A new way of looking at Asparagus staging?

[Pecan]

...The boost stage looks to be a lot like my knocked- together example. single mainsail surrounded by aerospikes with radial engines...

Is there any particular reason why you're using the mainsail and radial engines - all of which have fairly awful ISPs? Clusters of 48-7Ss and/or aerospikes are much more efficient.

[GoSlash27]

I goofed somewhere and gave it way more DV than it needed for the mission. By the time I was circularized and zeroed out my inclination, it had hardly used any of the fuel in the injection stage.

The transstage worked perfectly as designed and there was more boost stage DV than I needed. It'll need some tweaking...

[GoSlash27]

Is there any particular reason why you're using the mainsail and radial engines - all of which have fairly awful ISPs? Clusters of 48-7Ss and/or aerospikes are much more efficient.

That they are, but unfortunately I have to meet the acceleration requirement using the available real estate. Generally when you break the job down into a small DV budget for a stage, the higher thrust to weight of a lighter engine can override the Isp advantage of a heavier but more efficient engine. I'm not a fan of any of the big radius engines, but in this case I need the kilonewtons.

Best,

-Slashy

[nohelmet]

.

So what if you set up the center stage to accelerate the payload at a desired rate, while the radially dependent stages merely have enough thrust to lift themselves? Looked at this way, the radial stages become not "boosters", but actually "massles fuel tanks".

Has anyone experimented along these lines?

Best,

-Slashy

I tried exactly that and found that I had better results with higher -TWR boosters.

My understanding is that even though those "self-supporting tanks" do not exert downwards force, their mass still adds to the inertia and makes it hard for the center rocket to accelerate fast enough. So we end up going pretty slowly and spending a lot of time (and fuel) getting up.

[Alistone]

For optimal ascent you want to *ideally* be as close to terminal velocity as possible. This means accelerating to 100 m/s as quickly as possible. Your core may have a T/W ratio of 3, but if there is 6 times as much mass on the radial boosters which all have a T/W ratio of 1 then your vessel only has a T/W ratio of 1.29 (core has 3 thrust units and 1 mass unit; radials have 1 thrust unit and 1 mass unit. Total of 7 mass unit and 9 thrust units; 9/7 nets 1.29 T/W).

Suddenly it will take you 30+ seconds to get up to 100 m/s and you'll have been suffering excessive gravity losses due to your slow acceleration to terminal velocity. Your rocket has been running for 30 seconds and you are still below 1500m; a similar vehicle with better T/W ratio at the start is already at 4,000m and moving faster.

As you drop radial tanks your T/W ratio improves; which is good because above 12,000m the higher T/W ratio will allow you to reduce your gravity losses significantly as you can "gravity turn" toward the horizon more instead of thrusting straight up against gravity.

But during your climb from 1,000m to 12,000m your speed only needs to increase from 100 m/s up to 250 m/s. That's only 150m/s increase during the 60 seconds of trudging through thick atmosphere. If you go faster than terminal velocity you lose more to atmospheric drag than you save in gravity losses. That slow trudge requires a pretty low T/W ratio (fine with about 1.15 T/W). Much lower than the initial burst to get up to 100 m/s within the first 1000m.

Different parts of your ascent are optimal with different T/W ratios. For this reason you may be better off adding some extra fuel tanks to the stages that are consumed during the 1,000 - 12,000m atmosphere portion of your flight because the reduced T/W they cause doesn't cost you any real performance. Similarly it is a good reason to use small SRB at liftoff to ensure you quickly reach 100m/s. You find that during the slow trudge through atmosphere stage it may even pay off to have a radial stage that begins with less than 1 T/W ratio because as it loses fuel it brings the average T/W closer to the optimal needed while providing more fuel for future stages.

The efficiency of asparagus is approximate; it keeps a relatively consistent T/W ratio by jettisoning dead weight engines at the same time it jettisons the empty tanks. The reason this is good is because in atmosphere you don't just want as high a T/W ratio as possible due to excessive air resistance losses. But because it is only approximate you are forced to adjust your speed to match terminal velocity by throttling back during the thick atmosphere portion and the extra engines power you aren't using is essentially just additional dead weight causing gravity losses.

The other reason to jettison the booster engines is due to their ISP. Most launch engines have a high T/W ratio, but have a relatively low ISP. High ISP engines tend to be heavier and require a certain run period; a certain amount of fuel to pass through them before they pay back that added weight. The radial engines tend to have low ISP but a very high T/W ratio. The T/W is highly beneficial for the lifting power it provides lower stages of a rocket in escaping a gravity well. But in space the higher ISP of other engines makes them much better for orbital maneuvers and if possible you prefer to jettison the ineffecient low ISP engine prior to actually reaching orbit.

[weezl]

I had an interesting thought earlier and I'd like to share it here and let you folks kick it around...

The advantage of asparagus staging in terms of efficiency is that virtually none of the weight involved is "dead" weight. The disadvantage is that it's payload capacity is ultimately limited by how much payload the last remaining stage can accelerate acceptably.

So what if you set up the center stage to accelerate the payload at a desired rate, while the radially dependent stages merely have enough thrust to lift themselves? Looked at this way, the radial stages become not "boosters", but actually "massles fuel tanks". They provide additional DV by donating their unburned fuel to the core booster, rather than the usual practice of providing additional thrust. If this line of reasoning is correct, an asparagus booster designed in this fashion should be noticeably more efficient than common asparagus designs.

As an example, say you're designing an asparagus booster as a transstage for a 50t payload/injection stage combo. You want to generate 2,000 DV at 1G. So you design the core stage to generate maybe 700DV while lifting 20T + whatever it's fuel is. The radial stages have the same amount of fuel as the core, but only enough thrust to lift themselves. Each stage pair would then be expected to keep the core booster running to generate another 200+450+650 and ultimately fulfill the DV requirement.

This should also cut down on the inherent instability problems since the bulk of the thrust is behind the payload instead of spread out.

Has anyone experimented along these lines?

Best,

-Slashy

Most of my 'big' Tier 2 lifters work on basically this principle.. for the initial ascent stage (where I mostly use this style of staging): I size the inner-most part of the asparagus stage for TWR = 2.0, then place radial tanks with engines sized to keep the TWR at 2 (approximately). for a 40 ton lifter this means a mainsail/jumbo tank in the center, then the next layer out has poodles or skippers (depending on tank size and the exact size of my upper payload), etc.

One thing to note.. I do spreadsheets to figure my staging.. I don't use an add on. I wanted to understand the rocket equation from the inside out.. so I implemented a spreadsheet that pre figures the total dV for a given stage and shows the TWR for all combined engines on that stage. I can throw a basic design together VERY quickly using the spreadsheet, by just copying my desired parts to the proper place in the sheet. Its not super-fancy at this point, but it certainly gets the job done. Since I typically design for low risk vehicles, I don't worry much about how the ISP changes between air and vacuum.. I just use the air values for all stages that are either at liftoff or in the transition.. this gives me extra dV for emergencies and monkey-based-accidents (I mess up).

[Duxwing]

I will test this alternative arrangement for cost-effectiveness relative to standard asparagus.

-Duxwing

[GoSlash27]

Most of my 'big' Tier 2 lifters work on basically this principle.. for the initial ascent stage (where I mostly use this style of staging): I size the inner-most part of the asparagus stage for TWR = 2.0, then place radial tanks with engines sized to keep the TWR at 2 (approximately). for a 40 ton lifter this means a mainsail/jumbo tank in the center, then the next layer out has poodles or skippers (depending on tank size and the exact size of my upper payload), etc.

One thing to note.. I do spreadsheets to figure my staging.. I don't use an add on. I wanted to understand the rocket equation from the inside out.. so I implemented a spreadsheet that pre figures the total dV for a given stage and shows the TWR for all combined engines on that stage. I can throw a basic design together VERY quickly using the spreadsheet, by just copying my desired parts to the proper place in the sheet. Its not super-fancy at this point, but it certainly gets the job done. Since I typically design for low risk vehicles, I don't worry much about how the ISP changes between air and vacuum.. I just use the air values for all stages that are either at liftoff or in the transition.. this gives me extra dV for emergencies and monkey-based-accidents (I mess up).

Same here. Spreadsheets rawk! I also have a part in there where I reversed the rocket equation and applied it to each engine. I just put in a payload, required DV, number of engines and local gravity and it spits out how much fuel (including the mass of the tank) I would need, the recommended number of engines to make various G levels, total vehicle mass, etc. for each engine. I can tell at a glance which combo would be most efficient for a given stage.

Best,

-Slashy

[Pecan]

You can't get better than starting with tavert's engine charts.

Stick that in your spreadsheet with your desired number of stages, just like I do '-0

... except that, in fact, I rarely end-up with optimal designs. 15% payload-ratio is to be expected, if not required, but part-count and build considerations usually mean I compromise quite a bit from optimal - there are some excellent 18%+ designs around.

[weezl]

You can't get better than starting with tavert's engine charts.

Stick that in your spreadsheet with your desired number of stages, just like I do '-0

... except that, in fact, I rarely end-up with optimal designs. 15% payload-ratio is to be expected, if not required, but part-count and build considerations usually mean I compromise quite a bit from optimal - there are some excellent 18%+ designs around.

Since right now in career mode there is no concept of cost of a rocket.. I tend to build lifters based on general lift needs (5 ton/10 ton/20 ton/40 ton) and don't worry about the wastage. I'm expecting this to change in the next release.. which is another reason why I built my spreadsheet (the first being understanding how the Rocket equation works in mixed ISP engine situations).

There are certain situations where I'll tailor the rocket to the payload (long range very light probe is a good for instance).. but for repetitive use, you can't beat standardized lifters.

[NecroBones]

So what if you set up the center stage to accelerate the payload at a desired rate, while the radially dependent stages merely have enough thrust to lift themselves? Looked at this way, the radial stages become not "boosters", but actually "massles fuel tanks". They provide additional DV by donating their unburned fuel to the core booster, rather than the usual practice of providing additional thrust. If this line of reasoning is correct, an asparagus booster designed in this fashion should be noticeably more efficient than common asparagus designs.

...

Has anyone experimented along these lines?

Yep, in fact I've seen Asparagus thought through this way before in other threads. I've done designs before where as long as the fuel tanks lifted themselves, I felt that their thrust was sufficient. It's a good idea, generally speaking.

[Jouni]

Since right now in career mode there is no concept of cost of a rocket.. I tend to build lifters based on general lift needs (5 ton/10 ton/20 ton/40 ton) and don't worry about the wastage. I'm expecting this to change in the next release.. which is another reason why I built my spreadsheet (the first being understanding how the Rocket equation works in mixed ISP engine situations).

We still don't know what kind of rockets are going to be efficient in the career mode. If engines are expensive, we'll build rockets with low initial TWR. If fuel is expensive, we'll go for the traditional asparagus solution with high TWR and high payload fraction. If decouplers and fuel lines are expensive, we'll probably want to minimize the number of boosters.