressure drop at fuel burn-through. This pressure drop will immediately send a signal to cut the oxidizer flow and trigger the staging sequence.

Quote

Assuming of course, that a successful deployment system is found.

I'm all for pneumatic deployment using residual HTP. RCS thrusters can be repurposed as open-cycle pneumatic pushers using a simple airtight seal.

53 minutes ago, wumpus said:

I'm really wondering about "just parachute down".  Their velocity at staging is roughly the delta-v they (+sustainer) are providing.  A good place to start is to divide the stages evenly by delta-v, which means that a "half stage" should either provide 1/4 to 1/3 of the delta-v.  I'd have some doubts about it hitting the atmosphere at ~2000m/s, let alone the rest of the flight down.  Perhaps a back burn could help things.  Expect a few drogues or something like spaceship 1, as the parachute will be heavy enough if it only has to handle landing duty.

Landing on land sounds iffy (especially the "flying over land" license), although a fairly hefty aluminum tube on it's side should be able to land in the desert easily enough.  Landing on water sounds easier (just have enough empty space above the booster, preferably made of fiberglass) assuming you can get to it by boat, but this lowers the acceptable max-Q the rocket can sustain.

Tumbling terminal velocity shouldn't be too terribly high but this is definitely an issue to consider. 

Edited July 11, 2017 by sevenperforce

[Kerbart]

Something you might want to involve early on in your plans, as a lot of other things seem to depend on it, is flight control. Obviously you're limited to autonomous guidance, and you'll need a way to figure out attitude (pretty tricky to figure when under acceleration), velocity and altitude. Those are not exactly trivial to retrieve, it seems, especially at a tight budget. It's not like there's a magic box that can give it to you.

[DerekL1963]

3 hours ago, sevenperforce said:

14 hours ago, DerekL1963 said:

The big thing it adds is a nightmarishly stupid level of complexity.  Not in the wiring harness and control, but in installing and checkout.  And a variable number of injection points just screams "combustion instability" to me.

Yes, checkout is difficult. However, combustion instability won't be a problem; the injection is behind the catalyst bed, so as soon as the HTP hits the catalyst bed it will decompose anyway.

0.o  The issue isn't whether the HTP is decomposed or not - it's uneven flow at the inlet face of the cat pack and thus potentially uneven flow at the outlet face.

[sevenperforce]

One nice thing about hybrids is that you can increase thrust to pretty much whatever you want by varying chamber length and throat diameter, without the mass penalty you get from increasing a liquid-rocket engine design.

Let's say we use the Lambda 4S dV budget as a design target. I want to see if we are anywhere in the right ballpark. The Lambda 4S essentially had five stages. With the 1+2+2 first stage, a hybrid second stage, and a solid kick stage, as proposed by @TheEpicSquared, we're looking at a similar dV layout for our LV.

If we want a total of around 6 km/s of dV on the upper hybrid stage plus our kick stage and we are guesstimating 4 kg payload (avionics + aeroshell + cubesat), then let's look at what we can do with White Lightning. Let's assume a propellant fraction of 80% on a hand-cast SRB and a vacuum specific impulse of 220 seconds. A little iterative modeling in Excel and I've got a sweet spot at 35 kg of solid propellant (any more than that, and too much impulse is lost on the hybrid stage lifting the weight of the SRB casing). The 300-s hybrid stage gives 2.67 km/s and the kick stage gives 3.08 km/s, for a total dV of 5.655 km/s.

Just with these two stages, we'd already be dealing with a high-suborbital spaceflight.

1 hour ago, Kerbart said:

Something you might want to involve early on in your plans, as a lot of other things seem to depend on it, is flight control. Obviously you're limited to autonomous guidance, and you'll need a way to figure out attitude (pretty tricky to figure when under acceleration), velocity and altitude. Those are not exactly trivial to retrieve, it seems, especially at a tight budget. It's not like there's a magic box that can give it to you.

There are Android phones with accelerometers and gyroscopes; I wonder if someone could write an app to calculate telemetry in real-time based solely on that.

