Project High Velocity Interplanetary Passenger Spacecraft

[T.Neo]

Yeah... the only problem with that acceleration range, is that it will have to take up a large part of the journey, meaning it cuts into the cruise phase when I can use artificial gravity (instead of spending a few hours at 1 G, I spend four days at 0.05 G and then don't have a lot of time to implement artificial gravity anyway)...

What you're saying essentially is that I should have a low thrust ship- but I want the ship to have high thrust, because for me it's a matter of personal pride (yeah :uhh. There are plenty of ships that are structurally and morphologically designed around engines that accelerate the ship at hundredths of a G. This isn't one of them.

Interestingly enough, I found out that the original performance I had (7 840 000 m/s) was far more than I needed. I only needed 380 000 m/s, but this was also after I increased the diameter of my propellant tanks (I need a bit of extra fuel after I fixed my mass- parts of the ship were made of air). That and settling on 1 G instead of 1.5 G brought my thrust power down to an insane ~3 terawatts per engine (better than an insane 39.2 terawatts).

The mass figures I came up with were interesting- I based the masses of the pressurised components off the densities of existing and historial spacecraft, like the ISS and Mir. And I think I may have been a little pessimistic with the truss at something like 100 kg/meter, but I felt it needed to be a bit beefy and withstand torsional stresses as well. Which annoys me even more, because the rest of the ship is fine, but the engines are not.

In comparison, the Discovery II had a 310 ton reactor, and a jet power of 4.83 gigawatts. I have a jet power per engine of around 3000 gigawatts. If the mass is scaled up with the power, it turns out you need something like a 200 000 ton reactor. Now, given that it can be He3-He3 fusion instead of He3-D fusion, and that the reactor could be a bit more advanced, it could be a bit less, but it would still be quite a high amount.

If I reduced my thrust power to 115 gigawatts (like the Saturn V first stage) I would be able to only accelerate at 0.018 G. And that is already leaving me no time for spinning during cruise or even time to decelerate.

 

[n72.75]

Have you considered variable ISP based on throttle setting?

 

[T.Neo]

I don't see how it would solve the problem, since it would take too long to burn at low thrust, and not be able to take long enough burning at high thrust.

EDIT:

To illustrate the point of Epic Fail, attached is HVIPS next to the fusion reactor from the Discovery II.

Below that is HVIPS next to the fusion reactor from the Discovery II, and attached to how large such an engine would have to be to achieve the same power/m^3 and power/ton characteristics. In short, that's how large the 200 000 ton engines would have to be.

 

[Izack]

I see your point. :lol:

It would be a waste of a mesh to just scrap the project, though. Could you scale the entire vessel up to lessen the performance hit from the so-called Epic Fail?

 

[T.Neo]

Scaling the entire vessel up won't remove the nasty ratios though.

And it is partially also like scaling a house to twice the size to increase the amount of people that can live inside, but scaling the faucets and electrical outlets 2x as well.

A few nasty comparisons between this thing and the Discovery II:

Physical engine density:
HVIPS single engine: 0.217 tons/m^3
Discovery II reactor: 1.645 tons/m^3

Mass flow:
HVIPS single engine: 44kg/s
Discovery II reactor: 0.08kg/s

Megawatts generated per m^3 of engine:
HVIPS single engine: 26 090 MW/m^3
Discovery II reactor: 9.47 MW/m^3

Megawatts generated per ton of engine:
HVIPS single engine: 120 000 MW/ton
Discovery II reactor: 15.58 MW/ton

Newtons of thrust per ton of engine:
HVIPS single engine: 660 000 N/ton
Discovery II reactor: 89.54 N/ton

So... yeah. My engines were the spaceflight equivalent of trying to fit a 16-inch naval gun into a 9mm pistol.

But I'm looking at the figures for the Daedalus interstellar probe concept... the first stage had a thrust of 7 540 000 newtons, and an exhaust velocity of 1 600 000 m/s. And a thrust power of just under 40 terawatts.

