Discussion EARLY WIP: Multipurpose Exo-Atmospheric OTV

[Eagle1Division]

This is a concept I've been on for maybe half a year now, it's a shuttle that works for all the Galilean moons, and with some simple field mods (reflective foil for sun protection, like the foil used on the Apollo landers) also works on Luna. An "Exo-Atmospheric Orbital Transportation Vehicle".

No futuristic technology, just ol'fashioned LOX-LH2 engines, chosen because of the vast abundance of water ice on the Galilean moons, and MMH/N204 OMS engines.

If all goes well, this should be the first part of a series of vessels, followed by a very heavy cis-lunar LOX/LH-2 transport (almost could say cruise liner), and a similar vessel using VASIMR drives for Jupiter's cislunar space.

Anyways, it's very early and somewhat ugly in terms of appearance, but we're orbinauts, it's the thought behind the vessel that counts
And pictures speak 1,000 words, cliche' but true.

I'll divide this into 4 sections, Overview, Engines, Cabin, and Mission.

OTV Overview:
Mission: Orbital lift/transport
Bodies of operation:
Luna (Moon)
Ganymede
Callisto
Europa
Io (Extra radiation shielding req.)
Normal crew: 4
Maximum crew: 7
Payload: 70,000 kg
Vehicle weight: 60,000 kg
Full Payload Takeoff weight: 352,400 kg
Total Delta-Vee: ~4,887.7 m/s
Mass Ratio:
(As per mass of empty vehicle+payload)
1.27 MMH/N204
2.44 LOX/LH-2

Engines:
1x Main Engine, 2x Rotary Main Engines (RME's):
Propellant: LOX/LH2
Thrust: 1,235.3 kN each
ISP: 460 seconds

2x OME:
Propellant: MMH/N204
Thrust: 422.5 kN each
ISP: 370 seconds

1x Retro/Hover
Propellant: MMH/N204
Thrust: 659.7 kN
ISP: 370 seconds

Space shuttle orbiter (missing it's OMS RCS pods and OME's...) in the background for size comparison.

No payload bay doors needed with no atmosphere, it's designed to lift 70 tons, the vehicle itself weighs about as much.

On the nose is the retro engine and an RCS nose assembly.

Behind it is a cabin, with 18 days' endurance with a crew of 7. The OTV can handle 7 crew, but normally operates with 4, a Commander, a Pilot, Flight Engineer and Navigator.

In blue are the LOX tanks, in red are the LH2 tanks for the EPS, 3x the mass of the Space Shuttle's EDO package, for 18 days of power from it's 3 fuel cells.

Behind it is the payload track. A strong support beam to handle the weight of the vehicle under it's peak acceleration of 3 G, and to anchor the payload onto. Above is the robotic arm and radiator.

The four spherical tanks are two MMH and two N204 tanks. This is the earlier design where the retro engine uses the OMS MMH/N204 propellant. I'm thinking of revising the design so that it uses LOX-LH2...

Behind that is the LOX/LH2 tank.

And finally, on the very back end are the three main engines, and, not yet modeled, the two OME's, APU's, and RCS units.

Cabin notes:
This is from later on, and a lot of stuff is still WIP. Notice the hatch is more like the cabin door on a pressurized airliner (50,000 feet altitude has same physiological effects as space), and the little red box next to it is sort of mini cupola for a quick visual examination of the area immediately around the airlock door before going through it.

Engines:
At the very front is the Retro engine, this, along with the lower two rear RMEs, allows the vehicle to hover. The three LOX-LH2 engines on the rear form a triangle like the STS, but the bottom two are different:

A roller and a track allow them to rotate 90* downwards, along with the nose engine doing the same, this allows the vehicle to hover for liftoff and landing. Shown here is an RME. The track allows 90* of rotation, gimballing provides a further 12* in any direction.


