Rocketry General Thread

General
813

A larger planet would mean that. A slightly larger planet would alter the economics, and it would depend on how badly the civilization on that planet wanted to get into space.

Here’s a guy who did some math on the problem. There are piles of simplifying assumptions.

This is the chart he includes for launching a one ton mass into low planetary orbit:

Surface                         First        Total       Saturn V 
Gravity   Stages                Stage      Mass, t     Equivalent
 0.5           2             1x RL-10          4.5
 1.0           3             1x   H-1         49.4          0.02
 1.5           3             1x   F-1        249.2           0.1
 2.0           4             5x   F-1       1329.0           0.5
 2.5           5            40x   F-1       8500.9             3
 3.0           6           274x   F-1      50722.2            17
 3.5           7          2069x   F-1     331430.9           100
 4.0           8         20422x   F-1    2836598.4           950
 4.5           8        392098x   F-1   47 million         15000
 5.0           9    3.5 million   F-1  391 million        130000
 6.0          11    400 million   F-1   38 billion      millions
10.0          18        2.88e19   F-1      1.65e21  quadrillions

On the 2g planet, you’d need a rocket half the thrust of the Saturn V to get just one ton into orbit. Yuri Gagarin’s Vostok 1 capsule massed nearly 5 tons. Is that untenable?

In contrast, if you live on the 0.5g planet, space travel is easy. Would those people look at our situation and declare it untenable for space travel? Or do all civilizations figure out a way to get into orbit?

Corollary: Can a civilization develop in an environment where space travel is untenable?

814

I’ll do my best! Boy is this going to be a long post. I wish the forums supported writing math better and I apologize in advance.

The short answer is yes for practical purposes. Since you read about space travel as a hobby I’m going to assume you know what deltaV is, if not it’s basically a metric of how much you can change your orbit. It’s easier to measure it in terms of energy/velocity rather than distance because distance between 2 points varies in an orbit.

Knowing this, given an objects mass we can solve for the velocity necessary to obtain an orbit. Orbital Velocity also varies dependent on altitude, but that is bounded below by sqrt(G*M/R) where G is the gravitation constant, M is the body in question, and R is the distance from the center of mass, or radius + altitude.

So we could conceive of planets that are both more massive and that are larger that would require more deltaV to reach orbit. That may or may not also translate to a ‘taller’ atmosphere that also represents a lower bound to the orbital velocity. For example, we could orbit at 50km, but the atmosphere presents too much drag and it’s simply not possible without continued thrust, so we target higher orbits where there is less drag.

Example time!

Lets take a look at Earth 2.0. Earth 2.0 is everything Earth is but twice as large. Twice the mass and twice the radius! This makes it easy for me because I already know that Earth 1.0 requires 7.8 km/s of deltaV to orbit at around ~400km, but practically requires around 9.5 km/s of deltaV from losses due to drag and gravity. This means that Earth 2.0 requires 15.6 km/s and I’ll assume it takes 19 km/s for simplicity. In all likelyhood it’s more due to the fact the atmosphere is probably thicker and you take twice the gravity losses per second. Ouch.

Looking at a deltaV map to save myself from doing the math, just to land on the moon requires 9.5 + 3.2 + .68 +1.73 or 15 km/s. It would require another 1.73 to get back to low lunar orbit, then ~ 1.6 to lower your periapsis into the Earth atmosphere and aerobrake to the surface. This totals us at 18.44 km/s. It’s debatable, but using all 5 stages of Saturn V it still might not be able to reach orbit on Earth 2.0.

So with current chemical rocket technology, we might be able to reach orbit on Earth 2.0.

To make it as simple as possible, the reason for the : [quote=“Kindred, post:812, topic:445”]
“fuel adds weight which needs more fuel” problem
[/quote]

is because of how the rocket equation works.

The rocket equation, as stated above in another post of mine, is deltaV = Isp * g * ln(m1/m0)

Isp is like the mpg of your rocket engine, but it’s really just exhaust velocity/g. We normalized it to seconds during the cold war so we could compare the stats of rocket engines between us, the europeans, and the russians without doing all these conversions between 5,000 m/s or whatever that is in ft/s.