And the mass of the first stage at staging is 1 690 tons. Which is a thrust/mass of 4460 newtons per ton and a power/mass figure of 23 670 MW/ton! And that is for the entire first stage, tankage and all, rather than the engine alone.

Of course, wiki could either be wrong, or the Daedalus study could have simply been incorrect (or have used over-optimistic figures, or whatever). But if that is correct, I think it is very attractive and interesting- might have something to do with either the dynamics of the inertial confinement fusion, or an ablative layer inside the nozzle.

Either way, it would be a tad difficult to implement into the vessel as I have it now, since I am set up for thousands of tons of slush hydrogen, not a small-ish bucket of helium-deuterium ice pellets.

And there is the issue of thrust stream spreading as well. If it is anything like VASIMR, the ship would be under hideous effects from the thrusters. But it is unfortunately a good thing as well, in that the more the thrust spreads, the less destructive it becomes to other objects in space (and the more politically correct the spacecraft is).

 

[Tommy]

I like the design of this ship quite a bit. It looks very good so far, and this type of vessel is in short supply - and most of the existing ones are getting rather outdated.

I believe I can shed a bit of light on the navigation issues. It's not the amount of delta-V that causes problems - it's the length of the burn. IMFD and TransX both assume a "point thrust" when calculating trajectories. In other words, they assume that the entire delta-V is applied instantaneously. In practice, of course, delta-V is applied over time, and there will be a discrepancy between the planned vector and the vessels actual vector when the burn is complete. The longer the burn, the larger the discrepancy. IMFD uses a couple types of "burn integration" which help ensure that the actual vector is as close as possible to the planned vector, but it's not perfect and the longer the burn takes, the less effective it becomes. For a 5 minute burn it's plenty close enough, but for a 5 hour (or longer!) burn it doesn't help as much.

I'm not very familiar with AGMFD, but it looks like it may work well for this craft. It's designed for prolonged burns at about 1 G, a coast period, and a braking phase. While I think it was designed for longer burns and shorter coast phases than you are talking about, I think it's worth looking into.

 

[Sky Captain]

According to article about nuclear salt water rocket http://www.npl.washington.edu/AV/altvw56.html
over 400 GW of thrust power would be possible with relatively straightforward technology in compact and lightweight package. If those claims are legitimate then few TW of power per engine might be plausible with future technology. Suppose the fusion reactor were designed in a way that it is only skeletal frame holding the superconducting coils with ~80% being empty space to allow most of the waste heat radiate freely into space then cooling recquirements would be minimized allowing to ramp up the power.

Fast spaceflight is a power guzzler there is no way around that.

 

[Izack]

Uh oh, you've brought in the deathspewer. :lol:

 

[T.Neo]

Uh oh, you've brought in the deathspewer.

Actually, deathspewer is fine if you aren't trying to launch the thing from the surface of a planet- the exhaust will eventually leave the solar system and contaminants falling onto the surface of a planet will be minimal.

My issue here is whether any radioactivity from the exhaust of an NSWR, or say an open-core gas NTR would affect the rest of the vehicle and the crew. Of course, if this a non-issue if the actual exhaust is impacting the ship (i.e. that would be the end of the ship).

Fast spaceflight is a power guzzler there is no way around that.

Yeah... I figured that out the hard way.

I honestly don't know how NSWR does it, and does it with (what I assume is) is a physical nozzle. The second figure for NSWR (4 700 km/s exhaust velocity version) would have a thrust power of 31 terawatts!

RisingFury kindly provided an equation for deducing the radius of an engine:

Sqrt[P * e / (4*Pi*K*T^4)]

Where:
P = power
e = emissivity of the surface of the engine
K = Stefan's constant (5.67e-8)
T= temperature of the facing material in Kelvin.

