DRACO: NASA and DARPA nuclear propulsion collaboration

[InterestedEngineer]

Another problem with using NTR as a deep space tug, Mars Transit, or what-not.

How are you going to fuel the H₂?

Starship's cargo volume is 1000m³.   Assuming the double hulled insulated tank doesn't take any extra room that's only 71t of H² per launch.

That's 2.5-3 times the cost per kg to LEO for LH₂ than for LCH₄/LOX, simply because the density of LH₂ is so bad Starship can't max out its mass launch capabilities.

Even assuming there's a "free"-ish way to get back (there's not), doubling the Isp can't make up for the extra cost to get the LH₂ to LEO.

NTR is simply a waste of money, or at best, a backup plan if Starship program fails utterly.  Since a likely way for Starship program to fail for deep space missions is to not figure out LEO refueling, and LH₂ is going to be even harder to transfer than LCH₄/LOX, as a backup plan NTR is terrible.

[Asteroza]

DRACO will be fairly conventional (NERVA derived engine, conventional tankage/structure), which might be a limiting factor? Once you have demonstrated NTR in space, the relative engine maturity (well implied TRL maturity) might allow more room for mass savings as the designers play around with the design space.

A route to improving the tug aspect would be a dual engine tractor configuration towing propellant tank bags constrained by a tensegrity truss tail. Easy to double bag the tank with substantial spacing between walls. That has the potential to substantially drop tank/structural mass.

[InterestedEngineer]

DRACO will be fairly conventional (NERVA derived engine, conventional tankage/structure), which might be a limiting factor? Once you have demonstrated NTR in space, the relative engine maturity (well implied TRL maturity) might allow more room for mass savings as the designers play around with the design space.

A route to improving the tug aspect would be a dual engine tractor configuration towing propellant tank bags constrained by a tensegrity truss tail. Easy to double bag the tank with substantial spacing between walls. That has the potential to substantially drop tank/structural mass.

Since it has to brake and come back from any sort of boost or `tug` activity, it's a no-win situation for NTR unless the Isp is somewhere north of 1500.

You save a little bit by having say 5% mass tanks instead of 7-10% mass tanks that is typical for H₂.  The shielding and mass of the NTR itself alas doesn't change, and it'll take a lot of shielding to make the NTR human rated.  NERVA was for example 18t, no shielding

H₂ tank mass analysis:

https://www.nasa.gov/pdf/382034main_018%20-%2020090706.05.Analysis_of_Propellant_Tank_Masses.pdf

[sevenperforce]

Another problem with using NTR as a deep space tug, Mars Transit, or what-not.

How are you going to fuel the H₂?

Starship's cargo volume is 1000m³.   Assuming the double hulled insulated tank doesn't take any extra room that's only 71t of H² per launch.

That's 2.5-3 times the cost per kg to LEO for LH₂ than for LCH₄/LOX, simply because the density of LH₂ is so bad Starship can't max out its mass launch capabilities.

I tend to agree with your conclusion, but now you've got me musing about the feasibility of sending up H₂O in Starship instead of H₂ and using a solar prop depot to crack it into liquid hydrogen and liquid oxygen. The liquid oxygen could be used to refill Starship (necessitating fewer total prop launches) for other operations; it's fine if some tankers launch with only CH₄ as a fuel-only tanker will have more total dV for the volume.

Of course energy requirements are an issue. A quick BOTE estimate could be useful. Suppose Starship delivers 150 tonnes of liquid water to the prop depot. That can be split into...16 tonnes of hydrogen and 134 tonnes of LOX. Okay, no, that won't work.

[edzieba]

Another problem with using NTR as a deep space tug, Mars Transit, or what-not.

How are you going to fuel the H₂?

Starship's cargo volume is 1000m³.   Assuming the double hulled insulated tank doesn't take any extra room that's only 71t of H² per launch.

That's 2.5-3 times the cost per kg to LEO for LH₂ than for LCH₄/LOX, simply because the density of LH₂ is so bad Starship can't max out its mass launch capabilities.

I tend to agree with your conclusion, but now you've got me musing about the feasibility of sending up H₂O in Starship instead of H₂ and using a solar prop depot to crack it into liquid hydrogen and liquid oxygen. The liquid oxygen could be used to refill Starship (necessitating fewer total prop launches) for other operations; it's fine if some tankers launch with only CH₄ as a fuel-only tanker will have more total dV for the volume.

