"What's a donkey barbeque?"--- I asked of an elderly supervisor at a bank once upon a time. She had told me that they just had one, so I was a little curious about what it was. Well, it's a polite way of saying they had an ass chewing.
It turns out that ass chewing is something I like to do so much. I'm always holding a donkey barbeque on this here blog on a nearly daily basis. I rip everybody, including the great Elon Musk.
Mighty Quinn, my ass!
Donkey barbeques can be held for yours truly too. Today's barbeque is about a chart that was too damned busy. Let's make it a bit simpler that even a caveman can get it. We all like to dumb things down these days. I don't know why I can't get in on the act.
Below is the chart that shows various isp combos for a given delta v, which is to Earth orbit. Everybody, including caveman, are probably familiar with how these work. The following chart can show you why they have to use "stages" for rockets. The delta v requirement is just too doggone tough to make in one single stage.
So, let's look at this chart again and see what we can see from it. For example, to launch from the ground to orbit using a rocket with 300 isp will require that you use 96% of the mass as fuel. That pretty much rules out getting to orbit because you need rocket engines and tanks and what have you. That may take up more than 10% of the launch mass, and you don't even have that much to spare.
Going down the color code isp list, we find methane/lox next at 380 isp. But even that one doesn't get it below 90%. Liquid hydrogen at 450 fares a little better, but comes in at over 88%. That doesn't leave hardly anything for cargo at all. Plus, you don't get 450 at sea level anyway.
Now, if you go to nuclear thermal, you get 62%. That might be possible except for one thing. Nuclear reactors are heavy and their thrust to weight ratios aren't good enough.
An ion engine doesn't have the thrust either. They are restricted to deep space.
Fusion engines could definitely do it at a gaudy 1 million isp. The fuel required for orbit is less than 1%. Only problem is, they don't exist.
Now, that's what I was
chewing on Musk's ass about yesterday. A trip to Mars isn't going to be as challenging in delta v as getting to orbit, but it is still a challenge. You will need 78% fuel to get to Mars orbit from Earth orbit. If you take 10% for engines and whatnot, you'll only have 12% for everything else. Not much margin for a trip that will take months. It's not impossible, but getting large numbers of people all of the way to Mars requires a bit more cleverness that Musk is showing lately. But he's clever enough to figure it out.
I hope by simplifying the charts that this is more clear than I made it yesterday. Now, if you go to a Lagrange point, and refuel there, and from there to Deimos, Musk will be required to devote no worse than 61% for fuel, and that is for the short haul from Earth to the Lagrange point. People may find inconvenience for a short time to be bearable. For the longer haul to Deimos, he will be required to devote only 44% for fuel. That's almost half of his mass will be available for his passengers. It's still a tough haul, but he could make it, and not have his passengers in revolt.
He does even better with nuclear thermal for the long haul. This will bring the mass available for his passengers up to 70% or so. Not exactly an easy trip, but much more comfortable than 12%.
Einstein said that if you can't explain it to a six year old, you don't understand it. Six year old or caveman, maybe this explanation might do it. Now if Musk can get it, we may get somewhere.
Update:
Let's look at an example, the Space Shuttle. The boosters helped get the Shuttle to orbit by supplying up to 83% of its thrust at liftoff, but only fired for about 2 minutes out of the 8 minutes needed to get to orbit. The boosters may have separated at about 2200 meter/sec, leaving 7300 meters/sec to get to orbit. This shaved the necessary mass for fuel for a liquid hydrogen liquid oxygen engine from 88% to about 82%. Since the Shuttle massed at the pad at 4.4 million pounds, this enabled them to bring 6% more to orbit. Let's say that's about the weight of the Shuttle itself at about 260k lbs. The boosters enabled the Shuttle to get to orbit. Duh. That's how staging works, basically.
Can the cave man get it now? Or am I still not 'splaining good enough'ski?
Interesting to note also that drag and gravity losses add about 1600 meters sec to the orbital velocity. If you get around that problem, you can ( almost ) do away with the boosters entirely. But that is a different discussion.
Update:
Playing around with the Shuttle numbers, if you were to fire up an NTR instead of the SSME's of the Shuttle at SRB separation, you could put over 900k lbs in orbit. Of course, that would have been impossible for a number of reasons, but the point should be well taken. By lessening the delta v and increasing the ISP, you can drastically increase the amount of mass that arrives at a destination.
Of course, there are people at these organizations that do this for a living who know this stuff backwards and forwards. But who else except somebody like me who can and will put that information in front of you? Won't it make a difference in how you see things?
For example, the Shuttle is probably seen as the ultimate, but it is nowhere near what we can do. Now, if only we can be allowed to do it.
