28

To make a long story short, liquid hydrogen has a very low density of just 70 kg/m3. RP1, on the other hand, has a density very close to that of water - about 1000 kg/m3. This means that for the same mass of hydrogen fuel, you'd need a tank 14 times as large. Couple that with the need to keep LH2 cryogenic or lose it to boiloff, and it becomes a very complex ...


27

Chemical rockets will never have more than 600 seconds specific impulse. Storing free radicals in propellant to defeat this limit is impractical. Validated. Chemical rockets in use top out at 450-460 seconds, with a demonstrated test-stand record of 542 seconds. ~500km range rockets will use chlorine pentafluoride and a hydrazine derivative. As far as I ...


21

SpaceX looked at the design of a booster with fresh eyes. Their concern was cost and reusability. Different fuels, while more efficient are much more expensive to manage (two fuel systems), and develop (two different engines). They took a more pragmatic approach of can we build an effective booster that is cheap, vs super performant. An RL-10 cluster as ...


19

Short answer: no, it won't increase the performance of the rocket. Platinum (or other catalysts) can be used in fuel cells, where the purpose is to get a chemical reaction at (relatively) lower temperatures. That means without having to burn the gases, in other words. (It achieves this by reducing the "activation energy" of the reaction as the question ...


19

The problem is that the transition produces enough energy to boil the LH2. As explained on the old sci.space.history group: Skipping the gory quantum-mechanical details... there are two energy states of the hydrogen molecule, ortho and para. At room temperature, hydrogen is about 3/4 ortho. At liquid-hydrogen temperatures, the stable state is ...


18

It's theoretically possible to collect fuel from near empty space, the bussard ramjet is an example of an engine built to do just that. The principle is that you use magnetic fields to collect and concentrate hydrogen atoms from the near vacuum of space, and then a fusion rocket would turn some of this into propulsion for speed and a fusion reactor would ...


15

In the late 70's and early 80's I worked for a company named Unidynamics Defense Systems. We provided this subassembly. We were not the design originators, this design I am quite sure came from NASA. My guess as to who the upstream contractor was would land on either Martin Marietta or Thiokol. On this particular design my drawings did not indicate who the ...


15

It is easy enough to analyze this in terms of conservation of momentum, we'll assume the velocity through the atmosphere to be 7.8km/s, that is LEO orbital velocity - in actuality it'll be a little different due to rotation of the Earth and eccentricity of the atmosphere-grazing orbit, but not different enough to change the conclusion. Now assume the ship ...


13

...is the tank within a tank a sound engineering concept for rocket stages? I take this to mean that you are not talking about pressurant bottles or other small devices submerged in the propellant tanks. Instead you mean the large primary propellant tanks. Then, No, this is not a good idea. It doesn't need to be a double-walled, or "dewar-flask" tank....


12

The way you ignite a rocket engine depends a lot on the fuel/oxidiser combination. Some combinations, like Liquid Hydrogen/Liquid Oxygen (HydroLox) ignite very easily and are ignited using a glorified spark plug (this is the case of the RL10, used on the Centaur Upper Stage). The RP1 (Kerosene)/Liquid Oxygen mixture is harder to ignite, and requires more ...


11

I guess the generic answer for any mass separation system would be that you spin it, but mass separation isn't really effective for electrolysis since that requires larger separation between anode and cathode which leads to protonic ohmic losses, i.e. increased internal resistance. So a more optimal species separation is done not by their mass but by their ...


11

I think geoffc is correct. It's primarily about cost and if that is a major concern then Hydrogen is almost immediately off the table. It is good for high efficiency upper stages but as both other answers mentioned it's bad for a lower stage as the thrust is on the lower end and the tanks are heavier. You need a pretty advanced engine just to make a Hydrogen ...


11

SpaceX optimizes for cost over performance, and everything SpaceX does with respect to the Falcon architecture works towards that goal. SpaceX will gladly trade a few m/s of ΔV if a) it saves significant amounts of money and b) there's enough left over to complete the missions most of their customers need. One significant cost optimization is to ...


10

They do use sparklers: the Radially Outward Firing Initiators (ROFI), derived from the Shuttle system, are started at T-15 seconds. Note that a hydrogen fire is the expected result of this system; they just didn't expect it to be this extensive. On the NasaSpaceflight forum, someone suggested that the scorching of the insulation is caused by the hydrogen not ...


10

It can be made to work, but it would be quite different from the typical SF depiction. As Blake points out, you can't gain anything from mass that is already moving faster than rocket exhaust. However, you do not have to collect the gas at orbital velocity. In the extreme case you could slow to a complete stop and refuel while landed, as long as your fuel ...


10

At engine start the ramjets would provide no thrust - but at this stage of flight there are not excessive cooling needs, so there's no "excess hydrogen" to burn in them anyway. In the middle of the flight regime the air intake is decelerating more air than is needed, by reheating this extra air with the excess hydrogen in the ramjet burners, this extra ram ...


10

Each technology has their own strengths and difficulties. It's really difficult to explain this without going in to a great detail about how rocket engines work, but let me try and give a brief overview. There are a number of different ways you can power a rocket engine. In general, for the same fuel and engine size, the higher the chamber pressure, the ...


9

The melting point of hydrogen isn't much lower than its boiling point (6K), so the temperature isn't necessarily that much of an obstacle. However, using solid fuel requires either melting it, or burning it in place. Trying to burn solid hydrogen would likely result in the whole thing flash-boiling from the radiated heat, so that's a bit of a non-starter. ...


8

Blue Origin (BO) chosed LH2 for BE-3 rocket engine because of the performance. "Performance drove the decision to use hydrogen fuel in the BE-3," said Rob Meyerson in an interview in 2013, president and program manager in BO. Looking in to perspective, BO had ambitions for space exploration and they would not stop the development only with a suborbital ...


6

My understanding is that designing for multiple restarts isn't particularly difficult; it's just that most engines don't have the need for it, so they don't bother. The RL-10 mentioned in the Lunar COTS paper you linked has been restarted at least 7 times in a single real mission, and depending on the model, is rated for 10 starts and 4000 seconds of run ...


6

Even with hydrogen, the tank weight is quite small compared to the content weight. For some examples, you can look here (you may need to do some math to get the actual numbers): http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750004950.pdf You are right that the heat loss is less of a problem for larger tanks, but structural integrity, especially ...


6

For the SpaceX Falcon 9 the fuel lines are 12.5 cm/5" (link: youtube video, SpaceX factory tour by Elon Musk) in diameter, so those are pretty big hoses to attach manually. LH2 is under pressure, making for a heavier hose. So you probably need hoisting tools to get the hose into place. The attachment points for upper stages are fairly high up, meaning you'd ...


4

The benefit of a high altitude exit from a high velocity gun is the reduced density of the atmosphere as the payload/projectile at full speed, hits the air. Sea level is not a good place to suddenly hit the atmosphere at orbital velocity or even close to it. 20,000 feet the air pressure reduces enough to matter. Higher would be better. The question is ...


4

It is possible that the platinum would improve the performance, but not in the way you're thinking. Metal particles in rocket fuels have been used experimentally as a way of increasing the density impulse. This is discussed in chapter 12 of Ignition: an Informal History of Liquid Rocket Propellants by John D. Clark (available in PDF on the Internet if you ...


3

Giving the tank a positive charge is in the most extreme case to partially ionize it, but if it is possible to do it in another way, it is going to take a lot of energy anyway. Even with only a little energy usage, a bigger problem is to avoid heating of the hydrogen, and the resulting boil-off. I can not think of a way to charge either the tank nor the ...


3

Yes it is, and in fact, this is being done, to some extent, already. It does have a number of risks, however, and is typically only used for small items as a result of that risk. The Falcon 9 is the rocket that I am aware of that does this. There are a number of helium tanks, known as COPVs. These would need to be insulated more if they were not in the LOX ...


3

The issue is that the collection mechanism tends to produce more drag than thrust. Robert Zubrin and Dana Andrews showed that this makes most Bussard Ramjet-type starship propulsion impractical. In fact, this sort of mechanism is so good at producing drag, it actually does function well for decelerating a starship once it reaches its destination, meaning ...


3

ESA is currently (2018) studying a quite similar project. In short, it does not use oxygen but any gas in upper atmosphere to feed an ion thruster instead of flying with a tank of gas to feed this thruster.


3

Considered, yes. See e.g. this study from 2010: We discuss the applications of metastable metallic hydrogen to rocketry. Metastable metallic hydrogen would be a very light-weight, low volume, powerful rocket propellant. The main attraction of metallic hydrogen is its Isp: metallic hydrogen has a theoretical Isp of 1700 s. And this paper from 2008: ...


3

Yes, the fuel would be used as a monopropellant because as soon as you excite the metallic hydrogen, it will be just H molecules which will quickly bond to each other to Form H2, creating mass amounts of energy. If you convert it back to molecular hydrogen, all the energy required to produce it is released. No burning would be necessary, it would just be the ...


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