Fenris - SSTO candidate?

[JoeSchmuckatelli]

Quote

 

 

Just ran across this 

https://www.wired.com/story/the-rocket-motor-of-the-future-breathes-air-like-a-jet-engine/

Seems interesting.

Quote

Based on his computer models, Davis says he expects to achieve over 600 seconds of specific impulse during the tests

Any idea why the graphic shows the engine as an arc?l

 

Edited June 26, 2020 by JoeSchmuckatelli

[wumpus]

600s seems a little low for a SSTO candidate, but considering GNOM had 550s I suspect that most of the advantage is from air augmentation.  Perhaps they are using intake air as oxidizer, but it can't be much with that Isp (much, much less than X-43).

The unavoidable problem with a SSTO is that you essentially subtract the mass of your high-thrust booster engines (plus their propellant tanks) from your final payload (with traditional engines this means a trivial payload, with 600s Isp it might be enough to matter).  It isn't clear how light and powerful these engines will be.

[K^2]

With good ascent trajectory you only need about 9.5km/s. So that's about 1.6x the exhaust velocity at 600s. So you only need 75% of your takeoff weight to be propellant. Very reasonable for SSTO.

Honestly, even with just conventional rocket on LH2, I_SP is pretty close to ideal. The two challenges that prevent SSTO from being the simplest way of getting to orbit is needing different nozzle bells for different altitudes and being able to maintain efficiency at full throttle and throttling down to about 10%. And the moment you start fiddling with using different engines for different parts of the ascent trajectory, two stage is just cheaper, and then you might as well go kerlox.

If you have an engine that provides a weighted average of 600s throughout ascent and has the right throttle range, yeah, SSTO sounds like a good way to go.

[KSK]

11 hours ago, JoeSchmuckatelli said:

Just ran across this 

https://www.wired.com/story/the-rocket-motor-of-the-future-breathes-air-like-a-jet-engine/

Seems interesting.

Any idea why the graphic shows the engine as an arc?l

The graphic looks like an old SABRE graphic lifted from the former Reaction Engines site. I don't recall the reason for the curvature and unfortunately the Reaction Engines site has gone a lot more corporate and is full of techno-teasers whereas before it had quite a lot of solid information about the SABRE concept. When I was writing about an airbreathing rocket engine for my KSP fiction, the old website is where I got all the technical details from.

As for the Fenris engine, the Wired article is fairly lightweight so it's hard to tell what Mountain Aerospace have actually got here but, as noted by their critics in the article, just sucking the air in through the front is avoiding most of the hard problems in building an airbreathing rocket engine. I wish them the best of luck though and it's quite possible that the Fenris engine will deliver - it's just hard to tell that from the long-on-hype-but-short-on-detail Wired article.

Oh - and personal peeve. It's fazed goddammit. Things can be in or out of phase. To be unafraid of the odds stacked against you is to be unfazed.

Edited June 27, 2020 by KSK

[JadeOfMaar]

18 hours ago, JoeSchmuckatelli said:

Any idea why the graphic shows the engine as an arc?l

The article, while it's about a new engine of the same kind, uses an image of the SABRE engine because everyone knows what SABRE is and does, and evidently, no proper image of Fenris has been taken (apart from that view of the intake and some pipes), likely for several good reasons.

Note that Fenris is said to be hour-glass shaped. Very different from SABRE.

 

[wumpus]

12 hours ago, K^2 said:

With good ascent trajectory you only need about 9.5km/s. So that's about 1.6x the exhaust velocity at 600s. So you only need 75% of your takeoff weight to be propellant. Very reasonable for SSTO.

Honestly, even with just conventional rocket on LH2, I_SP is pretty close to ideal. The two challenges that prevent SSTO from being the simplest way of getting to orbit is needing different nozzle bells for different altitudes and being able to maintain efficiency at full throttle and throttling down to about 10%. And the moment you start fiddling with using different engines for different parts of the ascent trajectory, two stage is just cheaper, and then you might as well go kerlox.

If you have an engine that provides a weighted average of 600s throughout ascent and has the right throttle range, yeah, SSTO sounds like a good way to go.

