This post started as a 7 tweet tweet book . Several people asked me to put it together in a way that can be permanently linked. This is the result.
I'm assuming there is a valid business case. The business concept with cost estimates and identified customers etc... needs to work before any of this matters. I assume that any business evaluation will include the business fundamentals. I personally believe that a low cost dedicated nanosat launcher makes business sense and I have personally worked on such a business plan. I see a lot of launcher startups being funded so I must assume that others agree the business case might work.
What I see looking out at the world is a complete failure in technical evaluation of the various launcher startups. I'm trying to provide a rough guide for technical evaluation of a launcher startup.
To state the obvious, the first goal of any launcher company will be to get payloads into orbit,
that is the hard part of the business. Added services, responsiveness, cool features etc... are all irrelevant until you have a vehicle getting to orbit.
So what does it take to get to orbit and how does that differ from getting to space?
The official line of space is 100Km or 328K ft. In the scale of the earth a tiny distance. Suborbital vehicles, like Space Ship 1, and Blue Origin's New Shepard are suborbital. They go to space, but they do not go into orbit. This may be useful as a tourist vehicle, and for limited scientific experiments, but its not really relevant if you are trying to build an orbital vehicle.
The potential energy necessary to get to space (straight up) is (approximately)
where m is mass, g is acceleration due to gravity and h is height. So for one kg
1*9.81*100,000m = 981,000 nm of energy.
The minimum speed to be in orbit is around 7500 m/sec
so for the same 1 Kg in orbit we add an additional
0.5*7500*7500=28,125,000N-m of energy.
So the total is ~ 29,106,000 NM or 29 times the energy of a suborbital vehicle.
So if a space company says we have gotten to space and we will be in orbit real soon now; realize it's like an airline company offering you a ride from San Francisco to New York City, yet the farthest their plane has ever flown is from SFO to Sacramento.
So what does it take to actually get to orbit:
- Mass Fraction (Hard)
- Motor performance (Hard)
- Guidance (Medium Hard and getting easier)
- Regulatory approval (Medium Hard)
Mass fraction is the ratio of propellant mass to empty weight. For example a 2L coke bottle has a mass fraction of about 40. I.e., the bottle full of coke weights 40 times what it weighs empty.
This is a good mental model for a rocket, the mass ration of the upgraded Falcon 9 is estimated to be 25:1.
Any orbital vehicle using chemical propulsion will look like a big tank.
So when evaluating a potential launcher company you need to ask what is the Mass Fraction of hardware you have on hand and can show me. When evaluating this don't accept paper numbers, insist on knowing what the numbers are for the hardware on hand.
Rocket motors (assuming they work at all) have one major and two minor performance parameters.
- ISP. It's usually stated in seconds. IE a motor with an ISP of 300 will make 300lb sec of thrust for one lb of propellant.Orbital vehicles have had rocket motors varying in ISP from 220 (shuttle big solids) to 450 (high stress staged combustion Lox/H2 SSME) Lox RP1 motors are in the 300 to 330 range.
- Thrust to Weight: What is the ratio of motor weight to motor thrust. If the rocket motor is accounted for in the Mass Fraction of the overall vehicle, this is sort of irrelevant. Its also a bit hard to score as the value varies depending on where you draw the line between the rocket motor and the rest of the vehicle. The Main propellant valves for instance, do they belong to the tank or the Motor?
- Burn Duration: The rocket motor need to last long enough to get you to orbit. This is not really relevant to most liquids, but for small solids, it's really hard to increase this number due to the physics of solid propellant burning and insulation requirements.
I've marked this part of the four fundamentals as Hard. I'd actually score the motor as Medium hard, but getting all the instrumentation and test together to optimize and really know what your motor is really doing pushes this into the hard category.