If we have a power of 3.135 terawatts, a facing material with an albedo of 99.5% (emissivity of 0.005) and a suitable temperature of 1 824 Kelvin (albedo and temperature data is from the Beryllium plasma facing material of the Discovery II), you get a radius of 44.6 meters, or a diameter of around 90 meters.

So whether the structure is skeletal or not, you're going to have to be that far from the power source to have that surface temperature. A skeletal structure would certainly help with removal of the waste heat, but in this case it also exposes a good deal of the rest of the ship to the brightness of the reactor/engine. As to what the waste heat of the engine would be, it would likely be quite a bit for a fusion engine like that of the Discovery II.

Fusion is a pain. You can't just get energy out of it like chemical, or even fission. Everything has to be set up just right for it to work. That's why we had our first fission reactor in the 1940s, whereas we'll be lucky to have a breakeven fusion reactor by the 2040s.

I'm still looking at the Daedalus... that nozzle is pretty huge, but the mass figures are really optimistic...

 

[Sky Captain]

I honestly don't know how NSWR does it, and does it with (what I assume is) is a physical nozzle. The second figure for NSWR (4 700 km/s exhaust velocity version) would have a thrust power of 31 terawatts!

Maybe these figures are based on very optimistic assumptions. I think NSWR is just a concept and there is no serious engineering studies done about it.

On the other hand average power output of a nuclear pulse engine during a burn using one 20 kt explosion per second would be ~100 TW, larger designs utilizing thermonuclear explosions would probably have power output in PW range and engineers who worked on the project thought it was doable with technology available in sixties and wouldn't even recquire any active cooling. As far as I know Project Orion was the most detailed study on a high power high velocity interplanetary spacecraft ever done and no potential show stopper problems were found (at least on paper and small scale testing).

 

[T.Neo]

On the other hand average power output of a nuclear pulse engine during a burn using one 20 kt explosion per second would be ~100 TW, larger designs utilizing thermonuclear explosions would probably have power output in PW range and engineers who worked on the project thought it was doable with technology available in sixties and wouldn't even recquire any active cooling. As far as I know Project Orion was the most detailed study on a high power high velocity interplanetary spacecraft ever done and no potential show stopper problems were found (at least on paper and small scale testing).

The original Orion had propulsion units with a yield of 0.14-0.15 kilotons though, and one of those each second would be a power of around 585.76 gigawatts. I can see a lot being lost out of the system, especially if the bomb does not direct the yield.

Here's a fun fact: the equivalent of 0.75 kilotons of TNT are being released in each of my engines per second. :lol:

The reason nuclear pulse propulsion won't work here is because the ship would be under the stress of jarring accelerations, even with the shock absorbers integral to an Orion design. The structure would be under plenty of strain (graphite epoxy doesn't do well under impact stresses) and it would not be very comfortable for the passengers during a 5-6 hour period...

Flipping around the equation to deduce nozzle size from temperature and power, so that we can determine temperature from power and nozzle size:

T = 4th root of[(P*e)/(4*Pi*K*r^2)]

Where:
P = power
e = emissivity of the surface of the engine
K = Stefan's constant (5.67e-8)
T= temperature of the facing material in Kelvin.

For Daedalus, with a 90 meter wide nozzle and a thrust power of 40 terawatts, this equates to a temperature of 3431.25 K. This is of course assuming the same emissivity as the plasma facing material of the Discovery II, which is a different material at a lower temperature.

Molybdenum melts at 2896 K, so it could be potentially work if there was perhaps some sort of cooling system at work, or if the material albedo could be increased.

The alloy proposed for the Daedalus nozzles does exist, and is claimed to have twice the strength of pure molybdenum at temperatures over 1300 C.