45 minutes ago, DerekL1963 said:

0.o  The issue isn't whether the HTP is decomposed or not - it's uneven flow at the inlet face of the cat pack and thus potentially uneven flow at the outlet face.

Fair point.

[Kerbart]

1 hour ago, sevenperforce said:

There are Android phones with accelerometers and gyroscopes; I wonder if someone could write an app to calculate telemetry in real-time based solely on that.

I'm sure there are Arduino accelerometers and gyros. Reading out from a phone through an app might not give you good enough refreshment rates. There are even complete inertial navigation units available. Hooking that up to a (cheap) smartphone for telemetry purposes might not be a bad idea; or if you plan on retrieving the payload, you could even write it to a memory stick for post-flight analysis (at the very least as a backup).

 

[sevenperforce]

On to the first/parallel stage(s).

Let's assume we use @TheEpicSquared's 1+2+2 configuration, with a core sustainer and two pairs of side boosters. GLOW is 1.2 tonnes. Determining when to throttle down and how far to throttle down is itself a major optimization problem, but let's start by assuming the following sequence, just to get ballpark performance:

1. Launch: core at 100%, booster pair A at 100%, booster pair B at 100%
2. 25% propellant consumption: core at 50%, booster pair A at 50%, booster pair B at 100%
3. Booster pair B burnout: separation, core at 50%, booster pair A at 100%
4. Booster pair A burnout: separation, core at 100%

At this rate, we should expect to get 336 m/s from launch to throttledown, 817 m/s from throttledown to booster pair A burnout, 471 m/s from booster pair A burnout to booster pair B burnout, and 245 m/s remaining on the core, for a total of 1.87 km/s off the first stage. Now, the sooner we can downthrottle and the deeper we can downthrottle, the better, but TWR will become an issue if we downthrottle too low or too early. Combustion instability is also a reason not to downthrottle too aggressively.

With 1.87 km/s for the parallel/staggered first stage and 5.655 km/s for the second stage and kick stage, we're already up to 7.52 km/s, which is DEFINITELY in the neighborhood of orbital flight. Of course, we have to subtract out gravity, pressure, and drag losses. But it's certainly in the right ballpark. And there's nothing saying we can't make a slightly wider stage than HELOS-3 and pack more propellant in, especially because we don't need the ultra-extreme fineness ratio typical of low-isp, high-thrust amateur rockets (amateur rockets make up in part for their poor isp by their very high thrust, cutting down on gravity drag but necessitating an ultra-streamlined profile to minimize more significant aerodrag losses).

If our final design is generally in this ballpark, we can also get an idea of the minimum thrust rating we'll need in order to make it all work. In this configuration, the lowest TWR condition is at secondary booster burnout, when the core is lifting its own partially-fueled mass plus the mass of the upper stage, kick stage, and payload. In order to ensure sufficient TWR at this point in the ascent, I would suggest that an entirely full core ought to be able to lift an upper stage, a kick stage, and the payload with no less than a 2:1 TWR. Crunching the numbers, this means a naked stage TWR at least 4.4 and a single-motor thrust of at least 10.5 kN, about the same as the HELOS-3. Launch TWR would be 4.56:1, and hybrid-stage mass flow at 100% is 3.57 kg/s with a burn time of 47.3 seconds at full throttle.

EDIT: I added in fuel incorrectly; correction below.

Edited July 11, 2017 by sevenperforce

[TheEpicSquared]

7 minutes ago, sevenperforce said:

On to the first/parallel stage(s).

Let's assume we use @TheEpicSquared's 1+2+2 configuration, with a core sustainer and two pairs of side boosters. GLOW is 1.5 tonnes. Determining when to throttle down and how far to throttle down is itself a major optimization problem, but let's start by assuming the following sequence, just to get ballpark performance:

1. Launch: core at 100%, booster pair A at 100%, booster pair B at 100%
2. 25% propellant consumption: core at 50%, booster pair A at 50%, booster pair B at 100%
3. Booster pair B burnout: separation, core at 50%, booster pair A at 100%
4. Booster pair A burnout: separation, core at 100%

At this rate, we should expect to get 336 m/s from launch to throttledown, 817 m/s from throttledown to booster pair A burnout, 471 m/s from booster pair A burnout to booster pair B burnout, and 245 m/s remaining on the core, for a total of 1.87 km/s off the first stage. Now, the sooner we can downthrottle and the deeper we can downthrottle, the better, but TWR will become an issue if we downthrottle too low or too early. Combustion instability is also a reason not to downthrottle too aggressively.