Cabin:

Pink: Flight Deck
(STS FD is temporary drop-in, actual flight deck will be different)
Green: Living space
(Sleeping bags, personal items, kitchennete, etc.)
Red: Airlock
Orange: RMS control station
Blue: Lavratory
(I almost forgot this bit...)
Light Blue (on bottom): Crawlspace
(Allows access to ECLSS N2, O2 and all water tanks without an EVA)

The red circle is an overhead window for the docking control station. All the windows have covers similar to the covers on the ISS's Cupola, to protect from micrometeorite damage, so the windows don't have to be replaced nearly as often. Sapphire windows are expensive!
(Other windows not modeled yet)

Typical Mission:
Normal crew: 4
Maximum crew: 7
Payload: 70,000 kg
Vehicle weight: 60,000 kg
Full Payload Takeoff weight: 352,400 kg
Tankage:
187,413 kg LOX/LH2
34,986 kg MMH/N204
Body: Callisto

Pre-Ignition:
Vehicle is loaded, fueled, and prepped to launch. APU's, EPS, ECLSS, and other vehicle systems are all started and running. APU's are set to HIGH mode, draining twice the fuel as normal to generate the power necessary to rotate the 2 main engines and hover engine while they're under power.
The 2 rotary main engines and hover engine are rotated downwards.

Vertical Ascent:
The two rotary main engines are ignite at a sub-liftoff throttle. Once ignition is confirmed, and engines are ready for throttle-up, the forward MMH/N204 engine ignites and all are throttled-up in sync for liftoff. The vehicle accelerates upwards at 2.5 m/s^2.
While accelerating upwards, "Yaw Program" is initiated and the vehicle Yaws by RCS to the launch heading. Once ~100 m/s of vertical velocity is achieved (gravitational drag for the 114 second ride to orbit), the two rotary main engines throttle down and rotate back to horizontal over the course of 7 seconds, putting the vehicle into the "Horizontal Ascent" mode.

Horizontal Ascent:
The center main engine is ignited and throttled-up simultaneously with the two rotary main engines, once they are in position. The APU is set to NORM mode, now consuming 1 kg of LOX+LH2 per minute, now that the rotary engines are in horizontal.
The vehicle accelerates with a pitch of 0*, so that it's vertical velocity is ~0 m/s once it reaches orbital velocity, at an altitude of ~5 km. Slight pitch adjustments may be used by the GNC to achieve this, there is enough Delta-Vee for a tolerance of 5* off-center pitch. Engine will continue burning until Apoapsis is at ~250 km (standard minimal), or whatever mission orbit altitude is required.

Post-MECO:
Retro/hover engine rotated back to horizontal for on-orbit ops. Radiator and antenna deployed, and the OMS puts the vehicle on proper course. MPS system is purged, and APU's are set to LOW until near the OME burn, when they'll be set to NORM, then shut off once OMS-2 is complete. OMS-2 burn is always required for orbit circulization.

On-Orbit:
The OMS system has:
25 m/s allocated to orbit circulization
32 m/s allocated to the climb to 250 km
143 m/s allocated to plane alignment burns (~5.4* degrees) (Note: almost all orbits are equatorial over Galilean moons.)
50 m/s allocated to free maneuvering/course adjustment
25 m/s allocated to RCS
40 m/s allocated to de-circulizing the orbit prior to Powered Descent.

Powered Descent:
APU's are restarted and set to HIGH, MPS system purged prior to ignition. Radiators, RMS, and antenna are stored. The OME's drop the orbit periapsis to 3-5 km over the landing site, and the retro/hover engine rotates to 90* vertical. At a predetermined distance from the landing site (For Callisto, this is 77,612 meters), the 3 main engines are ignited, and the vehicle decelerates to full-stop 3-5 km over the landing site within a certain minimum radius from the pad. Center engine is CO, and the two RMEs rotate to 90* with a synchronized ignition and throttle-up with the forward hovering engine, entering the "Final Approach" phase of the flight.