 

[Sky Captain]

The original Orion had propulsion units with a yield of 0.14-0.15 kilotons though

Wasn't 0.15 kt pulse units used only while in atmosphere because atmosphere would provide lots of extra reactional mass and once in vacuum 20 kt pulse units fired. As far as nuclear bombs go 0,15 kt would be inefficient because most of the plutonium wouldn't fission. IIRC lowest effective yield is somewhere around 15 - 20 kt when nearly all plutonium is used up. Some time ago I read a paper about Project Orion and it was claimed that lower yield explosions would be used in atmosphere and more powerful in vacuum, I just don't remember the exact numbers.

The reason nuclear pulse propulsion won't work here is because the ship would be under the stress of jarring accelerations, even with the shock absorbers integral to an Orion design. The structure would be under plenty of strain (graphite epoxy doesn't do well under impact stresses) and it would not be very comfortable for the passengers during a 5-6 hour period...

.

Yeah such long fragile structure wouldn't hande well such uneven acceleration. You would want something as compact as possible to be able to take hammer blows of nuclear pulse drive.

Molybdenum melts at 2896 K, so it could be potentially work if there was perhaps some sort of cooling system at work

I think some sort of active cooling system would be the key factor to prevent meltdown when operating at so high thrust power.

 

[T.Neo]

Wasn't 0.15 kt pulse units used only while in atmosphere because atmosphere would provide lots of extra reactional mass and once in vacuum 20 kt pulse units fired. As far as nuclear bombs go 0,15 kt would be inefficient because most of the plutonium wouldn't fission. IIRC lowest effective yield is somewhere around 15 - 20 kt when nearly all plutonium is used up. Some time ago I read a paper about Project Orion and it was claimed that lower yield explosions would be used in atmosphere and more powerful in vacuum, I just don't remember the exact numbers.

Perhaps... it makes sense. I got the 0.15 kt figure from wiki, which also stated the smallest achievable bomb yield would be 0.03 kt. Doesn't say anything about efficient fission of the plutonium though.

If that is the energy produced by each bomb, I think that a lot of it is getting lost to space... either that, or there's something going on with the propulsion system that I haven't got a clue about.

Yeah such long fragile structure wouldn't hande well such uneven acceleration. You would want something as compact as possible to be able to take hammer blows of nuclear pulse drive.

Yeah... you want a big, stocky, robust and heavy spacecraft for that. Would perhaps work well for... I dunno, some sort of military spacecraft, but not for this thing.

I think some sort of active cooling system would be the key factor to prevent meltdown when operating at so high thrust power.

Seconded. Modern chemical rockets do this, and I already have a resource of cryogenic fluid aboard the ship... the question though is how much the active cooling system could do to improve things...

 

[jedidia]

An active cooling system is a must, otherwise you'll never get the heat out of the engine. Of course it only helps as far as you have enough radiator area to get the heat out of the ship, but to get it from the engines to the radiators in the first place you need a cooling system that can transport enough heat per second away from the engine so it doesn't reach critical temperature.

I don't know how much increasing the albedo of the material helps. If it's constructed so the heat gets reflected away from the ship, ok. But that might be tough to do. If a high albedo results in heat being trapped and wandering around between engine parts, it might be worse than to suck it all up and pump it away.

 

[T.Neo]

High Thrust Testbed

Partially just because I'm messing around, and partially to redeem my, uh, personal pride, I've been messing around with a new spacecraft... I call it High Thrust Testbed. It doesn't do much at all, other than (hopefully) accelerate at 1 G or a sizable fraction of it.

Crew size is 2, Dv is maybe 200 km/s. Engine mass is 200 tons, radiator mass is 310 tons. Certain elements, such as RCS nodes, propellant tanks, some engine parameters and cockpit setup is borrowed from HVIPS.

Of course, I've run into the problem of my ship now being under compression, which means a very massive support structure. My idea was to have a light, hollow support structure with a lot of section depth, but I have a feeling it's still too flimsy. Structural mass calculator gives a mass of around 30 tons for my support structure, though I have a feeling it's going to be a good deal more. Of course, I could come up with something interesting like making the radiators my support structure. The problem is: more mass, the slower the ship can go (slower both in terms of overall acceleration and Dv).

My main hurdle with HVIPS now is thrust stream diversion. If it's anything like VASIMR, the ship is toast, unless I angle the engines at something like... 45 degrees. I can still get 70% effective thrust with that, but that limits my acceleration even further.

And these radiators are an utter, utter pain. These ones don't have to work as hard as those on HVIPS, and I really don't have a place to put such huge radiators onto HVIPS.

 

[Pyromaniac605]

Are those radiators or solar panels? Either way loks like it'd make a great stack thruster. :thumbup:

Darren

 

[River Crab]

Are those radiators or solar panels? Either way loks like it'd make a great stack thruster. :thumbup:

Darren

No, no! Where is the pride in that!? :lol:
(Those are radiators of course.)

Anyway, it's definitely a unique concept for an Orbiter vessel, sorry to see your previous design "fail". Though honestly it still looks really cool, I'd be happy with it no matter what propulsion it would end up with. That is, if you could ever allow yourself to give it some weaker engine.

As for having 1G acceleration, I really do hope for some such vessel, though. Would be a blast to burn with such speed and not feel guilty about using impossible propulsion. :thumbup: Perhaps this is to be some sort of realistic Torchship?

 

[T.Neo]

Yeah, they're radiators. Huge radiators. There isn't really any need for solar panels aboard the ship, since the drive can provide power (originally I was going to use a nuclear reactor as well, but that would incur even more radiator area).

Anyway, it's definitely a unique concept for an Orbiter vessel, sorry to see your previous design "fail". Though honestly it still looks really cool, I'd be happy with it no matter what propulsion it would end up with. That is, if you could ever allow yourself to give it some weaker engine.

The problem with giving it a "weaker engine" is that the design then doesn't work out (it spends too long thrusting).

My two main problems with HVIPS are radiator size, and thrust dispersion. And even the thrust dispersion I can handle, with something like 70% effective thrust. The radiators are a bit trickier though. And I would also have to cover basically my entire ship in reflective shielding. Not to shield from the brightness of the exhaust, but primarily to shield from the brightness of the engines. I could probably accomodate the shielding quite well though. I might be able to do 0.8 G, or 0.5 G, or even 0.25 G. But 0.25 G is better than 0.05 G.

As for having 1G acceleration, I really do hope for some such vessel, though. Would be a blast to burn with such speed and not feel guilty about using impossible propulsion. Perhaps this is to be some sort of realistic Torchship?

Probably not with this mass fraction and ISP. Project Daedalus had an exhaust vel of 10 600 km/s on the first stage, compared to my 380 km/s. The downside to a higher ISP is that you have a higher thrust power for a given thrust, and thus you need a larger nozzle.

One thing that this has taught me, is that it is possible to have high thrust-high ISP, high thrust power propulsion, if you give it what it needs. Even the DG's engines are in the range of real concepts, but they are just portrayed unrealistically. Scientifically, a propulsion system similar to mine might be plausible, but there are a whole host of engineering problems to it- as we've seen with fusion power in general.

Atomic Rockets has an equation for figuring out how much Dv you need to execute a brachistochrone trajectory (see torchship section here);

Now, just how brawny a rocket are we talking about? Take the distance and acceleration from above and plug it into the following equation:

transitDeltaV = 2 * sqrt[ D * A ]

where
transitDeltaV = transit deltaV required (m/s)


The rocket will also have to match orbital velocity with the target planet. In Hohmann orbits, this was included in the total.

orbitalVelocity = sqrt[ (G * M) / R ]

where
orbitalVelocity = planet's orbital velocity (m/s)
G = 0.00000000006673 (Gravitational constant)
M = mass of primary (kg), for the Sun: 1.989e30
R = distance between planet and primary (meters) (semi-major axis or orbital radius)


If you are talking about missions between planets in the solar system, the equation becomes

orbitalVelocity = sqrt[1.33e20 / R ]