The most important part is the ln(m1/m0). This is where that issue of adding fuel which needs more fuel to propel is described in the math of rocketry. This is called the mass fraction. m1 is the spacecraft + fuel and m0 is the spacecraft empty. Because you’re taking a logarithm of this, even if you double the amount of fuel, it does not result in a similar increase of deltaV. ln(2*m1/m0) is the mathmatical representation of that and you can plug in numbers in a calculator if you want to play around with that. Just know that m1 > m0 for obvious reasons.

The other alternative to increasing your deltaV is to increase the Isp, which is the exhaust velocity of your engine. Basically, make your propellant leave the rocket faster, and it imparts more energy for a given quantity. This makes sense because kinetic energy = .5mv^2.

There is a theoretical upperbound on the amount of energy stored in chemical reactions due to the potential energy of chemical bonds. So for a given propellant mixture there’s only so much you can do. It turns out hydrogen and oxygen is one of the best bipropellants for a variety of reasons. I won’t go into it unless you ask a specific question, but if you’re interested in rocket fuels I recommend reading Ignition! by Clarke. It’s available for free as a pdf around the internet. The maximum Isp of hydrolox is something like 500s give or take 20%. We’ve managed to create an engine(Space shuttle main engines/RS-25) with an Isp of 455! That’s pretty damn close to the max.

So I’ve been pretty scattered about this because there is so much material to cover so let me try to pull the pieces together. To get enough orbital velocity on a greater planet you need to use a combination of: Splitting up the rocket into multiple stages(creating better mass fractions with each stage produces better overall deltaV), increasing the mass fraction of each stage(More fuel per stage and lighter structures, engines, electronics, etc), and increasing the Isp to maximum possible(This could be done using hydrolox, or exotic tripropellant mixes, or nuclear rockets). Those 3 techniques each come with their own challenges and Earth 2.0 would be an absolute nightmare to get off of.

There’s a lot I skimmed over or just didn’t mention so if something isn’t clear let me know.

Part 2. Nozzles.

There is a simple analogy that kind of glosses over the details but I hope it will suffice. Ignore the combustion chamber and pressures and all of that. When you burn fuel, its product wants to exit. We want those products to exit exactly downward if our rocket is moving straight up. This is because otherwise the exhaust might move a bit sideways and you don’t get 100% of the momentum possible out of that reaction due to the sine losses of that. I hope that makes sense. Basically if you’re trying to go straight up, but you have particles not moving straight down, you won’t get the most out of those particles, thus reducing the overall energy transferred to the rocket which is a reduction in it’s Isp.

Now we can talk about pressure. The ambient pressure helps push those exhaust particles back downward because that’s what pressure does. It pushes stuff from all directions. I really hope you can visualize it because I’m having trouble explaining it. Without pressure, like in a vacuum, there’s nothing to stop a greater percentage of the exhaust from moving at an angle, so it requires a larger/longer nozzle to make sure a greater percentage moves straight down. This is called the expansion ratio of nozzles if you want to do more indepth reading.

Part 3. [quote=“Kindred, post:812, topic:445”]
Also I don’t really understand the need for pressurizing the gas before the combustion chamber… is it to help combustion? To reduce the size/weight of the combustion chamber?
[/quote]

I don’t follow. There isn’t a gaseous step before the combustion chamber. Gas likes to react with things and propellant reacting before the combustion chamber would be bad news. Is this related to the COPV bottles of helium SpaceX uses to pressure their fuel tanks? If so, the reason is that, unsurprisingly, as you burn propellant it will leave the fuel tank. Try sucking on an empty water plastic bottle and see what happens. The helium is to keep the fuel tanks at the same pressure so the tank doesn’t implode. Very necessary.

Edit: @JB47394 mentioned economics and is pretty much the reason why pushing the limits that I mentioned would be difficult. It would be very impractical.

Edit2: here I am trying to get to sleep hours later and I realize I screwed up my math. Orbital velocity and general gravity for Earth2.0 should be about sqrt(2)*Earth. This explains the slight descrepency between barely making it for me and only 4 stages with JBs source. Check your math people!