With 1.87 km/s for the parallel/staggered first stage and 5.655 km/s for the second stage and kick stage, we're already up to 7.52 km/s, which is DEFINITELY in the neighborhood of orbital flight. Of course, we have to subtract out gravity, pressure, and drag losses. But it's certainly in the right ballpark. And there's nothing saying we can't make a slightly wider stage than HELOS-3 and pack more propellant in, especially because we don't need the ultra-extreme fineness ratio typical of low-isp, high-thrust amateur rockets (amateur rockets make up in part for their poor isp by their very high thrust, cutting down on gravity drag but necessitating an ultra-streamlined profile to minimize more significant aerodrag losses).

If our final design is generally in this ballpark, we can also get an idea of the minimum thrust rating we'll need in order to make it all work. In this configuration, the lowest TWR condition is at secondary booster burnout, when the core is lifting its own partially-fueled mass plus the mass of the upper stage, kick stage, and payload. In order to ensure sufficient TWR at this point in the ascent, I would suggest that an entirely full core ought to be able to lift an upper stage, a kick stage, and the payload with no less than a 2:1 TWR. Crunching the numbers, this means a naked stage TWR at least 4.4 and a single-motor thrust of at least 10.5 kN, about the same as the HELOS-3. Launch TWR would be 4.56:1, and hybrid-stage mass flow at 100% is 3.57 kg/s with a burn time of 47.3 seconds at full throttle.

@sevenperforce Mind telling me what GLOW is? Also, how do you get those numbers? Could you tell me the formulas you use?

This second stage would also be hybrid, using our HTP+napalm configuration, yes? In this case, what would be the relative size of the first and second stages? Purely guessing here, I'd say that it would have to be around the same size or just a bit smaller than the first stage, since it still has to produce the majority of the 5.655km/s of Dv, but it's already high in the sky which reduces gravity and aerodrag losses. It would also be lighter than the first stage, since we could eliminate the chutes and their deployment system, saving mass. The RCS could go at the bottom, so it would be far away from the center of mass (which would be closer to the top of the stage due to the kick motor+payload). Just before burnout, the small amount of remaining HTP could be redirected to the RCS system to spin up the rocket for spin stabilization during the kick motor burn.

Also, what type of communication would we use? From the KSP Community CubeSat thread, S-Band seems ridiculously expensive, so it seems like VHF is the way to go. Unless we want a live feed of our rocket going up, I don't think an S-Band system is necessary. VHF can still take pictures, in the event that we would want cameras on the rocket. Speaking of which, are onboard cameras really necessary? I'm against it for rockets this small, since the weight of a camera makes up a lot more of the entire rocket's mass with a small rocket than with a big one. And we'd get all the telemetry we'd need anyway, so we don't need visual confirmation, IMO.

[Steel]

11 minutes ago, TheEpicSquared said:

@sevenperforce Mind telling me what GLOW is? Also, how do you get those numbers? Could you tell me the formulas you use?

 

I believe it's a play on GTOW (Gross TakeOff Weight) for aircraft, so I think it stands for Gross Launch (Off) Weight. Could have just been reading that wrong all this time though!

38 minutes ago, sevenperforce said:

On to the first/parallel stage(s).

Let's assume we use @TheEpicSquared's 1+2+2 configuration, with a core sustainer and two pairs of side boosters. GLOW is 1.5 tonnes...

Also, with a launch weight that low you're beating a Lambda 4S by almost an order of magnitude.

[sevenperforce]

23 minutes ago, TheEpicSquared said:

@sevenperforce Mind telling me what GLOW is? Also, how do you get those numbers? Could you tell me the formulas you use?