You'd need to throttle if you wanted a retropropulsive SSTO landing, and you'd still only need about 25% throttle (basically the inverse of your mass fraction, maybe less depending on how hard you want to hoverslam).  And of course if the thing is crew-rated, you'd have to keep TWR from creeping much past 3 (but that's far less throttle than needed for a hoverslam, somewhere between 50%-75% and probably something the RS-25 can do).

Even if it can't SSTO, I have to wonder how hard (assuming the engines were already developed) it would be to design a booster to lift Starship (or the upper stages of New Armstrong or similar) and return to launch.  Certainly you'd need the 10% throttle (possibly going the way of Merlin and only using a single engine), but it would allow a heavier booster to still be efficient and maintain costs (assuming that mating them together each time didn't break the bank).

[Nuke]

9 hours ago, KSK said:

The graphic looks like an old SABRE graphic lifted from the former Reaction Engines site. I don't recall the reason for the curvature and unfortunately the Reaction Engines site has gone a lot more corporate and is full of techno-teasers whereas before it had quite a lot of solid information about the SABRE concept. When I was writing about an airbreathing rocket engine for my KSP fiction, the old website is where I got all the technical details from.

As for the Fenris engine, the Wired article is fairly lightweight so it's hard to tell what Mountain Aerospace have actually got here but, as noted by their critics in the article, just sucking the air in through the front is avoiding most of the hard problems in building an airbreathing rocket engine. I wish them the best of luck though and it's quite possible that the Fenris engine will deliver - it's just hard to tell that from the long-on-hype-but-short-on-detail Wired article.

Oh - and personal peeve. It's fazed goddammit. Things can be in or out of phase. To be unafraid of the odds stacked against you is to be unfazed.

the skylon design operates at a high angle of attack so it bends down so the shock cones align with airflow, also its neccisary to angle the engine down in order to line up with the plane's cg, since its essentially one big fuel tank. the result is an engine curved 15 degrees, which i see as being neither efficient nor practical. the wide spacing in the engine nacelles is somewhat of a concern, losing one of those at any phase of flight would be catastrophic. keeping the engines more inboard would make such scenarios less catastrophic potentially allowing for an rtb without the loss of payload. the skylon is essentially a paper airplane they used to promote their engine design, i dont think the final design would look anything like that and its probably going to go into someone else's space frame. 

ultimately i see this as mostly useful for small crew shuttles or other small payloads. maybe some kind of space fighter. heavy lift largely reusable rockets would be better for large payloads though. 

Edited June 27, 2020 by Nuke

[KSK]

6 minutes ago, Nuke said:

the skylon design operates at a high angle of attack so it bends down so the shock cones align with airflow, also its neccisary to angle the engine down in order to line up with the plane's cg, since its essentially one big fuel tank. the result is an engine curved 15 degrees, which i see as being neither efficient nor practical. the wide spacing in the engine nacelles is somewhat of a concern, losing one of those at any phase of flight would be catastrophic. keeping the engines more inboard would make such scenarios less catastrophic potentially allowing for an rtb without the loss of payload. the skylon is essentially a paper airplane they used to promote their engine design, i dont think the final design would look anything like that and its probably going to go into someone else's space frame. 

Yeah, looks like the curved engine design has been dropped, at least if the current graphics on their website are anything to go by. As you say, the final design will probably be different again.

[K^2]

7 hours ago, wumpus said:

You'd need to throttle if you wanted a retropropulsive SSTO landing, and you'd still only need about 25% throttle (basically the inverse of your mass fraction, maybe less depending on how hard you want to hoverslam).  And of course if the thing is crew-rated, you'd have to keep TWR from creeping much past 3 (but that's far less throttle than needed for a hoverslam, somewhere between 50%-75% and probably something the RS-25 can do).