Of course energy requirements are an issue. A quick BOTE estimate could be useful. Suppose Starship delivers 150 tonnes of liquid water to the prop depot. That can be split into...16 tonnes of hydrogen and 134 tonnes of LOX. Okay, no, that won't work.

If you're going to crack in orbit, you might as well skip H2O entirely and just crack LCH4 instead. 286 kJ/mol for H2O vs. 75 kJ/mol for CH4 (https://doi.org/10.3390/en14113107), or 286 kJ/mol of H2 produced from water vs. 150 kJ/mol of H2 produced from CH4 means you come out ahead on energy input per unit final propellant. Saves adding an entire new water tank and water handling subsystem to the propellant lifters vs. using the LCH4 tank and handling hardware of stock tankers to offload it to a H2 production depot.

[Robotbeat]

It makes no sense to do either of those things. LH2 on the ground is just $3-10/kg. That’s already as cheap as Starship ever hopes to get. Using water would mean you need to launch 9 times as much mass, and even methane requires 4 times as much mass.

[edzieba]

It makes no sense to do either of those things. LH2 on the ground is just $3-10/kg. That’s already as cheap as Starship ever hopes to get. Using water would mean you need to launch 9 times as much mass, and even methane requires 4 times as much mass.

Depends on whether your lifter is mass-limited or volume-limited. If Starship can lift ~150 tonnes of LCH4 (probably more as the propellant residuals are the payload so things like the PAF, deployment hardware, etc can be mitted for the tanker) but can only lift 71 tonnes of LH2 due to volume limitations (even assuming no volume used for insulation/cryocoolers/etc), that means each LCH4 launch gets you 20 tonnes of H2. 3x launches for the same mass sounds bad, but when when you are trying to pump up launch numbers to approach the launch cost = prop cost asymptote, and your prop costs are ~$1.35/kg vs. ~$10/kg, and you save the entire cost of building out LH2 handling infrastructure and building a dedicated LH2 tanker variant, its probably much closer to parity if not coming out ahead. Particularly if you already want LCH4 in orbit anyway for other reasons, so your LH2 customer only needs to foot the bill for the LCH4 they siphon off rather than footing the bill for an entire system architecture.

[Robotbeat]

Or, stretch Starship slightly.

Come on, just do a tiny bit of math and self-critical thinking and you’ll see it makes zero sense.

[TrevorMonty]

Technology to refuel and keep LH cool is already being developed by LM and Blue for their HLS tanker/spacetug.

[TrevorMonty]

It's 5710 km/sec from Earth LEO to Mars LEO per the solar system subway map.

For 700s, that's a mass ratio of 2.3 per the rocket equation.

For 100t of payload and 125t of dry mass that requires 300t of H₂ to orbit.  This

700ISP is for this demo vehicle, operational NTPs will be 850-900. Read the article.

Where does 125t drymass come from?.

[TrevorMonty]

Any Mars transporter is likely to be starting from EML1 or HEO with full tanks reducing its DV requirements by about 3km/s.

[Robotbeat]

…that also minimizes the IMLEO benefit of nuclear-thermal.

[TrevorMonty]

Can put extra DV savings towards a shorter travel time by going faster.

[InterestedEngineer]

It's 5710 km/sec from Earth LEO to Mars LEO per the solar system subway map.

For 700s, that's a mass ratio of 2.3 per the rocket equation.

For 100t of payload and 125t of dry mass that requires 300t of H₂ to orbit.  This

700ISP is for this demo vehicle, operational NTPs will be 850-900. Read the article.

Where does 125t drymass come from?.

when you are off by a factor 3 or more, a 25% improvement matters not.

When you plug 900s into the rocket equation, the mass ratio for NTR drops from 2.3 to 1.9.   Not a big improvement.

125t of drymass comes from:

40t for two engines (to get equivalent redundancy to Starship).
30t of shielding
30t of fuel tank.  (H2 tanks run about 10% of fuel mass).
25t of cargo hold.

I note I completely skipped orbital transfer of 100t of cargo.   25t assumes one just detaches the top of Starship and puts it on the NTR and also looks high because it has to survive 6g of ascent.  The logistics of moving cargo in orbits is a nightmare, fuel is going to be hard enough.  Positing some sort of transfer to lightweight 1g capable cargo hold is a logistical nightmare.

[Robotbeat]

Can put extra DV savings towards a shorter travel time by going faster.