Final Approach:
For Callisto, the pilot has 200 seconds of hover time and 90 m/s of reserve OMS propellant set aside for the final approach. The autopilot can land with far less, but this propellant is set aside for manual approach, should it be necessary. Once the vehicle touches down, all engines are CO, APU's are shut down, and the MPS is purged. EPS is set to low-power mode to keep the stored vehicle interior human-survivable, even if it is not occupied.



So far I'm thinking of changing the forward hover engine to use LOX/LH2, so if it runs out, it'll happen at the same time as the rotary main engines. Also I might increase the MMH/N204 amount to allow for more mission flexibility, which would also require increasing the amount of LOX/LH-2 to support it. But the mass ratio is below 3, even, so I'd think it still makes sense. Of course that would mean a ~5-second ignition and less reliability, but a more fuel efficient engine.

The APU's run on LOX/LH2 because hydrazine would require even more facilities to synthesize on the Galilean moons, I want to use LOX/LH-2 as much as possible since that can simply be extracted from the abundant water ice. Also, consuming only 1 kg a minute (the STS APU's consume 2 kg / minute, IIRC, of hydrazine. And they also must power the flight surfaces, which the OTV doesn't need to...), the APU fuel tanks can easily be refuelled from the main engine's propellant tank (Which has 187,413 kg...). There is a certain contingency amount of propellant, and the payload is very rarely going to be a full 70 tons, so if you need more APU fuel for whatever reason, there's plenty in the Main Engine's propellant tank for cross-feeding.
(APU fuel tank holds 75 kg, for a bit more than an hour of operation at NORM, and half an hour at HIGH. Ascent lasts 154 seconds, descent lasts a maximum of ~300 seconds)

 

[fireballs619]

Your image links appear to be broken, at least for me :shrug:

 

[Eagle1Division]

Your image links appear to be broken, at least for me :shrug:

Probably for the better xD
jk, I just had to fix the attachments. For some reason the forums dropped them.

EDIT: ('Cause I think if I edit the main page it'll drop the attachments...)

One issue I'd like some discussion on is the use of LH2. Vehicle endurance is 18 days, and Liquid Hydrogen will slowly leak out of any container made of matter... I think I found a source once that said 10% per day, but I don't think I'm remembering right, because that's a LOT, and I don't remember where it was...

Would it be plausible to freeze and compress that portion of the vessel to form solid hydrogen? After all, there's no atmosphere, we're near Jupiter, very far from the sun, and there's literally a giant gap in-between the tank and the warm habitat section of the spacecraft. So maybe that portion could be cooled to that level? Maybe even a hydrogen "slush" would help, so that much less evaporates through the tank every day.

 

[T.Neo]

The problems I can see are:

1. Hypergolics. I'd imagine they'd be a pain to synthesise out that far. Just use hydrolox- the performance is superior, though you will have to insulate it.

2. APUs. There was even a proposal to remove them from the Shuttle. If your APU runs on LOX/LH2, why not substitute it for a LOX/LH2 fuel cell and switch to electrical actuation? It could be more efficient and add a source of power for the ship.

3. You shouldn't worry that much about freezing your hydrogen. It'll become much, much harder to feed to the engines that way. You can insulate against boiloff and insolation is far lower out at Jupiter, which makes things easier.

4. Maximum acceleration of a whole 3G? Why so much? You're only taking off from the galilean moons, you don't need engines that are that powerful.

5. Retro and even 'main' (backwards-pointing) engines are not needed, and swivelling engines are needed even less. The LEM is a case in point. Gimballing engines isn't impossible (though it's never been done to such a severe extent, as far as I know), but it's (probably) just unecessary.

6. Co-opt the OMS to some other system. Even to the landing engines would work. Or use pressure-fed hydrolox, or even clustered RCS engines for redundancy.

7. You should avoid retracting panels/radiators/antennas wherever possible. You're not going into an atmosphere, you're just landing. Your primary problem would be some sort of regolith hitting the vehicle, but in a vacuum such regolith is going to go outwards, not cloud up around the vehicle. You can place radiators on the hull, for example, where they don't need to retract- potentially.

8. Is there radiation shielding of any kind?

 

[Tacolev]

Radiation shielding can easily become the biggest point. Jupiter's big old magnetic field means the radiation environment is extremely intense in the jupiter system.

 

[T.Neo]

Yeah, but it isn't as bad on Callisto- you could stand outside and not get a lethal dose of radiation in a scarily short amount of time.

Here is a good thread with info on radiation on the Jovian moons.

Nevertheless, some radiation shielding would help for long-term exposure, as well as travels to nearer-in locations or to one of the inner moons (perhaps Ganymede), though I have a feeling this vehicle is primarily intended for use on Callisto.

Still, it would help. I would personally be more comfortable in such a vehicle if it had a fair amount of radiation shielding- up to the task, of course, not absolutely overkill (unless it needs to be).

 

[Eagle1Division]

The problems I can see are:

1. Hypergolics. I'd imagine they'd be a pain to synthesise out that far. Just use hydrolox- the performance is superior, though you will have to insulate it.

You sorta need hypergolics for RCS, though. The point is using an engine that doesn't need an ignition system so it can fire hundreds or thousands of times, and fire with no fore-warning. I wonder what kind of ISP can be had with Cold Gas rockets? Esp. if the pressure is brought up really high. Maybe I could use LH2, and electrical heaters would heat it and cause it to expand into a high-pressure "holding tank". The holding tank would feed to the RCS engines...

2. APUs. There was even a proposal to remove them from the Shuttle. If your APU runs on LOX/LH2, why not substitute it for a LOX/LH2 fuel cell and switch to electrical actuation? It could be more efficient and add a source of power for the ship.

I wasn't entirely sure electrical motors could practically be used for it, but I guess so...

3. You shouldn't worry that much about freezing your hydrogen. It'll become much, much harder to feed to the engines that way. You can insulate against boiloff and insolation is far lower out at Jupiter, which makes things easier.

The main engines are only fired for powered descent and ascent, so the LH2 could be frozen while on-orbit and the OMS propellant(s) are used.

4. Maximum acceleration of a whole 3G? Why so much? You're only taking off from the galilean moons, you don't need engines that are that powerful.

It's really a matter of the mass ratio... I still want to minimize gravitational drag, though I guess twice Callisto's gravity would suffice. I'll probably work it so average acceleration during ascent is 5 m/s^2, or initial acceleration at liftoff is 3 m/s^2.

5. Retro and even 'main' (backwards-pointing) engines are not needed, and swivelling engines are needed even less. The LEM is a case in point. Gimballing engines isn't impossible (though it's never been done to such a severe extent, as far as I know), but it's (probably) just unecessary.

The LEM was also a specialty, non-mass produced vehicle. It was made to fit in the nose of a Saturn-V rocket, and as simple and light as possible.

The OTV is designed from a more economic, cargo-hauling, wide-use standpoint. Having the cargo bay so easily accessible, and the systems all close to the ground and arranged like they are, makes ground operations much, much easier than if a crane assembly was needed to access it, as an arrangement like the LEM would require. It's the difference in-between an airliner and a rocket in terms of servicing and accessing the cargo area.

6. Co-opt the OMS to some other system. Even to the landing engines would work. Or use pressure-fed hydrolox, or even clustered RCS engines for redundancy.

Same reason the shuttle has OME's, RCS just doesn't cut it for thrust. OME's are very reliable, too. Not to mention lightweight, and two is enough redundancy on engines as reliable as they are.

Hovering engines should probably use the same propellant as the main engines, anyways, since the main engines will have a higher impulse.

7. You should avoid retracting panels/radiators/antennas wherever possible. You're not going into an atmosphere, you're just landing. Your primary problem would be some sort of regolith hitting the vehicle, but in a vacuum such regolith is going to go outwards, not cloud up around the vehicle. You can place radiators on the hull, for example, where they don't need to retract- potentially.

The spine offers a large radiating area to set the radiator on. I was having it retract so it could survive the G-loading, but if the G's don't go very high, it could be stationary.

8. Is there radiation shielding of any kind?

Yes, that's why the cabin is rounded and smoothed. Less surface area, less shielding area, less mass. Aside from lead, it's also nicely lined with EPS Fuel Cell tanks (like those on the EDO package) that offer some shielding.

 

[T.Neo]

You sorta need hypergolics for RCS, though. The point is using an engine that doesn't need an ignition system so it can fire hundreds or thousands of times, and fire with no fore-warning. I wonder what kind of ISP can be had with Cold Gas rockets? Esp. if the pressure is brought up really high. Maybe I could use LH2, and electrical heaters would heat it and cause it to expand into a high-pressure "holding tank". The holding tank would feed to the RCS engines...

Buran used GOX and some hydrocarbon for its RCS, as far as I know. The advantage of not having to worry about producing and handling hypergolics probably outweighs the fact that they're hypergolic.

You can devise an RCS system that uses hydrolox, that could be quite reliable. Maybe. We don't have a lot of wealth in that area of expertise, but we also don't have much expertise in operating vehicles that fly from the moons of Jupiter...

The main engines are only fired for powered descent and ascent, so the LH2 could be frozen while on-orbit and the OMS propellant(s) are used.

Don't freeze it. It'd just cause trouble and have no worthwhile advantages that I know of.

It's really a matter of the mass ratio... I still want to minimize gravitational drag, though I guess twice Callisto's gravity would suffice. I'll probably work it so average acceleration during ascent is 5 m/s^2, or initial acceleration at liftoff is 3 m/s^2.

By not having to build the vehicle to withstand 3G, you'll probably save far more than you'll lose by having gravity losses that are just a tiny bit worse.

The LEM was also a specialty, non-mass produced vehicle. It was made to fit in the nose of a Saturn-V rocket, and as simple and light as possible.

The OTV is designed from a more economic, cargo-hauling, wide-use standpoint. Having the cargo bay so easily accessible, and the systems all close to the ground and arranged like they are, makes ground operations much, much easier than if a crane assembly was needed to access it, as an arrangement like the LEM would require. It's the difference in-between an airliner and a rocket in terms of servicing and accessing the cargo area.

That is a very poor justification for a layout you don't need. Plenty of 'horizontal' moon lander concepts exist, they don't swivel their engines.

I agree, keep the engines where they are- you might have more problems piping propellant, but you need balance. But they don't need to swivel at all. They can just be fixed (well, gimballing them would be nice, but for steering purposes of course).

Same reason the shuttle has OME's, RCS just doesn't cut it for thrust. OME's are very reliable, too. Not to mention lightweight, and two is enough redundancy on engines as reliable as they are.

RCS cuts it for Dragon. And that's why I suggested clustering them- for more thrust. Admittedly it'd add quite a bit of complexity though.

But there's still no reason for the OMS not to be hydrolox.

Just as an example of a hydrolox engine, the RL-10 is quite reliable. It is quite light. You could make it pretty rugged and have a lot of redundancy. It, or engines like it at least, have been made throttleable down to quite low levels.

Since your engines will have to throttle down to low levels for landing, you could just switch them on at a low setting on-orbit for OMS-type manuvers.

I'd personally suggest something like the RL-10. A ruggedised expander cycle engine.

Yes, that's why the cabin is rounded and smoothed. Less surface area, less shielding area, less mass. Aside from lead, it's also nicely lined with EPS Fuel Cell tanks (like those on the EDO package) that offer some shielding.

Not lead! Stuff hitting lead can spall out secondary radiation. You want something light, with a lot of hydrogen- like a polyethylene, or even water. That's better to defend against particle radiation. For gamma or X-rays, you want something like lead or tungsten.

Putting your fuel cell tanks there should help... but what about propellant tanks as well? You could add quite a bit of shielding, and you would help distribute the COG. I think that would help a lot on a vehicle like this.