Highly efficient ascent profile requires very fine tuning of TWR along ascent. It peaks at about TWR of 3, which is great because that's also about as high as you want it to go for crewed ascent, as you point out, and it happens after you burn off some of the fuel. So if you start at 100% throttle with TWR a little under 2, and start gravity turn until you hit TWR of 3, you're actually pretty close to perfect ascent. But after that, you need to throttle way down. There are ascent profiles that let you ride the rocket at pretty close to 100% throttle, but they are way less efficient, bringing you up to 10-10.5km/s requirement in worst cases. That's often an acceptable trade off in staged rocket with kerlox or solid boosters, but it's really not good for SSTO. Which is why ability to throttle the engine is one of the major tech challenges to SSTO. And if you're combining that with something like aerospike to combat the change in pressure, you end up with a very complex and heavy engine.

[Nuke]

1 hour ago, K^2 said:

Highly efficient ascent profile requires very fine tuning of TWR along ascent. It peaks at about TWR of 3, which is great because that's also about as high as you want it to go for crewed ascent, as you point out, and it happens after you burn off some of the fuel. So if you start at 100% throttle with TWR a little under 2, and start gravity turn until you hit TWR of 3, you're actually pretty close to perfect ascent. But after that, you need to throttle way down. There are ascent profiles that let you ride the rocket at pretty close to 100% throttle, but they are way less efficient, bringing you up to 10-10.5km/s requirement in worst cases. That's often an acceptable trade off in staged rocket with kerlox or solid boosters, but it's really not good for SSTO. Which is why ability to throttle the engine is one of the major tech challenges to SSTO. And if you're combining that with something like aerospike to combat the change in pressure, you end up with a very complex and heavy engine.

ive read that the sabre engine will be about the size of a bus. 

[K^2]

2 minutes ago, Nuke said:

ive read that the sabre engine will be about the size of a bus. 

Yeah, but you're saving with bringing a lot less oxidizer, which is a huge chunk of the fuel weight. So this might be a good trade. I was talking more about conventional LH2/LOX engine.

[wumpus]

1 hour ago, K^2 said:

Highly efficient ascent profile requires very fine tuning of TWR along ascent. It peaks at about TWR of 3, which is great because that's also about as high as you want it to go for crewed ascent, as you point out, and it happens after you burn off some of the fuel. So if you start at 100% throttle with TWR a little under 2, and start gravity turn until you hit TWR of 3, you're actually pretty close to perfect ascent. But after that, you need to throttle way down. There are ascent profiles that let you ride the rocket at pretty close to 100% throttle, but they are way less efficient, bringing you up to 10-10.5km/s requirement in worst cases. That's often an acceptable trade off in staged rocket with kerlox or solid boosters, but it's really not good for SSTO. Which is why ability to throttle the engine is one of the major tech challenges to SSTO. And if you're combining that with something like aerospike to combat the change in pressure, you end up with a very complex and heavy engine.

That's one option.

One is to simply get enough delta-v in enough atmosphere so the extra .5-1.0 km/s won't kill you (but yes, you really want the high Isp last and the low Isp first, this is a bit of a kludge).
Or you could have a low-thrust methlox engine and vacuum bell to get you the final kick (possibly tuned, RS-25 style to use for landing.   Somehow I doubt that an airbreathing rocket works well upside down in an atmosphere).

Or you could simply ignore the whole issue and stick with two stages.  I suspect that making a spacecraft that can be mated together inexpensively might be easier to solve than SSTO.

[sevenperforce]

Skylon is unworkable because the underexpanded engine plume torches the back end of the ship in rocket mode.

If they get this working and can make it good, great. It reminds me of a design I proposed a while back. The challenge is making reuse close with positive payload.

Any engine that is good for SSTO is better for the first stage of a TSTO.

[magnemoe]

3 hours ago, sevenperforce said:

Skylon is unworkable because the underexpanded engine plume torches the back end of the ship in rocket mode.

If they get this working and can make it good, great. It reminds me of a design I proposed a while back. The challenge is making reuse close with positive payload.

Any engine that is good for SSTO is better for the first stage of a TSTO.

This, everyday astronaut did an video about why SSTO is an bad idea.  Yes you could probably get it to work, throwing enough money on it but you are most likely to get something with an shuttle launch cost and electron payload. 
Earth is not Kerbin. our orbital velocity is  9.6 km/s, Kerbin orbital velocity is 2.4 km/s, so an X-15 might make it into orbit here with drop tanks, an falcon 9 would almost put the second stage into orbit in short its better than an fully disposable FH. 
In short, it an idea who might work down the line but its like designing an supersonic jet in the 19th century. 
Orion pulse nuclear should work, better push that, yes seriously. 
 