Figure the orbital velocity of the start planet and destination planet, subtract the smaller from the larger, and the result is the matchOrbitDeltaV

matchOrbitDeltaV = sqrt[1.33e20 / Di ] - sqrt[1.33e20 / Ds ]

If the rocket lifts off and/or lands, that takes deltaV as well.

liftoffDeltaV = sqrt[ (G * Pm) / Pr ]

where
liftoffDeltaV = deltaV to lift off or land on a planet (m/s)
G = 0.00000000006673
Pm = planet's mass (kg)
Pr = planet's radius (m)

At the time of posting, the distances to Mars and Saturn are 2.259 and 10.56 AU, respectively. We want to accelerate at 1 G, or around 9.81 m/s^2. So that is 3 647 200 m/s Dv to Mars, and 7 893 500 m/s Dv to Saturn, if my math is correct.

A ship with an ISP of 380 000 m/s would need a mass ratio of around 14 733 for a 1 G brachistochrone to Mars, and a mass ratio of 1 050 328 092 for Saturn. Clearly impossible.

A ship with an ISP comparable to that of Daedalus, however, could do a 1 G brachistochrone to Mars with a mass ratio of only 1.41, and a 1 G brachistochrone to Saturn with a mass ratio of only 2.105.

Of course, the downside problem is that a 20 meganewton engine with an exhaust velocity of 380 000 m/s has a thrust power of 3.8 terawatts, and an engine with similar thrust but an exhaust velocity of 10 600 000 m/s would have a thrust power of 106 terawatts.

That was my concept behind HVIPS... a craft that would bridge the gap between hohmann transfers and low-g brachistochrones, and true, 1 g brachistochrones. It had a transit velocity of only around 200 km/s, and thus could get away with a far less powerful propulsion system.

 

[River Crab]

Yeah, that page is where I got my idea of a Torchship. I didn't really mean something capable of a 1g Brachistochrone, but just an interplanetary ship that is as fast as realistically possible (without propulsion that requires new/impossible physics). One could call that a "real world" Torchship, maybe. Bridging that gap as you said.

 

[T.Neo]

Well, you can always do a lower G brachistochrone... this is essentially what craft like the Discovery II, or the VASIMR concept, does. But your crew won't be under 1 g of acceleration. You can create a centrifuge/outriggers for your crew, but it will incur extra complexity and/or mass.

The highest acceleration that is realistically possible is pretty much 1 G, since that is the maximum that your crew can comfortably sustain. You might be able to get it a little higher, but it starts to become painful pretty fast. An unmanned spacecraft could potentially sustain a far higher acceleration, but it becomes difficult engineering-wise (not only do you need to have higher thrust, but you also need to bulk up the spacecraft structure, which makes it more massive).

The thing about a torchship is, it is plausible in physics. But it would need huge advances in technological-know how and applied science. And that is not only for a 106 terawatt engine, or even the much less powerful 3.8 terawatt engine. It is applicable to an "easy" 3.8 gigawatt engine as well. Now, things get harder as you get orders of magnitude bigger, obviously. But even a 3.8 gigawatt fusion drive is impossible at the moment.

The concept behind HVIPS, is that it isn't thrust limited, but rather ISP limited. So it accelerates to it's top speed fast, and then rotates to produce artificial gravity during the cruise phase, because it can't burn the entire way to the destination. Now in the real world, you avoid the plethora of trouble associated with a high thrust engine, and you accelerate throughout the journey, or at least for a large part of it. But this means you need secondary structures to produce artificial gravity, and it essentially also makes the entire concept of HVIPS irrelevant; that of using the same structure that provides standoff distance from the engines, to produce artificial gravity when the engines are off.

You cannot spin the craft on the X axis when the engines are on, or else you won't get anywhere.

I also remembered another problem with HVIPS: the axis of spin is wrong for the mass distribution- at best that means adjusting the length of the trusses.