No, you get less DV for the same IMLEO if nuclear is used starting in HEO instead of LEO.

Also, I’d like to see a full architecture to compare. Dry mass penalty of NTR is insanely bad.

[InterestedEngineer]

An interesting article that claims for a basic deltaV optimization the optimum mass ratio is 5 and thus increased exhaust velocity isn't always helpful.

http://www.neofuel.com/optimum/

I note that Starship's mass ratio for a Mars transit will be about 4, and its launch mass ratio is about 6.

Getting a NTR to a mass ratio of 5 will be quite difficult.   It's impossible to get it above 10 given 10% tank mass of H₂.  I suspect 3-ish will be about as high as it gets, and getting that kind of tank to orbit will be prohibitive, not to mention getting the H₂ itself in 70-90t increments.

For example an NTR with 100t cargo capability, 75t of non-tank mass (40t dual redundant engines, 20t heat shield, 15t cargo framework), to get a  mass ratio of 5 requires 1100t of H₂, so final dry weight is 100+75 + 110 = 285t.

For a deltaV (at Isp 800) of 12.6km/sec, which requires a stupendously large tank of 15,000m³  (300m x 8m cylinder for example).   It also requires at least 11 Starship launches to fuel, assuming a super-stretch version that can hold 100t of H₂.  (it'd have to be 5 rings taller).

Contrast this to a maxed out Starship w/ Raptor 3 which has a mass ratio of 8 which has a deltaV of 7.6km/sec + 5km/sec of aerobraking or 12.6km/sec of deltaV.   It requires 8 Starship launches to fuel.

You can't replace aerobraking and refueled chemical rockets without an Isp given H₂of at least 1600.  ROI isn't massively positive until Isp reaches 2000.  Nuclear Lightbulb, IOTW, a materials science nightmare.

You can do all these things from LEO, HEO, etc, it doesn't change the math except that at some point you can't aerobrake any more (I think Mars is max 7-8km/sec but I can't find where that was derived) and then it starts to get very mission specific, hard to generalize.

[InterestedEngineer]

Here's a volume-limited version of the "minimum Isp to replace a Starship for Mars journeys" calculation.

Given it won't be easy to get anything larger than a Starship size tank into Space, and Starship's tank volume (Rv3 version) of 1600t converts to 1,600m³ plus the 1000m³ of cargo or 1,700m³ of storage.

That corresponds to 121t of LH₂.  I'm being very generous and assuming no insulation on such tank which would add a meter to the diameter (and I was using non-densified LCH4/LOX in the volume calcs)

Under the scenario 100t of cargo to Mars with the usual 40t of dual redundant engines, 20t of shielding, and 20t of cargo handling, tank mass of 12t (dry mass 192t),  and 5700 m/sec of deltaV from LEO to LMO, we get a mass ratio of 313/192 = 1.6.

This requires an Isp of 1240.

So given the limits of both lack of aerobraking AND of limited volume tanks that can be physically launched, an NTR that pushes 100t to Mars requires an Isp on the order of 1250 - 1750.

One would have to invent an NTR with collapsible tanks and  a good reusable TPS system to catch up with reusable chemical rockets to Mars or any other destination with an atmosphere.  Meanwhile those chemical rockets are way further down the mass production curve, and NTR will never catch up.

And adding the fact that transferring fuel will likely be far easier than transferring cargo while in LEO or HEO.  Someone call the SPCA

[Zed_Noir]

Here's a volume-limited version of the "minimum Isp to replace a Starship for Mars journeys" calculation.

Given it won't be easy to get anything larger than a Starship size tank into Space, and Starship's tank volume (Rv3 version) of 1600t converts to 1,600m³ plus the 1000m³ of cargo or 1,700m³ of storage.
<snip>

Think it is feasible to get an empty 13 meter external diameter tank on top of a Super Heavy with a stubby upper stage module (no payload section) to LEO to be outfitted with a propulsion module replacing the stubby stage module and a payload module on the top.

Back of the envelope calculation for a make up tank with internal dimensions of 42 meter high and 12 meter diameter is about 4296 m³ in volume not including internal tank fittings and plumbing.

[InterestedEngineer]

Here's a volume-limited version of the "minimum Isp to replace a Starship for Mars journeys" calculation.

Given it won't be easy to get anything larger than a Starship size tank into Space, and Starship's tank volume (Rv3 version) of 1600t converts to 1,600m³ plus the 1000m³ of cargo or 1,700m³ of storage.
<snip>

Think it is feasible to get an empty 13 meter external diameter tank on top of a Super Heavy with a stubby upper stage module (no payload section) to LEO to be outfitted with a propulsion module replacing the stubby stage module and a payload module on the top.

Back of the envelope calculation for a make up tank with internal dimensions of 42 meter high and 12 meter diameter is about 4296 m³ in volume not including internal tank fittings and plumbing.

Well to paraphrase some of our fellow enthusiasts here, it is physically possible within the current laws of physics, so sure, why not?

But from an engineering and logistics standpoint, very painful as to make it unlikely.  We have very little experience mating orbiting objects outside of simple docking.  So connecting all the plumbing in zeroG?   Matching the orbits?  Propelling the tank designed for all H₂ using CH4/LOX?

How are you going to make and transport 13m rings?  There are no jigs for it.  I suspect the transportation systems, which use up an entire roadway (both directions) to transport 9m rings, would not scale out to 13m.  Will the chopsticks go that wide?

Transferring fuel in orbit is logistically the most simple thing we can do, and we haven't really done it yet at scale.

You are still also having to transfer cargo in LEO, another logistical and engineering nightmare.  (well std short cargo containers would probably make it reasonably easy, but everyone balks at their mass when I bring it up).

All this pain... for what benefit?   The competition has a mass production line that pumps out Starships that probably have a marginal cost of < $30M (about 5x cheaper than an airliner).

My high school AP Physics paper was on NERVA.  I love the tech. It just has no place once low cost to LEO, orbital refueling, and aerobraking work short of the highly advanced lightbulb version.

[Zed_Noir]

Here's a volume-limited version of the "minimum Isp to replace a Starship for Mars journeys" calculation.

Given it won't be easy to get anything larger than a Starship size tank into Space, and Starship's tank volume (Rv3 version) of 1600t converts to 1,600m³ plus the 1000m³ of cargo or 1,700m³ of storage.
<snip>

Think it is feasible to get an empty 13 meter external diameter tank on top of a Super Heavy with a stubby upper stage module (no payload section) to LEO to be outfitted with a propulsion module replacing the stubby stage module and a payload module on the top.

Back of the envelope calculation for a make up tank with internal dimensions of 42 meter high and 12 meter diameter is about 4296 m³ in volume not including internal tank fittings and plumbing.

Well to paraphrase some of our fellow enthusiasts here, it is physically possible within the current laws of physics, so sure, why not?

But from an engineering and logistics standpoint, very painful as to make it unlikely.  We have very little experience mating orbiting objects outside of simple docking.  So connecting all the plumbing in zeroG?   Matching the orbits?  Propelling the tank designed for all H2 using CH4/LOX?

Well docking and propellant transfer is required for Starship to go beyond LEO. The LH tank is empty but pressurized with gases to maintain structural integrity. You misunderstood, the LH tank is on top of a propulsion module that is makeup of a Starship without the payload section.

How are you going to make and transport 13m rings?  There are no jigs for it.  I suspect the transportation systems, which use up an entire roadway (both directions) to transport 9m rings, would not scale out to 13m.  Will the chopsticks go that wide?

Making 13 meter rings will likely be more or less how SpaceX is making 9 meter rings now with new inhouse jigs.

Transport of the vertical stack with the LH tank on top of the propulsion module shouldn't be much different than the current transport of the 9 meter Starships and Super Heavy with the same footprint on the ground.

The chopsticks can be modified if not already capable of going wide enough to go around the LH tank. Remember that the grid fins on the Super Heavy are fixed. The Chopsticks lifts the stack with lifting points on the stubby propulsion module.

It is my understanding that SpaceX has consider an enlarged 13 meter diameter Starship stack. Mainly to send up propellants, AIUI.

Transferring fuel in orbit is logistically the most simple thing we can do, and we haven't really done it yet at scale.

As pointed out above, Starship required docking and propellant transfer to go beyond LEO.

You are still also having to transfer cargo in LEO, another logistical and engineering nightmare.  (well std short cargo containers would probably make it reasonably easy, but everyone balks at their mass when I bring it up).
<snip>

There is no transfer of cargo. Just docked the stack of payload modules onto the tank section with cargo aboard each module. Presumably each payload module is sized to fitted inside a Chomper Satellite Delivery Starship.