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815

Excellent and thorough explanation. Thank you so much!

816

This is assuming the use of chemical rockets, though.

With ENPP, well, sky is no more the limit

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817

That’s because chemical rockets are the practical first step. If chemical rockets aren’t practical, no research will go into them and it’s even harder to make the logical leap to something like Project Orion. It’s not impossible, it just gets exponentially more difficult the more massive your host planet is.

It’s difficult to predict the order of technological advancements with such different initial conditions. There are so many reasons why one technology might prevail against another. Maybe it’s actually more efficient on that planet to use lighter than air craft for mass transportation due the difficulty of more gravity + thicker atmosphere. Maybe their planet didn’t have a carboniferous era to store massive amounts of hydrocarbons for a modern civilization to exploit. There are so many what ifs that it’s hard to predict what and how technological advancements would evolve. Maybe they would love nuclear and go for Orion because of all that. Who knows and it’s all speculation at that point.

Just realize that it’s more difficult to make something like nuclear powered engines, such as NERVA or Orion, to work than a chemical rocket. We use what we use because it’s practical and economical. It’s hard to predict those criteria in a situation like this. I could ask why not just use an antimatter reactor to superheat and expel hydrogen. This provides high thrust and high specific impulse that would not only allow them to leave their planet easily enough, but also create generation ships to other solar systems. The reason is very same we haven’t: it’s damn difficult.

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818

I am not certain Orion is that much harder to work with than advanced chemical rockets.
Conceptually, it is very simple: blow something up below a plate, the plate will be thrown away. Children do it every day with firecrackers and small stones. We have had (modern) rocket technology for 75 years, and we could have built Orion 50 years ago. We didn’t do it for PR and diplomatic reasons, not technological ones
The biggest difficulty is to come up with a nuclear pulse unit.

And even then, you can sort of bypass this with conventional explosives in a thick atmosphere: use cannons on the ground to propel explosives and detonate them below the ship as it is rising, until it reaches orbit (in atmosphere, conventional explosives are powerful enough to propel the craft). Far more inefficient and you need pretty good cannons, but it should still work if nuclear pulse units take too much effort to develop initially.
A hybrid propulsion (HE cannon pulses in atmosphere, nuclear pulses in space) could also help mitigate fallout problems.
And if you have a higher gravity, you probably have a denser atmosphere. Spaceplanes with hybrid propulsion may become interesting (and if you go full nuclear, you can even use a nuclear thermal hybrid spaceplane).
Just because we developed chemical rockets to go for space first doesn’t mean it always has to be the first step to go to space.

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819

Hold up now, detonating that many nuclear warheads in earth atmosphere did raise a few eyebrows. Furthur, the G Forces that the crew would have had to withstand were pretty extreme, although I can’t recall if they would have been survivable.

Launching from higher atmosphere could make more sense in this case, as you pointed out, especially since it’s something we do now on our own tiny world.

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820

So is nuclear fusion.

The biggest difficulty with fusion is keeping the plasma stable.

There’s not much point in speculating about the development of heavy gravity worlds because we don’t know anything about them except that they’d have more gravity. Understanding the way that they play out is based on conjecture and speculation. Every point speculated towards the possibility of a nuclear bomb rocket can be countered by one speculated away.

Ultimately, the takeaway here is that if OUR planet was much more massive, the economics of chemical rocket flight would delay or even eliminate their use. We certainly wouldn’t build an Orion because just the use of nuclear power on the surface is a contentious issue, let alone the idea of setting off a bunch of tiny nukes to get lots stuff into space. On balance, as a society, we don’t believe that there’s a compelling enough reason to do so.

A damper was envisioned that would moderate acceleration to 2-4g. The unmanned version would throw stuff up at around 100g.

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821

Wow, a lot of text to read. Just skimmed over it, so sorry if what I say was already covered, but has anyone actually done the math on a gold mine on Mars using the cost associated with a fully reusable ITS?

In the presentation they claim they would need to decrease the current cost per Kg to Mars by FIVE MILLION PERCENT from current tech, so doing calculations using a current Falcon 9 and Dragon makes as much sense as calculating whether the gold mines in the 17th century in the Americas would make economical sense if you had to swim to Europe carrying the gold in your pockets. Spoiler alert, it would not.