The rocket needs to go where you want it to go. You'll also have to convince the regulatory agencies that you really do have control of where it's going so it won't be a hazard. As computer simulations get better and better getting this part right gets easier. It's really easy to have the simulation right, and have errors in the vehicle where the actuator direction is backwards or the simulation inertia scale is wrong lbs/kg anyone?
So, does the rocket company have a plan to verify and reality check the hardware with the simulation without destroying the rocket? I test flew my hovering rockets under a forklift so I could discover guidance errors without destroying the vehicle. The difference between this video and this video is the sign of a single term in the guidance equations.
If you're in the U.S. you are going to have to get FAA permission to launch your vehicle.
Where you launch from and how you mitigate risk to the general public are a big part of this process.
It's not really that hard, it just must be planned for in the development of the vehicle. It's not something you can just "Stick OK" a completed vehicle if you have given zero thought to things like ranges, range safety and EC (expected casualty) The FAA is going to care about what the rocket parts can hit if things do not go to plan. That's why I've always given very little credence to inland spaceports like the one in NM or Odessa as a place for orbital launches. If you launch from NM you're going to stage somewhere over the densely populated parts of Tx. Short of a planet wide emergency, like in Seveneves, it's never going to be allowed.
Putting it all together with the rocket equation
The rocket equation
Delta V= ln(MI/MF)*ISP*g
MI initial stage mass, ie the weight of everything....including upper stages and propellant
MF final stage mass ie the weight of everything minus all the burned propellant.
ISP the Motor ISP number. (Not really a fixed number improves with altitude)
g 9.81 m/sec acceleration due to gravity.....
Calculate this Delta V for each stage in the vehicle.
The Mass of the fully fueled stages above the one you are calculating Dv for must be included in both the initial and final weights.
Once you have calculated ALL of the stage Dv and added them up, if the number is > 9000m/sec then you have a chance at getting to orbit. This number will vary somewhat with the vehicle acceleration profile and size. I personally like this paper "How Small can a launch vehicle be" for evaluating concepts as a first order check.
If the number is not yet to 9000m/sec and the plan to fix mass fraction and ISP to get to 9000m/sec is not the number one issue for EVERY technical person working on the vehicle, I would be doubtful they will succeed.
Building a new Rocket is an exercise in creativity. There is not a formula that given inputs generates a rocket. Unless the engineering team shows the ability to design, build, evaluate, and repeat in a creative way, nothing else is going to matter. Building hardware is different than building software, rockets are hardware. When you talk to the CTO/Chief engineer/Chief Wizard without notes can he tell you what his mass fractions, ISP and current achieved weight vs planned weight numbers are?
If he can't then who in his organization can? If no one can, then they are not going to succeed.
Walk through the shop and engineering, do you see a scale? Does every engineer building a piece part know what his weight budget is and if he's going to make the weight budget?
Before XCOR failed, I personally thought about investing in their company several times, every time I got a tour of their facility, I was turned off by the complete lack of weight control. The half finished linx had many parts that look like they belonged on a truck, not a space craft. The complete lack of detailed weight control on all the minor parts spelled failure to me and I never invested for just this reason.
Eventually the team will need to learn operations, and as it grows it will need HR, middle management and all the other parts of a company. But unless it gets the creative engineering part right up front it's going to run out of $$ long before any of these things matter.
To quote from my tweet storm:
- Pretty Paint does not matter.
- Fancy Building does not matter.
- Previous Dinospace experience does not matter.
- Cool animations of a paper rocket do not matter.
- Until they have hardware that can do 9000m/sec DV, operational issues don't matter.
Why doesn't previous dinospace experience matter? Because other than SpaceX, all of the other launch providers had their creative how do we do the base design done in the 60's and 70's. The creative steely eyed missile men that made that fabulous leap to the moon and beyond are no longer part of the current dinospace environment. It's a very different thing to manage and evolve an existing system than it is to create a new one from scratch.
Please feel free to comment on errors or omissions or areas that need clarification in this post. I will try to correct, enhance and improve over the coming few weeks.