This second stage would also be hybrid, using our HTP+napalm configuration, yes? In this case, what would be the relative size of the first and second stages? Purely guessing here, I'd say that it would have to be around the same size or just a bit smaller than the first stage, since it still has to produce the majority of the 5.655km/s of Dv, but it's already high in the sky which reduces gravity and aerodrag losses. It would also be lighter than the first stage, since we could eliminate the chutes and their deployment system, saving mass. The RCS could go at the bottom, so it would be far away from the center of mass (which would be closer to the top of the stage due to the kick motor+payload). Just before burnout, the small amount of remaining HTP could be redirected to the RCS system to spin up the rocket for spin stabilization during the kick motor burn.

Also, what type of communication would we use? From the KSP Community CubeSat thread, S-Band seems ridiculously expensive, so it seems like VHF is the way to go. Unless we want a live feed of our rocket going up, I don't think an S-Band system is necessary. VHF can still take pictures, in the event that we would want cameras on the rocket. Speaking of which, are onboard cameras really necessary? I'm against it for rockets this small, since the weight of a camera makes up a lot more of the entire rocket's mass with a small rocket than with a big one. And we'd get all the telemetry we'd need anyway, so we don't need visual confirmation, IMO.

GLOW is Gross Lift Off Weight. All the numbers are determined by the rocket equation: dV = v_e*ln( m₀/m_f ). At each step, you take the total inert mass (payload + structure) plus the remaining propellant in each stage, and that's your m₀; then you subtract the propellant you burn, and the difference is your m_f. The natural log of this ratio, times the exhaust velocity (specific impulse times 9.8) gives you dV.

In the above design, the second stage, first stage, and boosters would share a common core. The second stage would admittedly be slightly less massive due to not having recovery gear, but I didn't factor that into my calculations.

EDIT: fixing my numbers.

Edited July 11, 2017 by sevenperforce

[sevenperforce]

Okay, so I completely screwed up the numbers on the first go-round because I only included half the propellant of the parallel boosters. Fixed that. Correct numbers follow.

• T-0:02. Valves open at low throttle for hypergolic ignition.
• T=0:00. Ignition times five confirmed, valves to full throttle. TWR is 3.5:1, GLOW is 1.5 tonnes
• T+0:12. +443 m/s. Core and one booster pair throttle down to 50%. Throttledown drops TWR from 4.11 to 2.88.
• T+0:48. +1,229 m/s. First booster pair jettison, second booster pair throttles back up to 100%. TWR drops from 4.37 to 3.79.
• T+1:05. +748 m/s. Second booster pair jettison, core throttles back up to 100%. TWR drops from 4.88 to 2.69.
• T+1:14. +245 m/s. Core burnout; TWR has climbed up to 2.93. Separation and S2 ignition.
• T+2:02. +2,570 m/s. S2 burnout; TWR climbed from 3.68 to 8.83. Spin-up, separation, kick stage ignition and orbital insertion. Kick stage is +3,085 m/s.

Cumulative dV is 8.32 km/s, which is definitely in the neighborhood of LEO. We can squeeze out more dV by deeper throttling, depending on the tradeoff between TWR, gravity drag, and aerodynamic drag.

If you use a bolted-together four-core sustainer cluster rather than a single-stick core sustainer, you get 8.65 km/s cumulative dV and slightly lower gravity drag losses.

Edited July 11, 2017 by sevenperforce

[sevenperforce]

1 hour ago, TheEpicSquared said:

Also, what type of communication would we use? From the KSP Community CubeSat thread, S-Band seems ridiculously expensive, so it seems like VHF is the way to go. Unless we want a live feed of our rocket going up, I don't think an S-Band system is necessary. VHF can still take pictures, in the event that we would want cameras on the rocket. Speaking of which, are onboard cameras really necessary? I'm against it for rockets this small, since the weight of a camera makes up a lot more of the entire rocket's mass with a small rocket than with a big one. And we'd get all the telemetry we'd need anyway, so we don't need visual confirmation, IMO.

I hate not having a camera. Maybe one on the first-stage core, if we can reasonably assume that it will survive recovery at least partially intact? That wouldn't unduly cut into payload budget but it would give a nice view of ascent. A camera on the terminal stage probably wouldn't work anyway, due to spin-stabilization and the weight of a live video transmitter.