[Spacescifi]

28 minutes ago, magnemoe said:

This, everyday astronaut did an video about why SSTO is an bad idea.  Yes you could probably get it to work, throwing enough money on it but you are most likely to get something with an shuttle launch cost and electron payload. 
Earth is not Kerbin. our orbital velocity is  9.6 km/s, Kerbin orbital velocity is 2.4 km/s, so an X-15 might make it into orbit here with drop tanks, an falcon 9 would almost put the second stage into orbit in short its better than an fully disposable FH. 
In short, it an idea who might work down the line but its like designing an supersonic jet in the 19th century. 
Orion pulse nuclear should work, better push that, yes seriously. 
 

 

Well... nuclear is bad for reasons we well know.

That's why I tend to kick the idea down into the future with antimatter catalyzed pulse propulsion.

The implication of this is also that nukes become ridicously small, so Earth had better be at peace or some random upset person is going to inconvinence a few people.

Even so, it is said that AM catalyzed nukes are just better, smaller but just as powerful with less radioactive fallout.

They are like a win-win provided we find workable storage... but leave that to the human minds of tomorow... with whatever new stuff they know then that we do not.

 

Perhaps once scifi tech like this becomes reality spaceship scifi will start dying?

As who needs to dream when we have the real deal?

Edited June 29, 2020 by Spacescifi

[Nuke]

9 hours ago, sevenperforce said:

Skylon is unworkable because the underexpanded engine plume torches the back end of the ship in rocket mode.

If they get this working and can make it good, great. It reminds me of a design I proposed a while back. The challenge is making reuse close with positive payload.

Any engine that is good for SSTO is better for the first stage of a TSTO.

that might be another reason to have your thrust vector 7.5 degrees off axis and the widely spaced engines. but yea skylon is a paper airplane designed to sell engines. 

however i still see an ssto as a light surface to leo craft, with multistage semi-reusable heavy rockets doing the heavy lifting. the advantages of being able to routinely operate with fast turn around. if combined with midair refueling, then you can pick your position for your orbital insertion manuver and are no longer constrained as much by launch windows. i wonder what the feasibility of refueling a space craft post re-entry to fly it back to its home space port under power. 

Edited June 30, 2020 by Nuke

[Spacescifi]

1 hour ago, Nuke said:

i wonder what the feasibility of refueling a space craft post re-entry to fly it back to its home space port under power. 

Hmmm... ever tried it in KSP? Someone should! I know just because someone did it in KSP DOES NOT mean they can do it in real life, but sometimes... you can LOL!

That is an excellent idea... yet it does also pose interesting challenges.

You have to either give the SSTO wings for lift to refuel... and also give it same safer modification of a nuclear air ramjet like project pluto.  Then you can refuel it in midair yeah.

The other option is no wings but some kind of helicopter powered via a nuclear reactor for powered launch and landing. Yet this had better be a lightweight SSTO. With some crazy high thrust chem or chem/solid mix rocket engines since that's the only way it has a snowball's chance on the sun of ever making it to orbital velocity.

Edited June 30, 2020 by Spacescifi

[Nuke]

44 minutes ago, Spacescifi said:

Hmmm... ever tried it in KSP? Someone should! I know just because someone did it in KSP DOES NOT mean they can do it in real life, but sometimes... you can LOL!

That is an excellent idea... yet it does also pose interesting challenges.

You have to either give the SSTO wings for lift to refuel... and also give it same safer modification of a nuclear air ramjet like project pluto.  Then you can refuel it in midair yeah.

The other option is no wings but some kind of helicopter powered via a nuclear reactor for powered launch and landing. Yet this had better be a lightweight SSTO. With some crazy high thrust chem or chem/solid mix rocket engines since that's the only way it has a snowball's chance on the sun of ever making it to orbital velocity.

i wouldn't doubt that its been done in ksp, though i haven't done it personally. this kind of thing seems like it would only really make sense for military applications, us space force would love to have this capability. 

[magnemoe]

5 hours ago, Spacescifi said:

 

Well... nuclear is bad for reasons we well know.

That's why I tend to kick the idea down into the future with antimatter catalyzed pulse propulsion.

The implication of this is also that nukes become ridicously small, so Earth had better be at peace or some random upset person is going to inconvinence a few people.

Even so, it is said that AM catalyzed nukes are just better, smaller but just as powerful with less radioactive fallout.

They are like a win-win provided we find workable storage... but leave that to the human minds of tomorow... with whatever new stuff they know then that we do not.

 

Perhaps once scifi tech like this becomes reality spaceship scifi will start dying?

As who needs to dream when we have the real deal?

The problem with antimatter catalyzed fusion is antimatter. 
Not only is it hard to make but it will explode if hot, cold, annoyed or bored. 

I say z-pinch look better. 
However both here is deep space engines. 

[JoeSchmuckatelli]

8 hours ago, magnemoe said:

The problem with antimatter catalyzed fusion is antimatter. 
Not only is it hard to make but it will explode if hot, cold, annoyed or bored. 

I say z-pinch look better. 
However both here is deep space engines. 

Do not annoy Happy Fun BallTM

[sevenperforce]

14 hours ago, magnemoe said:

Earth is not Kerbin. our orbital velocity is  9.6 km/s, Kerbin orbital velocity is 2.4 km/s, so an X-15 might make it into orbit here with drop tanks, an falcon 9 would almost put the second stage into orbit in short its better than an fully disposable FH. 

Pretty sure a Falcon 9 could put an entire second stage with payload into orbit on Kerbin. During the AMOS-17 launch, when SpaceX expended its used Block 5 booster, staging took place at 9520 km/hr or 2.64 km/s. Plus Kerbin's gravity drops off faster than Earth's, resulting in less gravity drag as you ascend.

12 hours ago, Nuke said:

however i still see an ssto as a light surface to leo craft, with multistage semi-reusable heavy rockets doing the heavy lifting. the advantages of being able to routinely operate with fast turn around.

The only reason you need a light, short-turnaround surface-to-LEO craft is for crew, and with crew you either need a heavy LES or extraordinarily high vehicle safety margins.

12 hours ago, Nuke said:

if combined with midair refueling, then you can pick your position for your orbital insertion manuver and are no longer constrained as much by launch windows. i wonder what the feasibility of refueling a space craft post re-entry to fly it back to its home space port under power. 

Entirely unfeasible. It's always going to be easier to just enter a phasing orbit and wait until your re-entry trajectory brings you down close to your home space port. Phasing orbits are essentially free, and refueling requires velocity-matching which requires you to be under power.

[Nuke]

2 hours ago, sevenperforce said:

The only reason you need a light, short-turnaround surface-to-LEO craft is for crew, and with crew you either need a heavy LES or extraordinarily high vehicle safety margins.

Entirely unfeasible. It's always going to be easier to just enter a phasing orbit and wait until your re-entry trajectory brings you down close to your home space port. Phasing orbits are essentially free, and refueling requires velocity-matching which requires you to be under power.

i was mostly thinking military applications where larger than usual safety margins are acceptable. say you have a space station which has been taken over by another government, and you need to send in a special ops team. granted were not quite where we need to do things like this yet. maybe they want to do a cover up and a properly lined up re-entry puts you in a position to be shot down by that country, so re-enter with different phasing, refuel, and rtb. you probably have to come up with a tom clancy esque situation where this capability is useful. 

crew is specifically the thing you need sstos for. especially if you want space travel to eventually be affordable to the masses. the skylon's approach to crew launch is a hab module in its cargo bay and the entire crew module could be jettisoned in an emergency and landed with chutes. biggest risk is engine failure at take off (and i have serious concerns about engine placement on skylon). a failure of one of the precoolers can be countered by switching to rockets giving you the option of a fuel dump and turn around. non-explosive failure of other parts of the engine might also be sufficient for a come around on the other engine (provided its placement is made more sane). explosive failure of an engine would probably trigger jettisoning the crew module. i think it could be made even safer than rockets with sufficient refinements.