The ITS could send 450 tons to mars if refueled in orbit. Assuming the same capacity of returning cargo to Earth after refueling in Mars(maybe even using in orbit refueling as well), that ends up at a return cargo of almost 20 Billion dollars worth of gold. You could pay for 10 boosters, 10 ships, 10 tankers, 10 billion worth of equipment and fuel, and still have a few billion in profit.

I mean, they want to bring the cost per ton to Mars to 140 thousand. A ton of gold costs 400 times that. Sure, there is also the return trip. but mars has a third of the gravity and would have a refueling plant, so the cost per ton from Mars to Earth would probably not be bigger than 100 times the cost of sending it there(14 million per ton of return).

So, if you send 200 tons of mining equipment to mars paying 28 million, and get 1 ton of gold on the return trip, paying 14 Million, that is a total cost of 42 million for every ton of gold you return. So anything better than a 200 to 1 ratio of equipment mass per mined gold mass results in profit. With these numbers, and assuming nothing blows up, of course.

Can you get 1 ton of gold with 200 tons of equipment? How about 10 tons with 2000 tons? I have no idea. It would depend on how easy it was to extract the gold, how far it was from the refueling plant, how much equipment the refining/etc would use, etc.

I would love to see a professional/economical analysis on this type of thing. And for people saying asteroids are better… Well, you cant aerobrake on an asteroid’s atmosphere, like the ITS plans to do to slow down from over 10km/s when going back to earth. Nor can you easily get carbon from it’s atmosphere to make fuel. But I don’t know, maybe taking an entire asteroid back using something like a massive solar powered ion engine would make sense. And you don’t have to escape a second gravity well, sure.

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822

Myanmar debris: ‘Mystery object lands at jade mine’ - bbc.co.uk

The cylindrical object, found on Thursday in Kachin state, is 4.5m (15ft) long and 1.2m wide

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823

Mars having a bad with there space flight also?

824

oof, rough buddy.

though admittedly spacex’s main selling point isn’t exactly reliability.

825

I wouldn’t say rough…It’s just PR and friendly banter between competitors. Launching rockets is a difficult business, especially when you’re on the cutting edge which was the cause of the Amos-6 anomaly(Unpredictable behavior of oxygen when at very low temperatures). Mistakes are bound to happen. I would bet this won’t be their last launch failure either.

It also undersells the fact that ULA also had a launch anomaly this year and their launches had to be delayed at least a month while they investigated why MECO occured a few seconds earlier than normal. This doesn’t sound like much but their centaur second stage had to use all of their reserve deltaV to achieve orbit. The launch was seconds away from mission failure.

I don’t pay attention enough to ESA/Arianespace stuff enough to know if they had any hiccups this year.

Either way, I know it’s just people having fun but it’s important to look at the big picture. It’s too easy to cherry pick your information and overlook a lot of important details these days.

826

Not much, that’s probably why you didn’t hear about it. Arianespace is often considered the most reliable launcher there is now.
Someone may want to confirm this, but IIRC the only Arianespace hiccups were from defective Soyuz (which, for some reason, lost much of their legendary reliability those last years)

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827

Russia’s economy has had a rough time and so has roscosmos in keeping their engineers paid. Less workers means longer shifts and more mistakes. At least that’s my best guess for the reasoning behind an uptick of Russian equipment failing in recent years.

828

Targeting return to flight from Vandenberg with the @IridiumComm NEXT launch on January 8. Update: https://t.co/15yMaiobpX

— SpaceX (@SpaceX) 2. Januar 2017
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829

Scott Manley provided a helpful explanation of the recent fueling test explosion.

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830

Static fire test successful
The launch is expected no sooner than Monday at 10:22 a.m. PST (1:22 p.m. EST; 1822 GMT).

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831

No launch today ;(

Launch moving due to high winds and rains at Vandenberg. Other range conflicts this week results in next available launch date being Jan 14.

— SpaceX (@SpaceX) 8. Januar 2017
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832

Waiting for this launch seems longer than waiting for the entire RTF announcement… :grin: