Bussard Ramjets

[shynung]

So, Bussard Ramjets.

In a classic rocket equation, it is always assumed that the propellant mass needed for deltaV burns are carried by the rocket at all times, because space is assumed to be devoid of all matter for use as propellant.

Except space isn't really empty. There are hydrogen particles floating about in space, whose density depends on what happens to be around. Stars, for example, emit stellar 'winds', which are actually hydrogen particles thrown off by the star's energy.

Enter the Bussard Ramjet. A strong laser or microwave emitter on the rocket's nose knocks off the electrons from the hydrogen particles, turning them into protons. This allows an electromagnetic scoop, thousands of square kilometers wide, to collect the protons thus obtained into the rocket's drive unit, where it is either fused with each other, reacted with antiprotons, or used as a thermal/electric thruster propellant, and then thrown out of the exhaust nozzle.

There you go. Large amounts of deltaV without the accompanying similarly large propellant tanks.

How would this kind of drive be realistically realized, given our current technology?

Edited September 3, 2015 by shynung

[CalculusWarrior]

Unfortunately, the Ramjet requires us to have extensive knowledge about how to contain and manage a proton-proton fusion reaction, the most difficult of all fusion reactions to maintain. I think our knowledge of nuclear fusion projects needs to improve by a significant amount before such a spacecraft can be considered plausible.

[shynung]

Unfortunately, the Ramjet requires us to have extensive knowledge about how to contain and manage a proton-proton fusion reaction, the most difficult of all fusion reactions to maintain. I think our knowledge of nuclear fusion projects needs to improve by a significant amount before such a spacecraft can be considered plausible.

If we're going to use the collected protons as fusion fuel, yes. Using a separate reactor to generate power and using that power to accelerate the collected protons seem to sidestep the issue.

It's not infinite fuel (reactor fuel is separately stored and procured), but still infinite propellant. That's still something quite useful to have.

[magnemoe]

If we're going to use the collected protons as fusion fuel, yes. Using a separate reactor to generate power and using that power to accelerate the collected protons seem to sidestep the issue.

It's not infinite fuel (reactor fuel is separately stored and procured), but still infinite propellant. That's still something quite useful to have.

Easiest way to use them would be to brake, you don't have to gather the protons just turn them.

[AngelLestat]

Angel, fact that ramming for matter for matter-antimatter drive is a bad idea is actually very easy to demonstrate. What might work way better is ramming for matter for an ion drive that you'll power with your matter-antimatter reactor. If you have no intention of going past 0.1c or so, that might actually work better than a pure matter-antimatter photon drive.

Because pure antimmatter annihilation only add good thrust at higher speeds when the drag is a lot higher?

About the ion + antimatter as energy source.. mmm I dont understand how..

I can imagine if the matter is close to the funnel center then you dont need to waste a lot of energy to redirect the space dust, but if its at few km away from the ship, not sure how you can restore the deltav lost accelerating that particle with an ion engine.

At least that is my assumption, you need a really big scoop at those speeds dont you? Or flying through a planetary nebula.

The good news that maybe you dont need a radiator because you will use the same space dust as colling.

[shynung]

Easiest way to use them would be to brake, you don't have to gather the protons just turn them.

Indeed. Though, simply braking isn't the only thing a ramscoop could do.

About the ion + antimatter as energy source.. mmm I dont understand how..

I can imagine if the matter is close to the funnel center then you dont need to waste a lot of energy to redirect the space dust, but if its at few km away from the ship, not sure how you can restore the deltav lost accelerating that particle with an ion engine.

In order for the scoop to collect anything at all, the particles have to be ionized. This is assumed to be done by a laser/microwave contraption on the ship's nose. An ion engine can use these ionized particles as propellant.

For the ship's engines to generate any useful thrust, all it has to do is to throw the collected particles out the nozzle at a higher velocity than the particles incoming towards the scoop, while maintaining the mass flow.

Example: ramscoop is picking up 50 kg of particles per second. Relative velocity of incoming particles in relation to the ship is observed to be 3,000 km/s (1% c). In order to generate thrust, engine exhaust velocity must be more than 3,000 km/s, implying a specific impulse higher than 306,000 seconds. Since mass flow must be constant, the engine must transfer more than ( 50 * ( 3,000,000 ^ 2 ) ) / 2 = 22,500,000,000,000 W/s = 22.5 TW/s of energy to the collected particles as kinetic energy.

Edited September 3, 2015 by shynung

[AngelLestat]

In order for the scoop to collect anything at all, the particles have to be ionized. This is assumed to be done by a laser/microwave contraption on the ship's nose. An ion engine can use these ionized particles as propellant.

[Bill Phil]

Isn't there a max velocity?

Due to the drag of the magnetic field or something?

Even though it has a max velocity, it shouldn't be thrown away as a concept.

[Rakaydos]

Hmm..

Ring shaped craft that ionzes interstellar hydrogen in the ring's path, pulls it into the emty center of the craft, and magnetically pushes off the ionozed "rail" it creates, while actually reducing interstellar drag.

[shynung]

But here you need to subtract the drag that each particle produce depending how far is from the scoop center.

Yes, I'd need to. What I'm saying is that the engine needs to generate enough energy to offset the combined drag from stopping the particles relative to the ship, and collecting the particles toward the ship. Assuming that the only propellant comes from the collected particles, mass flow is essentially constant; the energy spent by the reactor goes toward boosting engine exhaust velocity.

FIrst if those 3000 represent exhaust, you should divide by 10 to represent ISP.

That is 3,000 kilometer/second. Dividing by 9.81 (standard gravity, which is what I assume you wanted me to do) will get me 306 kiloseconds, which is equal to 306,000 seconds. I don't see where I'm wrong.

For comparison, SpaceX Merlin engine have a 3.05 km/s (3050 m/s) exhaust velocity.

Not sure how you call that an ION engine.. someone can explain this better? K2?

This isn't about the ion engine yet. This is the amount of power being fed to said ion engine. Basically, the ion engine needs to be fed more than 22.5 terawatts/second of electrical energy in order to impart enough thrust to offset the drag from the scoop.

Edited September 5, 2015 by shynung

[AngelLestat]

Ah sorry, when I saw 306000 seconds, I was imagine that was C (in km), but C in meters is 300,000,000, there my whole confusion. Also I dint imagine that you have into account the decimals from G.

About the power needed, not sure if that calculation is so easy to do, but I dont have much time now to check the math, but well, if that is the case, then we need to calculate the amount of antimatter to keep up with the needs.

There is a paper source talking about this? ion-scoop-antimatter?

[Chakat Firepaw]

Isn't there a max velocity?

Due to the drag of the magnetic field or something?

Yes, it's the exhaust velocity of the drive.

Even though it has a max velocity, it shouldn't be thrown away as a concept.

Once you combine it with the minimum velocity, you kind of have to toss it out as being that useful.

The low end of the minimum velocity range is ~0.01c, which is already putting you near the top end of Fusion and Antimatter-Thermal drives. For a drive that would actually be useful to combine with a Bussard ramscoop you need to use something like a Antimatter Beam Core.

[Bill Phil]

It's still useful after you've hit the minimum velocity.

[Chakat Firepaw]

It's still useful after you've hit the minimum velocity.

Only if one of the following conditions are true:

The minimum velocity is near the lower end of the range, (it runs from 0.01-0.06c depending on scoop geometry and the density of the interstellar medium), and we can build a Fusion or AM-Thermal rocket near the top end of their possible performance, (they cap out in the 0.02-0.03c range).

AM-Beam Core rockets actually work and we can build one.

Put simply, any drive which we know to be physically possible and that can use a Bussard ram scoop is at best marginal for use in a Bussard Ramjet.

[0111narwhalz]

How do Fission-Fragment Drives compare to Fusion/AM-Thermal? I think their specific impulses get pretty high (550,000s), if that's relevant. Barely any thrust, but that's fine. It's all about the /\ V, right? Not sure how the mass ratios look though.

[Chakat Firepaw]

How do Fission-Fragment Drives compare to Fusion/AM-Thermal? I think their specific impulses get pretty high (550,000s), if that's relevant. Barely any thrust, but that's fine. It's all about the /\ V, right? Not sure how the mass ratios look though.

They have the specific impulse, (some proposed designs hit 1.5Ms), but use the wrong fuel and propellant. Remember that a Bussard Ramjet has to use protium with small amounts of deturium and helium because that's what it's going to be scooping up. Fission-Fragment designs all use high-mass elements, (e.g. plutonium), as both fuel and propellant.

[shynung]

They have the specific impulse, (some proposed designs hit 1.5Ms), but use the wrong fuel and propellant. Remember that a Bussard Ramjet has to use protium with small amounts of deturium and helium because that's what it's going to be scooping up. Fission-Fragment designs all use high-mass elements, (e.g. plutonium), as both fuel and propellant.

The fuel in a Bussard ramjet depends more on design. A Bussard ramjet can use plutonium as a fuel to heat the the propellant it scoops up. Fusioning the propellant is entirely optional; what's mandatory is that energy must be injected into scooped propellant stream somehow.

[0111narwhalz]

The FFD isn't intended to compete with the ramscoop. Rather, it is to be used in-system to boost the ramscoop up to "ramming speed."

[Chakat Firepaw]

The fuel in a Bussard ramjet depends more on design. A Bussard ramjet can use plutonium as a fuel to heat the the propellant it scoops up. Fusioning the propellant is entirely optional; what's mandatory is that energy must be injected into scooped propellant stream somehow.

Fission-Fragment uses plutonium as both fuel and propellant. The whole idea is to fire the fission products right out the back.

Using it to heat hydrogen would be an NTR-Plasma or NTR-Gas and be pretty much useless in a Bussard Ramjet.

The FFD isn't intended to compete with the ramscoop. Rather, it is to be used in-system to boost the ramscoop up to "ramming speed."

Then you are right back to the problem of every drive we know is physically possible to build being at best borderline for use in a Bussard Ramjet. Some types of Fission-Fragment rocket could actually get the ship going _faster_ than a Bussard Ramjet.